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                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
                            ------------------------
 
                                  FORM 10-K/A
                                ---------------
 
   
                                AMENDMENT NO. 2
    
 
(MARK ONE)
 
  /X/    ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE
         SECURITIES EXCHANGE ACT OF 1934 [FEE REQUIRED]
 
                   FOR THE FISCAL YEAR ENDED OCTOBER 31, 1998
                                       OR
 
  / /    TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE
         SECURITIES EXCHANGE ACT OF 1934 [NO FEE REQUIRED]
 
        FOR THE TRANSITION PERIOD FROM ______________ TO ______________
 
                         COMMISSION FILE NUMBER 0-25540
 
                               STB SYSTEMS, INC.
 
             (Exact name of registrant as specified in its charter)
 
                   TEXAS                               75-1855896
      (State or other jurisdiction of               (I.R.S. Employer
       incorporation or organization)              Identification No.)
 
           3400 WATERVIEW PARKWAY                         75080
             RICHARDSON, TEXAS                         (Zip Code)
  (Address of principal executive offices)
 
       Registrant's telephone number, including area code: (972) 234-8750
 
          SECURITIES REGISTERED PURSUANT TO SECTION 12(b) OF THE ACT:
 
                                      None
 
          SECURITIES REGISTERED PURSUANT TO SECTION 12(g) OF THE ACT:
 
                    Common Stock, par value $0.01 per share
                                (Title of class)
 
    Indicate by check mark whether the registrant (1) has filed all reports
required to be filed by Section 13 or 15(d) of the Securities Exchange Act of
1934 during the preceding 12 months (or for such shorter period that the
registrant was required to file such reports), and (2) has been subject to such
filing requirements for the past 90 days. Yes /X/  No / /
 
    Indicate by check mark if disclosure of delinquent filers pursuant to Item
405 of Regulation S-K is not contained herein, and will not be contained, to the
best of registrant's knowledge, in definitive proxy or information statements
incorporated by reference in Part III of this Form 10-K or any amendment to this
Form 10-K. /X/
 
    The aggregate market value on January 20, 1999 of the voting and non-voting
common equity held by non-affiliates of the registrant was $91,444,815.
 
    Number of shares of registrant's Common Stock, par value $0.01 per share,
outstanding as of January 20, 1999: 12,606,787.
 
                      DOCUMENTS INCORPORATED BY REFERENCE
 
    None.
 
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ITEM 7. MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS
  OF OPERATIONS.
    
 
   
    Unless the context otherwise requires, the term "STB," "we" or "us" when
used in this report refers to STB Systems, Inc., a Texas corporation, and its
consolidated subsidiaries and prior affiliates.
    
 
    THIS REPORT CONTAINS CERTAIN FORWARD-LOOKING STATEMENTS WITHIN THE MEANING
OF THE FEDERAL SECURITIES LAWS. ANY STATEMENTS CONTAINED HEREIN -- INCLUDING
WITHOUT LIMITATION STATEMENTS TO THE EFFECT THAT STB OR ITS MANAGEMENT
"BELIEVES," "EXPECTS," "ANTICIPATES," "PLANS," "MAY," "WILL," "PROJECTS,"
"CONTINUES" OR "ESTIMATES," OR STATEMENTS CONCERNING "POTENTIAL," OR
"OPPORTUNITY" OR OTHER VARIATIONS THEREOF OR COMPARABLE TERMINOLOGY OR THE
NEGATIVE THEREOF -- THAT ARE NOT STATEMENTS OF HISTORICAL FACT SHOULD BE
CONSIDERED FORWARD-LOOKING STATEMENTS. ACTUAL RESULTS AND THE TIMING OF CERTAIN
EVENTS COULD DIFFER MATERIALLY FROM THOSE PROJECTED IN OR CONTEMPLATED BY THE
FORWARD-LOOKING STATEMENTS DUE TO A NUMBER OF FACTORS, INCLUDING WITHOUT
LIMITATION THOSE SET FORTH UNDER "RISK FACTORS" IN STB'S ANNUAL REPORT ON FORM
10-K FOR THE YEAR ENDED OCTOBER 31, 1998.
 
OVERVIEW
 
    We currently sell two broad categories of products, multimedia subsystem
products and specialized technology products. Our multimedia subsystem product
line includes a wide selection of multimedia accelerators designed for use in
mid-range to high-end PCs. Our multimedia subsystem product line also features
several complementary products, including DVD decoder subsystems and PC/TV
convergence subsystems. Our specialized technology products incorporate graphics
technologies and are primarily designed to enable one computer to control
simultaneously the display of multiple monitors.
 
    We sell our products to OEMs, the commercial market and the specialized
technology market. We sell multimedia subsystem products both to OEMs as
subsystems for their PC products and to the commercial market. Sales of
multimedia accelerators and other multimedia subsystems to OEMs typically
possess higher unit volumes and lower gross profit margins. Sales of multimedia
products to the commercial market typically have modest unit volumes and higher
gross profit margins than the sale of similar products to OEMs. Although sales
volumes of specialized technology products are relatively low, we realize higher
gross profit margins from the sale of these products than from the sale of
multimedia subsystem products.
 
    For the fiscal years 1998 and 1997, sales of our products to OEMs
represented approximately 80% and 79%, respectively, of total net sales. Sales
to the commercial market represented approximately 12% of total net sales for
fiscal years 1998 and 1997. Sales to specialized technology product markets
constituted approximately 7% of total net sales for fiscal 1998 and 8% of total
net sales for fiscal 1997. We derived the balance of total net sales primarily
from third party assembly services. Third party assembly services comprised
approximately 1% of total net sales for fiscal years 1998 and 1997. We export
our products through all of our sales channels. Export sales of our products
have grown moderately in recent periods. As a result, exports have increased as
a percentage of net sales to 28% in fiscal 1998 from 27% in fiscal 1997. Our
total gross profit margins and gross profits will likely fluctuate from period
to period as a result of our product mix, sales channel mix, component costs and
the competitive pricing pressures on our products.
 
    We recognize revenue upon shipment of our products. For products sold
through the commercial channel, we generally allow returns in the form of stock
rotation and price protection in the form of credits. Our current stock rotation
policies permit a commercial customer to return a portion of the products
purchased within specified time periods, if that customer places an order with
us for additional products of equal or greater value. We also provide price
protection to commercial channel customers in the form of credits for price
reductions on products remaining in customer inventories at the time of the
price reduction. We maintain reserves related to these programs, and we believe
that such reserves are adequate.
 
    We have no guaranteed supply arrangements with any of our suppliers. We
obtain most of the primary components of our products directly from the
component manufacturers. The primary components of our products consist mainly
of controller chips and memory chips. The prices of such components can change
significantly from time to time. In the past we have experienced, and may in the
future experience, increases in our unit component costs without being able to
increase the price of the related products. Such
<PAGE>
an increase in component costs could negatively impact our gross profit margins
and results of operations. In particular, occasional world-wide shortages of
memory and controller chips and international tariff disputes have in the past
resulted in substantial unit component cost increases that have materially
adversely affected our gross profit margins and our results of operations.
 
   
    On December 13, 1998, we entered into the 3Dfx Merger Agreement. The 3Dfx
Merger Agreement provides for the merger of a newly formed, wholly-owned
subsidiary of 3Dfx with and into STB (the "3Dfx Merger"). STB will be the
surviving corporation in the 3Dfx Merger and, upon consummation of the 3Dfx
Merger, will become a wholly-owned subsidiary of 3Dfx. In the event of the
consummation of the 3Dfx Merger, the combination of 3Dfx's and STB's operations
will result in many significant changes in STB's business and its related
results of operations and financial condition. In particular, the announcement
and consummation of the merger may disrupt STB's relationships with its
suppliers, some of whom are competitors of 3Dfx. For example, nVidia, which is a
major supplier of STB and whose graphics chips were incorporated into STB
products representing 63.9% of STB's net sales in fiscal 1998 (approximately
$170.1 million net sales), competes directly with 3Dfx. As a result of the
merger, STB plans to continue to offer for sale to its customers its current
products that use nVidia graphics chips, but does not expect to use nVidia's
graphics chips on any new products. Unless STB or the 3Dfx/STB combined company
can persuade STB's existing customers that are purchasing products using nVidia
graphics chips to purchase new products based on 3Dfx graphics chips, the
revenue derived from sales to such customers will be reduced significantly.
Similarly, while no suppliers of graphics chips to STB have as of the date of
this report indicated to STB that they intend to terminate their supplier
relationship with STB, to the extent that any such relationship is terminated or
curtailed and STB cannot persuade its existing customers who purchase products
containing any of those supplier's chips to purchase products containing 3Dfx
chips then revenues could be reduced significantly.
    
 
RESULTS OF OPERATIONS
 
    The following table sets forth certain items from our Consolidated
Statements of Operations as a percentage of net sales:
 
<TABLE>
<CAPTION>
                                                                           PERCENTAGE OF NET SALES
                                                                           YEAR ENDED OCTOBER 31,
                                                                       -------------------------------
                                                                         1998       1997       1996
                                                                       ---------  ---------  ---------
<S>                                                                    <C>        <C>        <C>
Net sales............................................................      100.0%     100.0%     100.0%
Cost of sales........................................................       83.4%      74.9%      80.4%
                                                                       ---------  ---------  ---------
Gross profit.........................................................       16.6%      25.1%      19.6%
                                                                       ---------  ---------  ---------
                                                                       ---------  ---------  ---------
Operating expenses:
  Research and development...........................................        4.0%       3.4%       2.4%
  Sales and marketing................................................        6.6%       7.4%       6.1%
  General and administrative.........................................        4.8%       5.3%       5.3%
                                                                       ---------  ---------  ---------
Total operating expenses.............................................       15.4%      16.1%      13.8%
                                                                       ---------  ---------  ---------
Income from operations...............................................        1.2%       9.0%       5.8%
Interest expense, net................................................        0.2%       0.8%       0.6%
                                                                       ---------  ---------  ---------
Income before income taxes...........................................        1.0%       8.2%       5.2%
Provision for income taxes...........................................        0.3%       2.8%       1.8%
                                                                       ---------  ---------  ---------
Net income...........................................................        0.7%       5.4%       3.4%
                                                                       ---------  ---------  ---------
                                                                       ---------  ---------  ---------
</TABLE>
 
                                       2
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FISCAL YEAR ENDED OCTOBER 31, 1998 COMPARED TO FISCAL YEAR ENDED OCTOBER 31,
  1997
 
    NET SALES.  Net sales increased by $66.8 million, or 33.5%, from $199.5
million in fiscal 1997 to $266.3 million in fiscal 1998. This increase resulted
from continued growth in all sales channels. Unit volume for fiscal 1998
increased by 25.7% over fiscal 1997. Our overall average unit selling prices
increased slightly, primarily as a result of increased product performance and
complexity and higher memory configurations. OEM channel sales increased $58.9
million, or 38.3%, from approximately $153.5 million in fiscal 1997 to
approximately $212.4 million in fiscal 1998. Sales growth in the OEM channel
primarily resulted from increased sales to existing customers. Commercial
channel sales increased $8.9 million, or 37.1%, from approximately $23.9 million
in fiscal 1997 to approximately $32.8 million in fiscal 1998. This increase in
sales to the commercial channel resulted primarily from sales of the award
winning Velocity 128, as well as the Black Magic Voodoo 2 and Velocity 4400
multimedia accelerators to established customers. Sales in the specialized
technology market experienced moderate growth, increasing from approximately
$15.2 million in fiscal 1997 to approximately $16.9 million in fiscal 1998, an
increase of $1.7 million, or 11.3%. Unit volume for specialized technology
products increased approximately 24.2%, while average unit selling prices
declined slightly as a result of increased competition in the market. An
increase in sales to existing customers also contributed to the moderate
increase in sales of specialized technology products.
 
   
    GROSS PROFIT.  Gross profit decreased by $5.8 million, or 11.6%, to $44.2
million in fiscal 1998, as compared to $50.0 million in fiscal 1997. For the
period, gross profit as a percentage of net sales declined to 16.6% from 25.1%.
The decrease in the amount of gross profit margin resulted primarily from
increased pricing pressure in the commercial and OEM markets and to a lesser
degree, the 10.3% decline in average selling prices in the specialized
technology market from fiscal 1997 to fiscal 1998. The decrease in gross profit
as a percentage of net sales resulted primarily from increased pricing pressure
on the Company's products and a decrease in higher margin specialized technology
products as a percentage of total sales.
    
 
   
    Price protection credits granted during the year ended October 31, 1998 were
$4.2 million compared to $1.3 million during the year ended October 31, 1997.
The increase in price protection resulted primarily from price protection
credits granted in the fourth quarter of 1998 for the Velocity 128 and Black
Magic Voodoo 2 products, totaling approximately $2.2 million. STB's inability to
obtain an adequate supply of SDRAM memory chips resulted in the late
introduction of these products and as a result, STB was forced to price protect
the products in the channel shortly after introduction to remain competitive.
    
 
   
    Amounts charged to expense for potential excess and obsolete inventory were
$6.4 million and $2.2 million during the years ended October 31, 1998 and 1997,
respectively. The relative increase in amounts charged to expense for potential
excess and obsolete inventory during 1998 was primarily related to $3.2 million
for non-recurring charges. These charges are comprised of a $2.0 million charge
associated with components provided by a supplier that did not meet
specifications (which charges and various related issues are the subject of
litigation between STB and the supplier) and a $1.2 million charge attributable
to slow-moving inventory specifically identified through a detailed obsolescence
review periodically conducted by management. The slow-moving inventory charge
was a result of technological advancements in the marketplace, changes in
product mix, engineering changes and other adverse market conditions. Management
believes the above described charges for excess and obsolete inventory are
considered unusual in nature and are not expected to represent any trend which
would have a material impact on future results of operations, although this is a
forward-looking statement and there can be no assurance in this regard.
Excluding these two specific non-recurring reserves described above, STB's
inventory obsolescence reserve increased from $1.4 million to $2.0 million from
fiscal year end 1997 to 1998 as a result of the increase in inventory levels and
the normal obsolescence rates associated therewith. The remaining $2.6 million
of the $6.4 million charged to expense in fiscal 1998 represents scrap arising
from the manufacturing process. This amount represents approximately 1.5% of
production, and is considered to be a normal part of the manufacturing process.
The increases in price protection credits
    
 
                                       3
<PAGE>
   
granted and amounts charged to expense for potential excess and obsolete
inventory contributed to the decrease in gross profit margin.
    
 
    RESEARCH AND DEVELOPMENT EXPENSES.  Research and development expenses
increased by $4.1 million, or 60.1%, to $10.8 million in fiscal 1998, as
compared to $6.7 million in fiscal 1997. This increase resulted primarily from
increased staffing levels at our corporate headquarters in Richardson, Texas, as
well as at our design centers in Austin, Texas, Eugene, Oregon and Belfast,
Northern Ireland. Expenses associated with new product development, software and
driver development and continued enhancement and support of our existing
products also contributed to the increase. Research and development expenses as
a percentage of net sales increased from 3.4% in fiscal 1997 to 4.0% in fiscal
1998.
 
    SALES AND MARKETING EXPENSES.  Sales and marketing expenses increased by
$2.9 million, or 19.8%, to $17.7 million in fiscal 1998, as compared to $14.8
million in fiscal 1997. This increase resulted from additional staffing and
commissions paid as a result of our growth and higher sales levels. A decrease
in commissions paid to independent sales representatives partially offset the
general increase in sales and marketing expenses. Increased advertising and
promotional expenses in the commercial channel, the specialized technology
market and the international market also contributed to the overall increase in
sales and marketing expense. Sales and marketing expense as a percentage of net
sales decreased from 7.4% in fiscal 1997 to 6.6% in fiscal 1998.
 
    GENERAL AND ADMINISTRATIVE EXPENSES.  General and administrative expenses
increased by $2.1 million, or 19.3%, to $12.7 million in fiscal 1998, as
compared to $10.6 million in fiscal 1997. The increase is due primarily to
expenses associated with our growth, including increased staffing, occupancy
costs and other general operating expenses. Expenses associated with data
processing, as well as increased goodwill amortization also contributed to the
overall increase in general and administrative expenses. General and
administrative expense as a percentage of net sales decreased from 5.3% in 1997
to 4.8% in 1998.
 
    NET INCOME.  As a result of the foregoing factors, net income decreased by
$9.0 million, or 83.9%, to $1.7 million in fiscal 1998, as compared to $10.7
million in fiscal 1997.
 
FISCAL YEAR ENDED OCTOBER 31, 1997 COMPARED TO FISCAL YEAR ENDED OCTOBER 31,
  1996
 
    NET SALES.  Net sales increased by $19.3 million, or 10.7%, from $180.2
million in fiscal 1996 to $199.5 million in fiscal 1997. This increase result
from continued growth in all sales channels. Unit volume for fiscal 1997
increased by 27.4% over fiscal 1996. At the same time, our average unit selling
prices continued to decline primarily as a result of declines in component
costs. OEM channel sales increased $8.0 million, or 5.5%, from approximately
$145.5 million in fiscal 1996 to approximately $153.5 million in fiscal 1997.
Sales growth in the OEM channel resulted primarily from increased sales to
existing customers. Commercial channel sales increased $4.1 million, or 20.8%,
from approximately $19.8 million in fiscal 1996 to approximately $23.9 million
in fiscal 1997. This moderate increase in sales to the commercial channel
resulted primarily from increased sales to established customers. Sales in the
specialized technology market experienced significant growth, increasing from
approximately $10.9 million in fiscal 1996 to approximately $15.2 million in
fiscal 1997, an increase of $4.3 million, or 38.9%. Increased sales to existing
customers and the sale of products to OEM workstation groups both contributed to
the increase in sales of specialized technology products.
 
    GROSS PROFIT.  Gross profit increased by $14.7 million, or 41.9%, to $50.0
million in fiscal 1997, as compared to $35.3 million in fiscal 1996. For the
period, gross profit as a percentage of net sales increased to 25.1% from 19.6%.
The increase in gross profit margin resulted primarily from increased sales of
higher margin specialized technology products and, to a lesser degree, increased
sales to the commercial channel. In addition, declines in component costs,
economies of scale resulting from higher production volumes and greater
manufacturing efficiencies also contributed to the increase in gross profit
margin. Decreasing unit sales prices partially offset the increase in gross
profit margins.
 
                                       4
<PAGE>
    RESEARCH AND DEVELOPMENT EXPENSES.  Research and development expenses
increased by $2.3 million, or 52.2%, to $6.7 million in fiscal 1997, as compared
to $4.4 million in fiscal 1996. This increase resulted primarily from additional
staffing levels at our headquarters in Richardson, Texas, as well as at our
design centers in Houston, Texas and Eugene, Oregon. During 1997 we expanded our
research and development efforts by establishing and staffing a design center in
Belfast, Northern Ireland. Expenses associated with new product development,
software development and continued enhancement and support of our existing
products also contributed to the increase. Research and development expenses as
a percentage of net sales increased from 2.4% in fiscal 1996 to 3.4% in fiscal
1997.
 
    SALES AND MARKETING EXPENSES.  Sales and marketing expenses increased by
$3.8 million, or 34.6%, to $14.8 million in fiscal 1997, as compared to $11.0
million in fiscal 1996. This increase resulted from additional staffing and
commissions paid as a result of our growth and higher sales levels, as well as
increased travel and operating costs. Increased trade show expense, as well as
increased advertising and promotional expenses in the commercial channel, the
specialized technology market and the international market also contributed to
the overall increase in sales and marketing expense. Sales and marketing expense
as a percentage of net sales increased from 6.1% in fiscal 1996 to 7.4% in
fiscal 1997.
 
    GENERAL AND ADMINISTRATIVE EXPENSES.  General and administrative expenses
increased by $1.1 million, or 11.9%, to $10.6 million in fiscal 1997, as
compared to $9.5 million in fiscal 1996. The increase resulted primarily from
expenses associated with our growth, including increased staffing and related
expenses and data processing costs. An increase in the allocation of certain
costs related to the Mexican manufacturing operation to cost of goods sold
partially offset the increase in general and administrative expenses. Facility
expansion at our headquarters and related occupancy costs, including rent and
insurance, also contributed to the overall increase in general and
administrative expenses. As a result of the increase in operating income,
expenses associated our profit sharing plan also increased. For the periods,
general and administrative expense as a percentage of net sales remained
unchanged at 5.3%.
 
    NET INCOME.  As a result of the foregoing factors, net income increased by
$4.7 million, or 77.2%, to $10.8 million in fiscal 1997, as compared to $6.1
million in fiscal 1996.
 
SEASONALITY
 
    Our quarterly operating results vary significantly depending on factors such
as the timing of new product introductions, adequacy of component supply,
changes in component costs, variations in our product mix, seasonal promotions
by us and our customers and competitive pricing pressures. Because the timing of
these factors may vary, the results of any particular quarter may not be
indicative of results for the entire year or any future period. In addition, the
PC market generally experiences weaker sales during the summer months.
 
LIQUIDITY AND CAPITAL RESOURCES
 
    Inventory and accounts receivable financing and manufacturing and other
equipment expenditures constitute our principal capital and liquidity needs. We
have generally financed these requirements and our operations through a
combination of cash generated from operations, trade credit from vendors, bank
borrowings and the proceeds from our public offerings. As a result of our rapid
growth in recent years and our capital requirements, we completed a secondary
public offering of 2,775,000 shares of STB Common Stock during the second
quarter of fiscal 1998. We used the net proceeds from our secondary offering to
reduce indebtedness and retained the balance for general corporate purposes. We
recognize that future growth, if any, may require additional capital,
particularly to support increased working capital needs, staffing requirements,
promotional expenses and manufacturing facilities and equipment requirements.
 
    Cash used in operating activities was $148,000 in fiscal 1998, primarily
attributable to increases in inventory and other assets, as well as decreases in
accounts payable, partially offset by decreases in
 
                                       5
<PAGE>
accounts receivable, as a result of declining revenues late in the fourth
quarter. Cash used in operating activities was $3.9 million in fiscal 1997,
resulting primarily from increases in inventory and accounts receivable, as a
result of higher sales, partially offset by increased earnings and increases in
accounts payable. At October 31, 1998, the Company's working capital amounted to
$83.7 million, compared to $31.4 million at October 31, 1997. Cash and cash
equivalents equaled $30.6 million and $3.9 million at October 31, 1998 and 1997,
respectively.
 
    In fiscal 1998, we invested $3.1 million in capital equipment, compared with
net purchases of equipment aggregating $9.6 million during fiscal 1997. Our
investment in equipment is primarily attributable to manufacturing equipment
additions and upgrades of existing equipment to support the increased demand for
our products. During the first quarter of fiscal 1998, we completed a move to a
new manufacturing facility in Juarez, Mexico, immediately adjacent to our
previous facility. We have retained one-half of the previous facility for
expansion. During the fourth quarter of fiscal 1997, we installed two new high
speed surface-mount assembly lines at our new facility, at a total cost of
approximately $6.3 million. We installed an additional line in the third quarter
of fiscal 1998, at a cost of approximately $2.9 million. We financed this
equipment through operating lease finance arrangements. During the fourth
quarter of fiscal 1996, we installed four surface-mount technology assembly
lines, at an approximate cost totaling $4.2 million. We financed this equipment
through traditional lease financing arrangements also. Our aggregate obligations
under all such equipment lease financing arrangements totaled approximately $9.2
million at October 31, 1998 (see Note 8 of Notes to Consolidated Financial
Statements). We expect that additional capital expenditures for similar types of
equipment may be necessary to support any additional future customer demand and
production requirements.
 
    We have a $40.0 million revolving credit facility ("Revolving Credit
Facility"), as well as a $3.0 million term loan ("Term Loan"). At October 31,
1998, no amounts were outstanding under the Revolving Credit Facility and $2.7
million was outstanding under the Term Loan. Principal amounts outstanding under
the Revolving Credit Facility bear interest at LIBOR plus 175 basis points
(6.989% at October 31, 1998). Amounts outstanding under the Term Loan bear
interest at LIBOR plus 250 basis points and are payable in 60 monthly
installments of principal and interest. Payment of principal and interest began
November 1, 1997. Formulas based on eligible accounts receivable determine
availability under the new Revolving Credit Facility. All indebtedness under the
Revolving Credit Facility matures on November 21, 1999, and indebtedness under
the Term Loan matures on November 1, 2002 (subject to renewal of the Revolving
Credit Facility through such date).
 
    In December 1997, we entered into a five-year agreement to construct and
lease a new corporate headquarters in Richardson, Texas. Construction on the
210,000 square foot facility was completed in December 1998, and we completed
our move into the facility during that month. We estimate the total cost of the
building and the land to be approximately $22.8 million. The lessor agreed to
fund the cost of the land and construction of the building. Rental payments
commenced upon occupancy. We estimate that we will pay approximately $225,000
per month in rent over a four-year period beginning in the first quarter of
fiscal 1999. This amount exceeds the expense of our previous headquarter
facilities, because local rental rates have increased and we have increased the
square footage of our corporate headquarters. The lease agreement also provides
that the amount of the lease payments is subject to adjustment based upon
prevailing interest rates. Consequently, an increase in prevailing interest
rates will increase the expense of our facilities. We have recently entered into
an interest rate swap agreement that fixes the interest rate on a majority of
our lease obligation at 7.55%. We are also seeking opportunities to sublease
that portion of our new headquarters that we do not expect to utilize
immediately. At the end of the initial five-year lease, we have the option to
renew the lease for an additional five years, pay off the underlying debt or
cause the building to be sold. In the event of a sale, the proceeds are to be
used to retire the underlying debt. Any excess will be paid to us. We are
generally responsible for any remaining unpaid balance owing on the underlying
obligation after the sale of the facility.
 
                                       6
<PAGE>
YEAR 2000 ASSESSMENT
 
    STATE OF READINESS:  STB has performed a company-wide evaluation to assess
the ability of its products and its information technology ("IT") and non-IT
systems to properly function and execute transactions in the Year 2000. STB's
Year 2000 Project is divided into three major sections: (a) Infrastructure,
which includes internal management information systems, computers, servers,
networks to support the business and any non-IT systems used in the operation of
the business; (b) Third party Suppliers, which includes those suppliers that
provide STB with components that are used in the manufacture of its products;
and (c) STB Products which includes those products that generate revenue for
STB. The Project has been divided into six phases: (1) Awareness and
Communication; (2) Inventory; (3) Assessment; (4) Renovation; (5) Testing; and
(6) Rollout. As discussed below, STB has substantially completed the first three
phases of the Year 2000 Project for its Infrastructure; the first phase for its
Third Party Suppliers and all phases for its Customer Products and Services. All
phases of the Year 2000 Project are expected to be completed by the third
quarter of 1999.
 
    INFRASTRUCTURE:  STB has completed an assessment of its IT and non-IT
systems and currently is in the renovation phase for these systems. STB has
completed the renovation of its IT hardware systems and expects to complete a
renovation of its various software systems by June 1999. The Renovation, Testing
and Rollout phases of the Project are expected to be complete by July 1999. STB
has distributed a letter to each of its vendors that supply systems or software
for its IT and non-IT systems to determine the systems' Year 2000 status. A
majority of the recipients have responded to the letter, and most of the
respondents have given assurances that their products and services are able to
function in the context of the Year 2000 Problem either currently or through
upgrades to existing systems. A majority of the total systems are either
compliant currently or have been upgraded. STB is assessing these responses and
will continue to communicate with vendors that are material to its operations to
gain satisfactory assurances. If such assurances are not obtained, STB will seek
alternatives, including contracting with other vendors.
 
    THIRD PARTY SUPPLIERS:  STB has taken a inventory of the components supplied
from third party suppliers that are used in conjunction with its products. STB
has contacted significant third party suppliers in an effort to assess the state
of their Year 2000 readiness. To date, a majority of the recipients have
responded to the letter, and approximately 20% of the respondents have given
assurances that their products and services are able to function in the context
of the Year 2000 Problem. Approximately 25% of those responding have not been
willing to certify the Year 2000 compliance of their products. STB is continuing
to obtain responses to the letter and at the same time is formulating a
contingency plan that includes identifying alternate suppliers in the event STB
is unable to obtain such assurances. Many of STB's components and services are
obtained from sources that are not the sole source for such items. Accordingly,
STB believes that alternative means are generally available that are Year 2000
compliant from which to obtain components and services. Other than as described
above, STB has not undertaken to employ any independent verification or
validation process to determine its third party suppliers' state of Year 2000
readiness.
 
    STB PRODUCTS AND SERVICES:  During 1998, all of STB's products that were
produced in the last five years were tested and confirmed as compliant. STB has
transmitted letters to its customers notifying them of their current year 2000
readiness status. In general, STB believes that the nature of the functionality
of its products do not entail any date type functions. Therefore STB believes
that, the products do not have any Year 2000 performance implications.
 
    COSTS:  To date, STB has spent approximately $170,000 relating to software,
training and labor costs for its Year 2000 Project, of which $100,000 was
incurred for replacement costs for non-compliant software systems as well as the
acceleration of replacement of certain other systems as a result of the Year
2000 Problem. STB currently estimates that its software, training and labor
costs through fiscal year 1999 relating to the Year 2000 Project will be
approximately $100,000. No assurances, however, can be given that these costs
will not exceed such amount or that STB will not have to use other sources for
these amounts.
 
                                       7
<PAGE>
STB has to date only used internal resources to assess its Year 2000 risks and
cost estimates and does not expect to use any independent verification or
validation process in this regard. None of STB's other IT projects has been
delayed due to STB's Year 2000 Project. Funds for the Year 2000 Project are
expected to be paid for out of operations.
 
    RISKS:  If STB does not successfully complete its Year 2000 Project, it
could, among other results, prevent it from receiving orders and delivering Year
2000 compliant goods to customers and prevent it from placing orders and
receiving sufficient quantities of supplies from vendors. This could have a
material effect on STB's ability to market, sell and implement its products,
which could have a material adverse effect on its financial condition and
results of operations. STB's most reasonably likely worst case Year 2000
scenario would likely involve the failure of a critical supplier's products to
be Year 2000 compliant, which could temporarily suspend the manufacture and
delivery of STB's related products while the problem was fixed or an alternate
supply source was identified. As indicated above, STB is formulating an informal
contingency plan that includes identifying alternate suppliers. In addition, as
a result of Year 2000 concerns, the PC industry as a whole may experience
declining growth rates and a decreased demand for PCs and PC related products.
There can be no assurances that third parties will be Year 2000 compliant in a
timely manner.
 
    CONTINGENCY PLANS:  STB has to date experienced no significant Year 2000
problems. Although STB has not adopted a formal contingency plan, it is
currently assessing alternatives, which may be implemented in the event Year
20000 issues arise.
 
    RECENTLY ISSUED ACCOUNTING PRONOUNCEMENTS
 
    In June 1997, Statement of Financial Accounting Standards No. 130,
"Reporting Comprehensive Income" ("FAS 130"), was issued. FAS 130 establishes
standards for reporting and display of comprehensive income and its components
(revenues, expenses, gains, and losses) in a full set of general-purpose
financial statements. It mandates that all items required to be recognized under
accounting standards as components of comprehensive income be reported in a
financial statement that is displayed with the same prominence as other
financial statements. FAS 130 is effective for fiscal years beginning after
December 15, 1997. STB will adopt FAS 130 in the year ending October 31, 1999.
Reclassification of financial statements for earlier periods provided for
comparative purposes is required upon adoption.
 
    In June 1997, Statement of Financial Accounting Standards No. 131,
"Disclosure About Segments of an Enterprise and Related Information" ("FAS
131"), was issued, FAS 131 establishes standards for the way that public
business enterprises report information about operating segments in annual
financial statements and requires that those enterprises report selected
information about operating segments in interim financial reports issued to
shareholders. FAS 131 is effective for financial statements for periods
beginning after December 15, 1997. STB will adopt FAS 131 in the year ending
October 31, 1999.
 
    On March 4, 1998, the Accounting Standards Executive Committee of the
American Institute of Certified Public Accountants issued Statement of Position
No. 98-1, Accounting for the Costs of Computer Software Developed or Obtained
for Internal Use ("SOP 98-1"). SOP 98-1 requires computer software costs related
to internal use software that are incurred in the preliminary project stage
should be expensed as services consumed in developing or obtaining internal-use
computer software; payroll and payroll-related costs for employees who are
directly associated with and who devote time to the internal-use computer
software project (to the extent of the time spent directly on the project); and
interest costs incurred when developing computer software for internal use
should be capitalized. SOP 98-1 is effective for financial statements for fiscal
years beginning after December 15, 1998. Accordingly, STB will adopt SOP 98-1 in
its financial statements for the year ending October 31, 1999. STB does not
believe the adoption of SOP 98-1 will have a material effect on its results of
operations or financial condition.
 
    On June 15, 1998, the Financial Accounting Standards Board (FASB) issued
Statement of Financial Accounting Standards No. 133, "Accounting for Derivative
Instruments and Hedging Activities"
 
                                       8
<PAGE>
(FAS 133). FAS 133 is effective for all fiscal quarters of all fiscal years
beginning after June 15, 1999 (November 1, 1999 for STB). FAS 133 requires that
all derivatives instruments be recorded on the balance sheet at their fair
value. Changes in the fair value of derivatives are recorded each period in
current earnings or other comprehensive income, depending on whether a
derivative is designated as part of a hedge transaction and, if it is, the type
of hedge transaction. STB is currently evaluating implementation of FAS 133 and
the effects the statement will have on its financial statements and disclosures.
STB believes that, due to the current limited use of derivative instruments,
adoption of the statement will not have a material effect on its results of
operations, financial position, capital resources or liquidity.
 
                                       9
<PAGE>
                                    PART IV
 
ITEM 14.  EXHIBITS, FINANCIAL STATEMENT SCHEDULES, AND REPORTS ON FORM 8-K.
 
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    (a)  1.            The following financial statements are filed as part of this report:
 
                       Report of Independent Accountants.
                       Consolidated Balance Sheets dated October 31, 1998 and 1997.
                       Consolidated Statement of Operations for the three years ended October 31, 1998.
                       Consolidated Statement of Changes in Shareholders' Equity for the three years ended
                         October 31, 1998.
                       Consolidated Statement of Cash Flows for the three years ended October 31, 1998.
                       Notes to Consolidated Financial Statements.
 
                 2.    Consolidated Financial Statement Schedules
 
                       Report of Independent Accountants on Financial Statement Schedule.                                S-1
 
                       Consolidated Valuation and Qualifying Accounts.                                                   S-2
 
                       All other schedules for which provision is made in the applicable accounting regulation
                         of the Securities & Exchange Commission are not required under the related instructions
                         or are inapplicable and therefore have been omitted.
 
                 3.    The following documents are filed or incorporated by reference as exhibits to this
                         Report:
 
                 2.1   Agreement and Plan of Reorganization by and between the Registrant and STB Systems, Inc.
                         dated as of December 13, 1998, and the related Stock Option Agreement (incorporated by
                         reference to Schedule 13D of 3Dfx Interactive, Inc. dated December 23, 1998 with
                         respect to the Company).
 
                 3.1   Amended and Restated Articles of Incorporation of the Company (incorporated by reference
                         to Exhibit 3.1 to the Company's Registration Statement on Form S-1 (Registration No.
                         33-87612))
 
                 3.2   Articles of Amendment to Articles of Incorporation of the Company (incorporated by
                         reference to Exhibit 3.1 to the Company's Quarterly Report on Form 10-Q for the quarter
                         ended April 30, 1997)
 
                 3.3   Amended and Restated Bylaws of the Company (incorporated by reference to Exhibit 3.2 to
                         the Company's Registration Statement on Form S-1 (Registration No. 33-87612))
 
                 4.1   Specimen of Common Stock Certificate (incorporated by reference to Exhibit 4.1 to the
                         Company's Registration Statement on Form S-1 (Registration No. 33-87612))
 
                 4.2   Amended and Restated Articles of Incorporation and Bylaws of the Company (see Exhibits
                         3.1, 3.2 and 3.3 above)
 
                 4.3   Right of First Refusal Agreement dated December 16, 1994 by and among the Company and
                         Messrs. Ogle, Balthaser and Sims (incorporated by reference to Exhibit 4.3 to the
                         Company's Registration Statement on Form S-1 (Registration No. 33-87612))
 
                 9.1   Form of Voting Agreement dated December 13, 1998 between the Company and certain
                         shareholders of 3Dfx Interactive, Inc., a California corporation (incorporated by
                         reference to Schedule 13D of 3Dfx Interactive, Inc. dated December 23, 1998 with
                         respect to the Company).
</TABLE>
 
                                       10
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<C>        <C>         <S>                                                                                        <C>
              **10.1   Employment Agreement dated November 1, 1996 by and between the Company and William E.
                         Ogle (incorporated by reference to Exhibit 10.42 to the Company's Registration
                         Statement on Form S-1 (Registration No. 333-14313)), as amended by that certain
                         Amendment to Employment Agreement for Executive Officer dated December 13, 1998
 
              **10.2   Employment Agreement dated November 1, 1996 by and between the Company and Randall D.
                         Eisenbach (incorporated by reference to Exhibit 10.43 to the Company's Registration
                         Statement on Form S-1 (Registration No. 333-14313)), as amended by that certain
                         Amendment to Employment Agreement for Executive Officer dated December 13, 1998
 
              **10.3   Employment Agreement dated November 1, 1996 by and between the Company and James L.
                         Hopkins (incorporated by reference to Exhibit 10.44 to the Company's Registration
                         Statement on Form S-1 (Registration No. 333-14313)), as amended by that certain
                         Amendment to Employment Agreement for Executive Officer dated December 13, 1998
 
              **10.4   Employment Agreement dated November 1, 1996 by and between the Company and J. Shane Long
                         (incorporated by reference to Exhibit 10.45 to the Company's Registration Statement on
                         Form S-1 (Registration No. 333-14313)), as amended by that certain Amendment to
                         Employment Agreement for Executive Officer dated December 13, 1998
 
               *10.5   Indemnification Agreement dated February 8, 1995 by and between William E. Ogle and the
                         Company (incorporated by reference to Exhibit 10.23 to the Company's Registration
                         Statement on Form S-1 (Registration No. 33-87612))
 
               *10.6   Indemnification Agreement dated February 8, 1995 by and between Randall D. Eisenbach and
                         the Company (incorporated by reference to Exhibit 10.24 to the Company's Registration
                         Statement on Form S-1 (Registration No. 33-87612))
 
               *10.7   Indemnification Agreement dated February 8, 1995 by and between James L. Hopkins and the
                         Company (incorporated by reference to Exhibit 10.25 to the Company's Registration
                         Statement on Form S-1 (Registration No. 33-87612))
 
               *10.8   Indemnification Agreement dated February 8, 1995 by and between J. Shane Long and the
                         Company (incorporated by reference to Exhibit 10.30 to the Company's Registration
                         Statement on Form S-1 (Registration No. 33-87612))
 
               *10.9   Indemnification Agreement dated February 8, 1995 by and between James J. Byrne and the
                         Company (incorporated by reference to Exhibit 10.28 to the Company's Registration
                         Statement on Form S-1 (Registration No. 33-87612))
 
               *10.10  Indemnification Agreement dated February 8, 1995 by and between Lawrence E. Wesneski and
                         the Company (incorporated by reference to Exhibit 10.29 to the Company's Registration
                         Statement on Form S-1 (Registration No. 33-87612))
 
               *10.11  Indemnification Agreement by and between Dennis G. Sabo and the Company (incorporated by
                         reference to Exhibit 10.11 of the Company's Registration Statement on Form S-3,
                         Registration No. 333-4684)
 
               *10.12  Indemnification Agreement by and between Bryan F. Keyes and the Company (incorporated by
                         reference to Exhibit 10.12 of the Company's Registration Statement on Form S-3,
                         Registration No. 333-4684)
 
               *10.13  Indemnification Agreement dated February 8, 1995 by and between Mark S. Sims and the
                         Company (incorporated by reference to Exhibit 10.26 to the Company's Registration
                         Statement on Form S-1 (Registration No. 33-87612))
</TABLE>
 
                                       11
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<C>        <C>         <S>                                                                                        <C>
               *10.14  Indemnification Agreement dated February 8, 1995 by and between William D. Balthaser Jr.
                         and the Company (incorporated by reference to Exhibit 10.27 to the Company's
                         Registration Statement on Form S-1 (Registration No. 33-87612))
 
               *10.15  Company's Amended and Restated 1995 Long Term Incentive Plan (incorporated by reference
                         to Appendix A of the Company's definitive Proxy Statement for the 1997 Annual Meeting
                         of Shareholders)
 
               *10.16  Company's Amended and Restated Stock Option Plan for Non-Employee Directors (incorporated
                         by reference to Appendix B of the Company's definitive Proxy Statement for the 1997
                         Annual Meeting of Shareholders)
 
               *10.17  STB Systems, Inc. 1995 Employee Stock Option Purchase Plan (as amended) (incorporated by
                         reference to Exhibit 10.38 to the Company's Quarterly Report on Form 10-Q for the
                         quarter ended January 31, 1996)
 
               *10.18  Amended and Restated Profit Sharing Incentive Plan (incorporated by reference to Exhibit
                         10.47 to the Company's Registration Statement on Form S-1 (Registration No. 333-14313))
 
                10.19  Lease Agreement dated December 6, 1988 by and between STB de Mexico S.A. de C.V.
                         (formerly known as Industrias Fronterizas de Chihuahua, S.A. de C.V.) (a subsidiary of
                         the Company, as lessee) and Complejo Industrial Fuentes, S.A. de C.V (as lessor),
                         including an Agreement for Modification dated February 25, 1994 by and between the same
                         parties (incorporated by reference to Exhibit 10.1 to the Company's Registration
                         Statement on Form S-1 (Registration No. 33-87612))
 
                10.20  Modification Agreement dated October 4, 1996 by and between STB de Mexico, S.A. de C.V.
                         and Complejo Industrial Fuentes, S.A. de C.V. (relating to the Lease Agreement filed as
                         Exhibit 10.1 hereto) (incorporated by reference to Exhibit 10.46 to the Company's
                         Registration Statement on Form S-1 (Registration No. 333-14313))
 
                10.21  Lease Contract dated October 4, 1996 by and between STB de Mexico, S.A. de C.V. (as
                         lessee) and Complejo Industrial Fuentes, S.A. de C.V. (as lessor) (incorporated by
                         reference to Exhibit 10.41 to the Company's Registration Statement on Form S-1
                         (Registration No. 333-14313))
 
                10.22  Amendment to Lease Agreement dated January 30, 1997, by and between STB de Mexico, S.A.
                         de C.V. (as lessee) and Complejo Industrial Fuentes, S.A. de C.V. (incorporated by
                         reference to Exhibit 10.22 to the Company's Annual Report on Form 10-K for the Fiscal
                         Year Ended October 31, 1997).
 
                10.23  Lease Agreement, as amended, dated July 8, 1986 by and between the Company (as lessee)
                         and Central Park Associates, Ltd. (as lessor) (incorporated by reference to Exhibit
                         10.2 to the Company's Registration Statement on Form S-1 (Registration No. 33-87612))
 
                10.24  Lease Agreement dated June, 1995, by and between the Company (as lessee) and Springcreek
                         Place, Ltd. (as lessor) (incorporated by reference to Exhibit 10.32 to the Company's
                         Annual Report on Form 10-K for the fiscal year ended October 31, 1995)
</TABLE>
 
                                       12
<PAGE>
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                10.25  Addendum to Lease Agreement dated March 7, 1996 by and between the Company (as lessee)
                         and Springcreek Place, Ltd. (as lessor) (incorporated by reference to Exhibit 10.37 to
                         the Company's Quarterly Report on Form 10-Q for the quarter ended January 31, 1996)
 
                10.26  Second Addendum to Lease Agreement dated March 7, 1996, by and between the Company (as
                         lessee) and Springcreek Place, Ltd. (as lessor) (incorporated by reference to Exhibit
                         10.1 to the Company's Quarterly Report on Form 10-Q for the quarter ended July 31,
                         1997)
 
                10.27  Sublease Agreement dated August 1996 by and between ADC Telecommunications, Inc. (as
                         sublessor) and the Company (as sublessee) (incorporated by reference to Exhibit 10.40
                         to the Company's Registration Statement on Form S-1 (Registration No. 333-14313))
 
                10.28  Tax Allocation and Indemnification Agreement dated December 16, 1994 by and among the
                         Company and Messrs. Ogle, Balthaser and Sims (incorporated by reference to Exhibit
                         10.15 to the Company's Registration Statement on Form S-1 (Registration No. 33-87612))
 
                10.29  Purchase Agreement dated December 17, 1996, by and between the Company and Gateway 2000,
                         Inc. (incorporated by reference to Exhibit 10.29 to the Company's Annual Report on Form
                         10-K for the Fiscal Year Ended October 31, 1997).
 
                10.30  Lease Agreement by and between the Company and Banc One Leasing Corporation dated October
                         30, 1996, together with related attachments (incorporated by reference to Exhibit 10.48
                         to the Company's Registration Statement on Form S-1 (Registration No. 333-14313))
 
                10.31  Participation Agreement dated as of November 14, 1997 among Asset XVII Holdings Company,
                         L.L.C., as lessor, STB Systems, Inc., as lessee and Bank One, Texas, N.A., as lender
                         (incorporated by reference to Exhibit 10.31 to the Company's Annual Report on Form 10-K
                         for the Fiscal Year Ended October 31, 1997).
 
                10.32  Lease and Development Agreement dated as of November 14, 1997 among Asset XVII Holdings
                         Company, L.L.C., as lessor, and STB Systems, Inc., as lessee (incorporated by reference
                         to Exhibit 10.32 to the Company's Annual Report on Form 10-K for the Fiscal Year Ended
                         October 31, 1997).
 
                10.33  Limited Notice to Proceed No. 1 dated as of December 18, 1997 executed by STB Systems,
                         Inc. and Austin Commercial, Inc. (incorporated by reference to Exhibit 10.33 to the
                         Company's Annual Report on Form 10-K for the Fiscal Year Ended October 31, 1997).
 
                10.34  Credit Agreement dated as of November 21, 1997 between STB Systems, Inc., and Bank One,
                         Texas, N.A. (incorporated by reference to Exhibit 10.34 to the Company's Annual Report
                         on Form 10-K for the Fiscal Year Ended October 31, 1997).
 
                10.35  First Amendment to Credit Agreement dated as of January 30, 1998 by and among the
                         Company, Bank One, Texas, N.A. and the Original Lenders as therein defined
                         (incorporated by reference to Exhibit 10.35 of the Company's Registration Statement on
                         Form S-3, Registration No. 333-4684)
</TABLE>
 
                                       13
<PAGE>
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                10.36  Lease Schedule No. 1000063250 dated as of October 31, 1997 to Master Lease Agreement
                         dated October 30, 1996 between Banc One Leasing Corporation and the Company
                         (incorporated by reference to Exhibit 10.36 of the Company's Registration Statement on
                         Form S-3, Registration No. 333-4684)
 
                10.37  Lease Schedule No. 1000063259 dated as of October 31, 1997 to Master Lease Agreement
                         dated October 30, 1996 between Banc One Leasing Corporation and the Company
                         (incorporated by reference to Exhibit 10.37 of the Company's Registration Statement on
                         Form S-3, Registration No. 333-4684)
 
                10.38  Lease Schedule No. 1000063905 dated as of December 15, 1997 to Master Lease Agreement
                         dated October 30, 1996 between Banc One Leasing Corporation and the Company
                         (incorporated by reference to Exhibit 10.38 of the Company's Registration Statement on
                         Form S-3, Registration No. 333-4684)
 
                10.39  Master Lease Amendment dated as of October 31, 1997 to Master Lease Agreement dated
                         October 30, 1996 between Banc One Leasing Corporation and the Company (incorporated by
                         reference to Exhibit 10.39 of the Company's Registration Statement on Form S-3,
                         Registration No. 333-4684)
 
                10.40  Selling Shareholder Agreement between the Company and each of Messrs. Ogle, Balthaser and
                         Sims (incorporated by reference to Exhibit 10.40 of the Company's Registration
                         Statement on Form S-3, Registration No. 333-4684)
 
                10.41  Underwriting Agreement by and among the Company, William E. Ogle, Mark S. Sims, William
                         D. Balthaser and CIBC Oppenheimer (in its own capacity and on behalf of an underwriting
                         syndicate) (incorporated by reference to Exhibit 1.1 of the Company's Registration
                         Statement on Form S-3, Registration No. 333-4684)
 
              **10.42  Letter Agreement dated December 11, 1998 between STB Systems, Inc. and Hoak Breedlove
                         Wesneski & Co.
 
                11.1   Intentionally omitted.
 
                21     Subsidiaries of the Company
 
                       (a) STB Assembly, Inc., a Texas corporation
                       (b) STB de Mexico, S.A. de C.V., a Mexican corporation
                       (c) Maquilados Continentales de Chihuahua, a Mexican corporation (an inactive shell
                           corporation)
                       (d) Symmetric Simulation Systems, Inc.
                       (e) STB Systems, Inc., a Delaware corporation (an inactive shell corporation)
 
               +23     Consent of PricewaterhouseCoopers LLP
 
              **24     Powers of Attorney (included on first signature page)
 
              **27     Financial Data Schedule
</TABLE>
 
------------------------
 
*   Management contract or compensatory plan or arrangement. The Company will
    furnish a copy of any Exhibit listed above to any shareholder without charge
    upon written request to Mr. Bryan F. Keyes, Secretary, 3400 Waterview
    Parkway, Richardson, Texas 75080.
 
**  Previously filed.
 
+   Filed herewith.
 
        (b) No reports on Form 8-K were filed during the last quarter of the
    period covered by this Report.
 
                                       14
<PAGE>
                                   SIGNATURE
 
   
    Pursuant to the requirements of Section 13 or 15(d) of the Securities
Exchange Act of 1934 as amended, STB has duly caused this Amendment No. 2 on
Form 10-K/A to be signed on its behalf by the undersigned, thereunto duly
authorized.
    
 
                                STB SYSTEMS, INC.
 
                                By:             /s/ WILLIAM E. OGLE
                                     -----------------------------------------
                                                  William E. Ogle
                                               CHAIRMAN OF THE BOARD
                                            AND CHIEF EXECUTIVE OFFICER
 
   
Dated: March 25, 1999
    
 
                                       15
<PAGE>
                                 EXHIBIT INDEX
 
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           EXHIBIT                                                                                                     PAGE
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<C>        <C>        <S>                                                                                           <C>
    (a)  1.           The following financial statements are filed as part of this report:
 
                      Report of Independent Accountants.
                      Consolidated Balance Sheets dated October 31, 1998 and 1997.
                      Consolidated Statement of Operations for the three years ended October 31, 1998.
                      Consolidated Statement of Changes in Shareholders' Equity for the three years ended October
                        31, 1998.
                      Consolidated Statement of Cash Flows for the three years ended October 31, 1998.
                      Notes to Consolidated Financial Statements.
 
                2.    Consolidated Financial Statement Schedules
 
                      Report of Independent Accountants on Financial Statement Schedule.                                   S-1
 
                      Consolidated Valuation and Qualifying Accounts.                                                      S-2
 
                      All other schedules for which provision is made in the applicable accounting regulation of
                        the Securities & Exchange Commission are not required under the related instructions or
                        are inapplicable and therefore have been omitted.
 
                3.    The following documents are filed or incorporated by reference as exhibits to this Report:
 
                2.1   Agreement and Plan of Reorganization by and between the Registrant and STB Systems, Inc.
                        dated as of December 13, 1998, and the related Stock Option Agreement (incorporated by
                        reference to Schedule 13D of 3Dfx Interactive, Inc. dated December 23, 1998 with respect
                        to the Company).
 
                3.1   Amended and Restated Articles of Incorporation of the Company (incorporated by reference to
                        Exhibit 3.1 to the Company's Registration Statement on Form S-1 (Registration No.
                        33-87612))
 
                3.2   Articles of Amendment to Articles of Incorporation of the Company (incorporated by reference
                        to Exhibit 3.1 to the Company's Quarterly Report on Form 10-Q for the quarter ended April
                        30, 1997)
 
                3.3   Amended and Restated Bylaws of the Company (incorporated by reference to Exhibit 3.2 to the
                        Company's Registration Statement on Form S-1 (Registration No. 33-87612))
 
                4.1   Specimen of Common Stock Certificate (incorporated by reference to Exhibit 4.1 to the
                        Company's Registration Statement on Form S-1 (Registration No. 33-87612))
 
                4.2   Amended and Restated Articles of Incorporation and Bylaws of the Company (see Exhibits 3.1,
                        3.2 and 3.3 above)
 
                4.3   Right of First Refusal Agreement dated December 16, 1994 by and among the Company and
                        Messrs. Ogle, Balthaser and Sims (incorporated by reference to Exhibit 4.3 to the
                        Company's Registration Statement on Form S-1 (Registration No. 33-87612))
 
                9.1   Form of Voting Agreement dated December 13, 1998 between the Company and certain
                        shareholders of 3Dfx Interactive, Inc., a California corporation (incorporated by
                        reference to Schedule 13D of 3Dfx Interactive, Inc. dated December 23, 1998 with respect
                        to the Company).
</TABLE>
 
                                       16
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           EXHIBIT                                                                                                     PAGE
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<C>        <C>        <S>                                                                                           <C>
             **10.1   Employment Agreement dated November 1, 1996 by and between the Company and William E. Ogle
                        (incorporated by reference to Exhibit 10.42 to the Company's Registration Statement on
                        Form S-1 (Registration No. 333-14313)), as amended by that certain Amendment to Employment
                        Agreement for Executive Officer dated December 13, 1998
 
             **10.2   Employment Agreement dated November 1, 1996 by and between the Company and Randall D.
                        Eisenbach (incorporated by reference to Exhibit 10.43 to the Company's Registration
                        Statement on Form S-1 (Registration No. 333-14313)), as amended by that certain Amendment
                        to Employment Agreement for Executive Officer dated December 13, 1998
 
             **10.3   Employment Agreement dated November 1, 1996 by and between the Company and James L. Hopkins
                        (incorporated by reference to Exhibit 10.44 to the Company's Registration Statement on
                        Form S-1 (Registration No. 333-14313)), as amended by that certain Amendment to Employment
                        Agreement for Executive Officer dated December 13, 1998
 
             **10.4   Employment Agreement dated November 1, 1996 by and between the Company and J. Shane Long
                        (incorporated by reference to Exhibit 10.45 to the Company's Registration Statement on
                        Form S-1 (Registration No. 333-14313)), as amended by that certain Amendment to Employment
                        Agreement for Executive Officer dated December 13, 1998
 
              *10.5   Indemnification Agreement dated February 8, 1995 by and between William E. Ogle and the
                        Company (incorporated by reference to Exhibit 10.23 to the Company's Registration
                        Statement on Form S-1 (Registration No. 33-87612))
 
              *10.6   Indemnification Agreement dated February 8, 1995 by and between Randall D. Eisenbach and the
                        Company (incorporated by reference to Exhibit 10.24 to the Company's Registration
                        Statement on Form S-1 (Registration No. 33-87612))
 
              *10.7   Indemnification Agreement dated February 8, 1995 by and between James L. Hopkins and the
                        Company (incorporated by reference to Exhibit 10.25 to the Company's Registration
                        Statement on Form S-1 (Registration No. 33-87612))
 
              *10.8   Indemnification Agreement dated February 8, 1995 by and between J. Shane Long and the
                        Company (incorporated by reference to Exhibit 10.30 to the Company's Registration
                        Statement on Form S-1 (Registration No. 33-87612))
 
              *10.9   Indemnification Agreement dated February 8, 1995 by and between James J. Byrne and the
                        Company (incorporated by reference to Exhibit 10.28 to the Company's Registration
                        Statement on Form S-1 (Registration No. 33-87612))
 
              *10.10  Indemnification Agreement dated February 8, 1995 by and between Lawrence E. Wesneski and the
                        Company (incorporated by reference to Exhibit 10.29 to the Company's Registration
                        Statement on Form S-1 (Registration No. 33-87612))
 
              *10.11  Indemnification Agreement by and between Dennis G. Sabo and the Company (incorporated by
                        reference to Exhibit 10.11 of the Company's Registration Statement on Form S-3,
                        Registration No. 333-4684)
 
              *10.12  Indemnification Agreement by and between Bryan F. Keyes and the Company (incorporated by
                        reference to Exhibit 10.12 of the Company's Registration Statement on Form S-3,
                        Registration No. 333-4684)
</TABLE>
 
                                       17
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           EXHIBIT                                                                                                     PAGE
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<C>        <C>        <S>                                                                                           <C>
              *10.13  Indemnification Agreement dated February 8, 1995 by and between Mark S. Sims and the Company
                        (incorporated by reference to Exhibit 10.26 to the Company's Registration Statement on
                        Form S-1 (Registration No. 33-87612))
 
              *10.14  Indemnification Agreement dated February 8, 1995 by and between William D. Balthaser Jr. and
                        the Company (incorporated by reference to Exhibit 10.27 to the Company's Registration
                        Statement on Form S-1 (Registration No. 33-87612))
 
              *10.15  Company's Amended and Restated 1995 Long Term Incentive Plan (incorporated by reference to
                        Appendix A of the Company's definitive Proxy Statement for the 1997 Annual Meeting of
                        Shareholders)
 
              *10.16  Company's Amended and Restated Stock Option Plan for Non-Employee Directors (incorporated by
                        reference to Appendix B of the Company's definitive Proxy Statement for the 1997 Annual
                        Meeting of Shareholders)
 
              *10.17  STB Systems, Inc. 1995 Employee Stock Option Purchase Plan (as amended) (incorporated by
                        reference to Exhibit 10.38 to the Company's Quarterly Report on Form 10-Q for the quarter
                        ended January 31, 1996)
 
              *10.18  Amended and Restated Profit Sharing Incentive Plan (incorporated by reference to Exhibit
                        10.47 to the Company's Registration Statement on Form S-1 (Registration No. 333-14313))
 
               10.19  Lease Agreement dated December 6, 1988 by and between STB de Mexico S.A. de C.V. (formerly
                        known as Industrias Fronterizas de Chihuahua, S.A. de C.V.) (a subsidiary of the Company,
                        as lessee) and Complejo Industrial Fuentes, S.A. de C.V (as lessor), including an
                        Agreement for Modification dated February 25, 1994 by and between the same parties
                        (incorporated by reference to Exhibit 10.1 to the Company's Registration Statement on Form
                        S-1 (Registration No. 33-87612))
 
               10.20  Modification Agreement dated October 4, 1996 by and between STB de Mexico, S.A. de C.V. and
                        Complejo Industrial Fuentes, S.A. de C.V. (relating to the Lease Agreement filed as
                        Exhibit 10.1 hereto) (incorporated by reference to Exhibit 10.46 to the Company's
                        Registration Statement on Form S-1 (Registration No. 333-14313))
 
               10.21  Lease Contract dated October 4, 1996 by and between STB de Mexico, S.A. de C.V. (as lessee)
                        and Complejo Industrial Fuentes, S.A. de C.V. (as lessor) (incorporated by reference to
                        Exhibit 10.41 to the Company's Registration Statement on Form S-1 (Registration No.
                        333-14313))
 
               10.22  Amendment to Lease Agreement dated January 30, 1997, by and between STB de Mexico, S.A. de
                        C.V. (as lessee) and Complejo Industrial Fuentes, S.A. de C.V. (incorporated by reference
                        to Exhibit 10.22 to the Company's Annual Report on Form 10-K for the Fiscal Year Ended
                        October 31, 1997).
 
               10.23  Lease Agreement, as amended, dated July 8, 1986 by and between the Company (as lessee) and
                        Central Park Associates, Ltd. (as lessor) (incorporated by reference to Exhibit 10.2 to
                        the Company's Registration Statement on Form S-1 (Registration No. 33-87612))
 
               10.24  Lease Agreement dated June, 1995, by and between the Company (as lessee) and Springcreek
                        Place, Ltd. (as lessor) (incorporated by reference to Exhibit 10.32 to the Company's
                        Annual Report on Form 10-K for the fiscal year ended October 31, 1995)
</TABLE>
 
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               10.25  Addendum to Lease Agreement dated March 7, 1996 by and between the Company (as lessee) and
                        Springcreek Place, Ltd. (as lessor) (incorporated by reference to Exhibit 10.37 to the
                        Company's Quarterly Report on Form 10-Q for the quarter ended January 31, 1996)
 
               10.26  Second Addendum to Lease Agreement dated March 7, 1996, by and between the Company (as
                        lessee) and Springcreek Place, Ltd. (as lessor) (incorporated by reference to Exhibit 10.1
                        to the Company's Quarterly Report on Form 10-Q for the quarter ended July 31, 1997)
 
               10.27  Sublease Agreement dated August 1996 by and between ADC Telecommunications, Inc. (as
                        sublessor) and the Company (as sublessee) (incorporated by reference to Exhibit 10.40 to
                        the Company's Registration Statement on Form S-1 (Registration No. 333-14313))
 
               10.28  Tax Allocation and Indemnification Agreement dated December 16, 1994 by and among the
                        Company and Messrs. Ogle, Balthaser and Sims (incorporated by reference to Exhibit 10.15
                        to the Company's Registration Statement on Form S-1 (Registration No. 33-87612))
 
               10.29  Purchase Agreement dated December 17, 1996, by and between the Company and Gateway 2000,
                        Inc. (incorporated by reference to Exhibit 10.29 to the Company's Annual Report on Form
                        10-K for the Fiscal Year Ended October 31, 1997).
 
               10.30  Lease Agreement by and between the Company and Banc One Leasing Corporation dated October
                        30, 1996, together with related attachments (incorporated by reference to Exhibit 10.48 to
                        the Company's Registration Statement on Form S-1 (Registration No. 333-14313))
 
               10.31  Participation Agreement dated as of November 14, 1997 among Asset XVII Holdings Company,
                        L.L.C., as lessor, STB Systems, Inc., as lessee and Bank One, Texas, N.A., as lender
                        (incorporated by reference to Exhibit 10.31 to the Company's Annual Report on Form 10-K
                        for the Fiscal Year Ended October 31, 1997).
 
               10.32  Lease and Development Agreement dated as of November 14, 1997 among Asset XVII Holdings
                        Company, L.L.C., as lessor, and STB Systems, Inc., as lessee (incorporated by reference to
                        Exhibit 10.32 to the Company's Annual Report on Form 10-K for the Fiscal Year Ended
                        October 31, 1997).
 
               10.33  Limited Notice to Proceed No. 1 dated as of December 18, 1997 executed by STB Systems, Inc.
                        and Austin Commercial, Inc. (incorporated by reference to Exhibit 10.33 to the Company's
                        Annual Report on Form 10-K for the Fiscal Year Ended October 31, 1997).
 
               10.34  Credit Agreement dated as of November 21, 1997 between STB Systems, Inc., and Bank One,
                        Texas, N.A. (incorporated by reference to Exhibit 10.34 to the Company's Annual Report on
                        Form 10-K for the Fiscal Year Ended October 31, 1997).
 
               10.35  First Amendment to Credit Agreement dated as of January 30, 1998 by and among the Company,
                        Bank One, Texas, N.A. and the Original Lenders as therein defined (incorporated by
                        reference to Exhibit 10.35 of the Company's Registration Statement on Form S-3,
                        Registration No. 333-4684)
</TABLE>
 
                                       19
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<C>        <C>        <S>                                                                                           <C>
               10.36  Lease Schedule No. 1000063250 dated as of October 31, 1997 to Master Lease Agreement dated
                        October 30, 1996 between Banc One Leasing Corporation and the Company (incorporated by
                        reference to Exhibit 10.36 of the Company's Registration Statement on Form S-3,
                        Registration No. 333-4684)
 
               10.37  Lease Schedule No. 1000063259 dated as of October 31, 1997 to Master Lease Agreement dated
                        October 30, 1996 between Banc One Leasing Corporation and the Company (incorporated by
                        reference to Exhibit 10.37 of the Company's Registration Statement on Form S-3,
                        Registration No. 333-4684)
 
               10.38  Lease Schedule No. 1000063905 dated as of December 15, 1997 to Master Lease Agreement dated
                        October 30, 1996 between Banc One Leasing Corporation and the Company (incorporated by
                        reference to Exhibit 10.38 of the Company's Registration Statement on Form S-3,
                        Registration No. 333-4684)
 
               10.39  Master Lease Amendment dated as of October 31, 1997 to Master Lease Agreement dated October
                        30, 1996 between Banc One Leasing Corporation and the Company (incorporated by reference
                        to Exhibit 10.39 of the Company's Registration Statement on Form S-3, Registration No.
                        333-4684)
 
               10.40  Selling Shareholder Agreement between the Company and each of Messrs. Ogle, Balthaser and
                        Sims (incorporated by reference to Exhibit 10.40 of the Company's Registration Statement
                        on Form S-3, Registration No. 333-4684)
 
               10.41  Underwriting Agreement by and among the Company, William E. Ogle, Mark S. Sims, William D.
                        Balthaser and CIBC Oppenheimer (in its own capacity and on behalf of an underwriting
                        syndicate) (incorporated by reference to Exhibit 1.1 of the Company's Registration
                        Statement on Form S-3, Registration No. 333-4684)
 
             **10.42  Letter Agreement dated December 11, 1998 between STB Systems, Inc. and Hoak Breedlove
                        Wesneski & Co.
 
               11.1   Intentionally omitted.
 
               21     Subsidiaries of the Company
 
                      (a) STB Assembly, Inc., a Texas corporation
                      (b) STB de Mexico, S.A. de C.V., a Mexican corporation
                      (c) Maquilados Continentales de Chihuahua, a Mexican corporation (an inactive shell
                          corporation)
                      (d) Symmetric Simulation Systems, Inc.
                      (e) STB Systems, Inc., a Delaware corporation (an inactive shell corporation)
 
              +23     Consent of PricewaterhouseCoopers LLP
 
             **24     Powers of Attorney (included on first signature page)
 
             **27     Financial Data Schedule
</TABLE>
 
------------------------
 
*   Management contract or compensatory plan or arrangement. The Company will
    furnish a copy of any Exhibit listed above to any shareholder without charge
    upon written request to Mr. Bryan F. Keyes, Secretary, 3400 Waterview
    Parkway, Richardson, Texas 75080.
 
**  Previously filed.
 
+   Filed herewith.
 
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