Patent Publication Number: US-6044414-A

Title: System for preventing a DMA controller from evaluating its DRQ input once a DMA operation has started until the DRQ input has been updated

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application relates to the following co-pending application entitled &#34;PC CORE LOGIC SERIAL REGISTER ACCESS BUS&#34;, Ser. No. 08/928,035, still pending filed Sep. 11, 1997, by Dale E. Gulick; which application is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to computer systems, and, more particularly, to direct memory access in computer systems. 
     2. Description of the Related Art 
     A typical personal computer (PC) system includes a microprocessor, associated memory and control logic and a number of peripheral devices that provide input and output for the system. Such peripheral devices typically include a display monitor, a keyboard and mouse, a floppy disk drive, a hard disk drive and a printer. The number of devices being added to personal computer systems continues to grow. For example, many computer systems also include modems, sound devices, and CD-ROM drives. 
     PC systems use one of several expansion bus architectures to facilitate communication between various components of the system and to provide the versatility needed to add additional components to the system. For example, the Industry Standard Architecture (ISA) provides an expansion bus for the 16-bit IBM AT personal computer. The Enhanced ISA (EISA) provide for systems utilizing 32-bit microprocessors such as the Intel 80386 and 80486 microprocessors. The Peripheral Component Interconnect (PCI) bus provides a bus architecture for 32-bit or 64-bit interconnection systems independent of processor generation or family. 
     The ISA bus, originally called the Advanced Technology (AT) bus, added the functionality needed for full 16-bit technology, but maintained compatibility with an older 8-bit PC bus. Because of its initial speed and data-path match with the 80286 microprocessor, the original ISA bus substantially out-performed the PC bus. The ISA bus has resisted replacement by newer bus architectures such as EISA and Microchannel, and remains commonplace in personal computer systems in use today. That is true in part because many devices that are designed to interface with the ISA bus are in widespread use today. Such devices are known as legacy devices since their design is based on older PC technology. Examples of such slower legacy devices include keyboards, and mouse(s), game ports, floppy drives, modems and printers connected respectively to serial and parallel communication ports, direct memory access (DMA) controllers, interrupt controllers and timers. Those legacy devices do not need the high speed throughput of the newer generation of buses such as EISA, Microchannel Architecture (MCA) and the Peripheral Component Interface (PCI) bus. 
     Although personal computer system speeds, and particularly microprocessor speeds, have increased dramatically, e.g. to 300 MHz and beyond, the speed of the ISA bus is limited to approximately 8 MHz. As higher speed processors were utilized, dedicated memory buses were added to personal computer systems because the ISA bus was too slow for the required high speed memory accesses. Video applications also became limited by the bandwidth of the ISA, so systems began to use a &#34;local bus&#34; for video applications. Although initially targeted at advanced video systems, new local bus specifications were made broad enough for handling other peripherals requiring high-bandwidth transfers such as mass storage devices and network interfaces. 
     The Peripheral Component Interconnect (PCI) bus is one example of a local bus specification. The VL bus is another local bus specification that has been less widely adopted. The PCI bus provides a high-speed interconnection system which runs more closely to microprocessor speeds than does a traditional expansion bus. And, although initially designed for 32-bit microprocessors, the PCI specification is broad enough to include the 64 bit data paths of the advanced processors. Legacy devices compatible with older bus architectures such as ISA connect to the PCI bus via a bus bridge circuit which provides for a translation between the protocols of the ISA and PCI buses. 
     Many present day personal computer systems contain both a PCI bus and an ISA bus. The PCI bus is used to connect to newer peripherals and/or those peripherals requiring a higher speed interface. The ISA bus is typically connected to legacy devices. Historically, interfaces to peripherals utilized a large number of discrete components. However, increased levels of integration has resulted in the logic necessary for interfacing to peripheral devices being combined into a relatively few integrated circuits (ICs), are sold as chip sets for the PC, such as such as Intel&#39;s 430 VX chip set. The integrated circuits include a plurality of terminals, pins, or leads, connecting the integrated circuit to the printed circuit board (PCB) to which the integrated circuit is mounted. The PCB functions as a system board. The terminals communicate input/output (I/O) signals between one integrated circuit and other integrated circuits or I/O devices coupled to the system board. These system boards often receive expansion boards to increase the capabilities of the computer system and to connect to peripheral devices, e.g., through the ISA bus. 
     Referring, to FIG. 1, an exemplary prior art computer system 100 is shown conforming to the above architectural approach of including both a PCI bus and an ISA bus. Computer system 100 includes processor 110 which is coupled to secondary cache 115 and memory 140. Bridge 120 provides an interface between the processor/memory system 105 and PCI bus 125. Bridge 120 provides a communication link between PCI devices 150, 160 and 165 and the processor/memory system 105. Although the PCI bus was originally intended for graphics, high speed graphics requirements have resulted in another specialized graphics bus called the Advanced Graphics Port Bus which can be utilized in place of the PCI bus for graphics applications. The PCI devices may be integrated circuits on the system board of computer system 100, expansion components connected to PCI bus 125 via expansion slots, or some combination thereof. A second bridge 130, provides a bus interface between PCI bus 125 and ISA expansion bus 135. In order to communicate with legacy devices which are designed to interface to the ISA bus, one approach, has been to provide super I/O chip 170 which provides the logic and pins to interface to legacy devices. 
     Super I/O chip 170 provides I/O terminals and control logic for commonly used legacy peripheral devices such as keyboards, IDE drive, IEEE parallel port, serial communication ports. One example of such an I/O chip is the National Semiconductor PC87306 SuperI/O™ chip. Thus, legacy devices can be included in the system by utilizing the bridge 130, the ISA bus, and super I/O chip 170. In order to integrate functional logic into as few parts as possible it would be desirable it integrate a bridge function with a legacy I/O chip. However, that can require a larger number of input/output pins than can be economically accommodated on a single chip. 
     SUMMARY OF THE INVENTION 
     It has been discovered to provide a processor system which accounts for the problem of high integration resulting in overly high pin counts by providing two integrated circuits coupled by a serial bus. In accordance with one embodiment of the invention a computer system includes first and second integrated circuits. The first integrated circuit includes a first input/output bus operating in accordance with a first protocol, such as ISA. The first input/output bus includes a plurality of address and data lines respectively providing address and data information. The second integrated circuit includes a plurality of functional blocks at least some of which interface to legacy devices. The first integrated circuit includes a plurality of additional functional blocks which interface to the first input/output bus. The first integrated circuit includes a host controller circuit, coupled to the first input/output bus and for coupling to a register access bus which includes a register data out and a register data in signal line. The register access bus connects the first and second integrated circuits. The host controller circuit receives address and data information from the first input/output bus and serially provides the address and data information to the data out line. A target controller circuit on the second integrated circuit is coupled to the register access bus. The target controller circuit receives the serially provided address and data information and provides the address and data information over another representation of the first input/output bus, which includes a plurality of internal address lines and a plurality of internal data lines coupled to the second functional blocks. The address information identifies an address location within one of the functional blocks in the second integrated circuit. 
     One problem associated with such a solution is to ensure that direct memory access information provided to the first integrated circuit is valid. Because of the inherent delay of a serial bus, such information may be invalid when the DMA control circuit expects the information to be valid. Accordingly, the present invention provides a computer system including a first and second integrated circuits which are coupled by a serial bus. A direct memory access (DMA) circuit is on the first integrated circuit and coupled to receive a direct memory access request (DRQ) input signal from the second integrated circuit. The value of the DRQ signal is provided from said second integrated circuit, across a signal line of the bus with the value being updated periodically the signal line; 
     Accordingly, a DMA synchronization control circuit on the first integrated circuit, prevents the DMA circuit from evaluating its DRQ input signal until after the DRQ input signal has been updated by said second integrated circuit after a DMA operation. In addition, the DMA acknowledge signal from the DMA controller circuit is mapped to an address indicative of the acknowledge signal, and the address is sent to the second integrated circuit. A decoder circuit on the second integrated circuit decodes the address and provides the appropriate DMA acknowledge signal according to the address received. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings, wherein the use of the same reference symbols in different drawings indicates similar or identical items. 
     FIG. 1 shows a schematic block diagram of a prior art personal computer system. 
     FIG. 2 shows a block diagram of a computer system which includes a South Bridge circuit and port expansion circuit in accordance with the present invention. 
     FIG. 3 shows a block diagram of the South Bridge integrated circuit and the Port Expansion integrated circuit of the personal computer architecture of FIG. 2. 
     FIG. 4 shows a seven line embodiment of the port expansion bus (PEB) connecting the South Bridge and the port expansion circuit of the computer system of FIG. 2 including a time slot portion and a register transfer portion. 
     FIG. 5a is a timing diagram showing a frame sync signal for the time slot portion of the port expansion bus (PEB). 
     FIG. 5b is a timing diagram showing the relation between the frame sync signal, the bit clock, Data In and Data Out on the port expansion bus. 
     FIG. 6 shows external gating of the PC speaker out signal. 
     FIG. 7 shows a timing diagram for the register transfer bus portion of the port expansion bus. 
     FIG. 8 shows a timing diagram for the DRQs being sent over the DRQ In signal line. 
     FIG. 9 shows a block diagram of the port expansion bus host controller. 
     FIG. 10 shows a more detailed block diagram of the host controller shown in FIG. 9. 
     FIG. 11 is a timing diagram illustrating restarting of the port expansion bus from a power-down condition. 
     FIG. 12 is a block diagram of the interrupt control logic in the South Bridge. 
     FIG. 13 is a block diagram of the DMA control logic in the South Bridge. 
     FIG. 14 is a timing diagram of a DMA transfer according to the invention. 
     FIG. 15 is a block diagram of the port expansion integrated circuit. 
     FIG. 16 shows the sequence of actions the host controller takes during a DMA operation involving the port expansion circuit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     FIG. 2 shows one embodiment of a personal computer system according to the present invention. The personal computer system comprises processor 110 which is coupled to cache memory 115 and memory 140. Bridge circuit 203 (North Bridge), provides an interface between the processor 110 and PCI bus 125. North Bridge 203 also provides an interface between the PCI bus 125 and memory 140. Bridge circuit 201 (South Bridge) provides an interface between PCI bus 125 and a plurality of devices and/or interfaces. A separate integrated circuit 205, connects to the South Bridge 201 via port expansion bus 207. The separate circuit 205 will be referred to herein as the port expansion circuit (PEC). The port expansion bus (PEB) is a seven pin serial bus. 
     Referring to FIG. 3, the South Bridge 201 and the port expansion circuit 205 are shown in greater detail. The South Bridge provides an interface 320 with the PCI bus. Interface 320 in one embodiment of the invention is a multiplexer structure providing a way to interface various blocks on the South Bridge with PCI bus 125. One such block is the ISA bridge 322. The bridge 322 creates an internal ISA bus 324 on the South Bridge. Full ISA compliance is not a requirement to interface to the various ISA resident blocks, but providing full ISA capability, including bus mastering capability may provide greater flexibility. The various ISA resident blocks on the internal ISA bus 324 in the South Bridge circuit include direct memory access (DMA) block 302 (e.g., dual 8237A compatible controllers), Programmable Interval Timer (PIT) 304 (e.g., an 8254 compatible counter/timer unit), AT port logic 306, programmable interrupt controller 308 (e.g., 8259A compatible), musical instrument digital interface (MIDI) 310, sound logic, (e.g., SOUND BLASTER) block 312, one or more universal asynchronous receiver transmitter(s) (UART) block 316, which for the embodiment shown is a 16550 UART, and keyboard/mouse logic 314. These blocks are representative, in that additional or fewer ISA resident functional blocks may actually be provided in any particular South Bridge embodiment. 
     The South Bridge also includes sideband signals 328 which are coupled to processor 110. Such sideband signals provide for legacy signals such as interrupts which can not be handled over the PCI bus. 
     In addition to the South Bridge having ISA resident blocks, PEC 205 also includes ISA resident blocks. For example, PEC 205 includes parallel port 353, game port 355, real time clock (RTC) 367, floppy disk drive controller 357. Such logic is well known in the personal computer industry and is not described further herein except where necessary for a better understanding of the claimed invention. The PEC also provides all of these ISA resident blocks with the necessary I/O terminals to connect to the legacy devices. Thus, for instance, the floppy control block 357 can be coupled to a floppy drive (not shown) and game port 355 can be coupled to a joystick (not shown) and parallel port 353 can be coupled to a printer (not shown) on the parallel port. 
     The PEC also provides a pin reflection function for the South Bridge. That means that the states of relatively slowly changing signals are transmitted over a time-division serial bus and recreated on the other side. That is, the PEC provides I/O terminals for some of the ISA resident blocks in the South Bridge such as UART 316, PIT 304, keyboard/mouse logic 314 and MIDI port 310. Output signals that would ordinarily be output by I/O terminals on the South Bridge to a legacy device, are instead transmitted to the PEC, and then output to the legacy device from I/O terminals on the PEC. Thus, serial port pins 359 provide the input/output pins for UART 316. Similarly, output pin 361 provides PIT 304 a connection to the PC Speaker. Keyboard/mouse pins 366 which connect respectively to a keyboard and mouse provide I/O pins for keyboard/mouse logic 314 in South Bridge 201. Finally, in the embodiment shown, MIDI pins 365 provide a connection to a musical instrument for MIDI port 310. 
     In order for the Port Expansion Circuit 205 to provide I/O function for the South Bridge integrated circuit, the output signals have to be sampled in the South Bridge and provided to the output pins of the port expansion circuit at a rate faster than the output signals change. Additionally, the input signals on the input pins of the port expansion circuit have to be sampled and provided to the interface logic blocks in the South Bridge integrated circuit at a rate faster than the input signals change. 
     Thus, the PEC provides several functions. One function is to provide I/O terminals for pin reflection. Another function is to provide both the functional logic and the I/O terminals for ISA resident logic blocks. Accordingly, to accommodate ISA resident functional logic blocks, the port expansion bus target controller block 351 recreates an internal ISA-like parallel bus in the port expansion circuit 205 which is described in greater detail in FIG. 14 and related discussion. The recreated parallel bus couples to ISA resident blocks in the port expansion circuit, such as parallel port 353, game port 355 and floppy control logic 357. 
     Serial bus 207, which couples the South Bridge integrated circuit 201 to the port expansion integrated circuit 205, provides both the pin reflection function by transferring the state of input/output signals across the bus and also provides for register access operations to ISA addressable registers in the PEC. The serial bus 207 also provides for additional functions which are described further herein. 
     Referring to FIG. 4, the port expansion bus provides a time slot bus for &#34;pin reflection.&#34; The time slot bus transfers frames of relatively slowly changing data between the South Bridge 201 and the port expansion circuit 205. In one embodiment each frame has 32 data bits. The time slot bus includes time slot data out signal line 401, time slot data in signal line 403, frame sync 405 and clock 407. The time slot bus transfers relatively slow signals on time slot data out signal line 401 from South Bridge 201 to be output by port expansion circuit 205. The signals are time multiplexed and are provided to the appropriate output pin and thus the output device according to the time slot on the bus. The time slot bus also operates to transfer signals that are received from relatively slow legacy devices such as a modem coupled to serial port pins 359, from the port expansion circuit to the South Bridge on time slot data in signal line 403. 
     Referring to FIG. 5a, the beginning of each frame for the time slot bus is determined by assertion of the frame sync signal at 501 which is synchronous with the bit clock. The frame sync signal is preferably a 50% duty cycle signal and is output from the South Bridge. The frame sync frequency (2.0625 MHz), in the embodiment shown, is 1/16 that of the bit clock which is a 33 Mhz 50% duty cycle clock (same frequency as the PCI clock). Frames are synchronized to the rising edge of the frame sync signal. Data on the time slot bus is synchronized with frame sync with time slot 0 on the bus corresponding to the rising edge of frame sync. As shown in FIG. 5b, a data bit is provided on both the rising and falling edge of the clock. Thus, data on signal lines 401 and 403 is transmitted on both the rising and falling edges of the clock. Accordingly, a 32 bit frame requires 16 bit clock periods. The data provided by the PEC on the rising edge of the bit clock is latched on the falling edge of the bit clock in the South Bridge as shown in FIG. 5b. In the embodiment shown, bit clock 407 is output from the South Bridge integrated circuit. 
     The clock, frame sync, time slot data in and data out signals are held high when the bus is deactivated. Note that other frame lengths, clock frequencies and duty cycles are of course possible. 
     An exemplary frame sent to and from PEC 205 is shown in Table 1. The bits transmitted to the PEC include bits 0-3 which are signals to be provided to the serial port pins on the PEC. Bits 0 and 1 are pin reflection signals including transmit data (TD) for serial ports 1 and 2, request to send (RTS/) and data terminal ready (DTR/) for port 0. The data transmit data bits for serial port 0 and 1 are repeated for bits 6 and 7. Port 1 RTS/ and DTR/ bits are provided in bits 8 and 9. MIDI data is provided on bits 12 and 26. Keyboard and mouse data is provided on bits 27 and 28. A mouse clock out bit is provided on bit 30 and a PC speaker on bit 31. The detailed operation the various ports and interfaces with respect to these control and data bits is well known in the art. The bus speed of the time slot bus should be sufficient to ensure that the various devices receive necessary data and control signals in a timely manner, i.e., the pin reflection function keeps up with the rate of change of the various interfaces. 
     
                       TABLE 1                                                     
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Bit #     Transmit to PEC                                                 
                         Receive from PEC                                 
______________________________________                                    
0         Serial port 0 TD                                                
                         Serial port 0 RD                                 
1            Serial port 1 TD                                             
                           Serial port 1 RD                               
2            Serial port 0 RTS/                                           
                         Serial port 0 CTS/                               
3            Serial port 0 DTR/                                           
                         Serial port 0 DSR/                               
4            Reserved              Serial port 0 DCD/                     
5            Reserved              Serial port 0 RD                       
6            Serial port 0 TD                                             
                           Serial port 1 RD                               
7            Serial port 1 TD                                             
                           Serial port 0 RI                               
8            Serial port 1 RTS/                                           
                         Serial port 1 CTS/                               
9            Serial port 1 DTR/                                           
                         Serial port 1 DSR/                               
10          Reserved               Serial port 1 DCD/                     
11          reserved               Serial port 1 RI                       
12          MIDI Tx Data       MIDI Rx Data                               
13          Reserved               Wake-up A                              
14          Reserved               Wake-up B                              
15          Reserved               Wake-up C                              
16          Serial port 0 TD                                              
                           Serial port 0 RD                               
17          Serial port 1 TD                                              
                           Serial port 1 RD                               
18          Reserved               Power button                           
19          Reserved               Sleep button                           
20          Reserved               Reserved                               
21          Reserved               Reserved                               
22          Reserved               Wake-up D                              
23          Reserved               Wake-up E                              
24          Serial port 0 TD                                              
                           Serial port 0 RD                               
25          Serial port 1 TD                                              
                           Serial port 1 RD                               
26          MIDI Tx Data       MIDI Rx Data                               
27          Keyboard Data Out                                             
                          Keyboard Data In                                
28          Mouse Data Out                                                
                             Mouse Data In                                
29          Keyboard Clock Out                                            
                         Wake-up F                                        
30          Mouse Clock Out                                               
                            Wake-up G                                     
31          PC Speaker           Wake-up H                                
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     Table 1 also shows the bits received from the PEC. Bits 0-11, 16 and 17, 24 and 25 relate to the serial port. Included in the signals received are receive data (RD), clear to send (CTS), data set read (DSR/), data carrier detect (DCD), and ring indicator (RI). The receive frame also includes MIDI receive data (bits 12 and 26) and several bits associated with power control features including wake-up bits 13-15, 22-23, and 29-21 and power and sleep buttons 18 and 19. The wake-up bits and power and sleep buttons relate to power management functions. For example, if the system is in a sleep state, the wakeup buttons can indicate which event caused the wakeup to occur. In addition, the receive frame includes data from the keyboard and mouse (bits 27 and 28, respectively). 
     Thus, every frame includes the data and control bits shown in Table 1. All bits in Table 1 are numbered with respect to the leading edge of frame sync. The time slot bus is &#34;protocol free&#34;, in that there is little overhead associated with transferring data. The bus simply continuously transfers frames between the port expansion circuit and the South Bridge. In the embodiment shown, the frames are contiguous in that there is no time between each frame, i.e., the start of bit 0 is one half clock period from the start of bit 31. However, other embodiments may provide some time period between frames so long as the time slot bus stays substantially continuously running to satisfy the speed requirements of the pin reflection approach described herein. Note that the I/O pin values are transferred every frame whether they change or not. Also note that transmit and receive data bits may be transmitted more than one time per frame. That further reduces latency for pin reflection purposes. At a data bit rate of 66 Mhz, the MIDI port in the South Bridge can transmit and receive data via the I/O pins on the PEC at an a rate of 4.125 Mhz (twice per frame). Note that the time slot bus does not operate while the port expansion circuit and South Bridge are in a power down mode to reduce power consumption. 
     An exemplary pin reflection function according to the invention is described as follows. The PC Speaker is connected to the counter-timer block 304 in the South Bridge and the frequency of the speaker is determined by the frequency of the digital signal output by the counter timer block 304 which is, e.g., an 8253 or 8254-2 equivalent timer/counter. Referring to FIG. 6, the PC speaker out signal 605 is gated external to PIT 601 by I/O port B (61h) bit (1) at gate 603. The PC speaker timer (timer 2) is enabled by I/O port B bit(0)(61h). At approximately the start of bit time 31 of each frame, the state of the counter timer&#39;s PC Speaker output is sampled in the South Bridge. Note that the state of the output of the counter timer may be sampled earlier than bit time 31. Bit position 31 in the frame is updated with the latest sampled value and transmitted to the port expansion circuit. At the port expansion circuit end, the state of the PC Speaker pin is latched during bit time 31. The PC Speaker pin on the port expansion circuit always reflects the latched value. Thus, the PC Speaker pin always reflects the state of the counter timer output, with a delay (and jitter) of up to approximately 485 nanoseconds assuming a data rate of 66 MHz and a 32 bit frame. 
     Various functions within a personnel computer system can utilize pin reflection as provided by the four pin time slot bus described herein. Such functions include the PC speaker, MIDI port, keyboard and mouse and serial ports. 
     Referring back to FIG. 4, the port expansion bus also provides a register transfer function, that is, the capability to read and write registers in ISA resident logic located in the PEC. In order to provide that register transfer function, the port expansion bus 207 includes a register transfer bus portion that includes register data out signal line 409 from the South Bridge to the PEC and register data in signal line 411 from the PEC to the South Bridge. 
     The register data out line 409 is output from the South Bridge. As with the time slot bus, data is transmitted synchronous to both edges of clock 407. However, register reads and writes on the register transfer bus are asynchronous to the frame sync signal. Instead, a start bit is used. Referring to FIG. 7, the communication protocol for the register access bus is shown. Start bit 701 initiates a transfer sequence and in the embodiment shown, is always initiated from the South Bridge. Following start bit 701, read/write bit 703 indicates the direction of transfer (write=1, read=0). The next bit 705, indicates the size of the read or write operation, (1=16 bits, 0=8 bits). A 10 bit address field 707 identifies the particular register in the PEC that is to be read or written. For a write operation, eight bit data 709, or sixteen bit data, 709 combined with data 711, are provided after the address field 707. 
     In order to account for possible delay in obtaining read data from e.g., the parallel port, a ready bit 731 alerts the South Bridge that read data is ready. In addition, one clock cycle may be provided between the address bits and the data bits during a read cycle at 730, to provide time for the PEC to decode the address and provide the contents of the selected register to the register transfer bus. The use of the ready bit 731 may make the one clock cycle 730 unnecessary. The ready bit is not necessary if all read data can be guaranteed to be ready. The delay caused by the lack of a ready bit during the read operation can be utilized in the South Bridge to control a CHRDY signal on the ISA bus which can stall the processor when data is not yet ready from e.g., the parallel port. Once the read operation is completed, the register data out pin is held low for at least one bit time (in this embodiment half a clock cycle) between transfers so that a new start bit can be recognized. In the exemplary embodiment, the register data out pin is held high when the bus is deactivated and low when active and idle. 
     In addition to providing read data, the register data in signal line provides, interrupt request (IRQ) information synchronized to the frame sync signal, whenever register reads are not taking place. That is, when the bus is idle or when register writes are taking place, the register data in line which is used only during reads, is available to send IRQ data. In the described embodiment, IRQ data is sent four times per frame synchronized to frame sync in a manner similar to the previously described time slot bus. An exemplary IRQ frame is shown in Table 2. 
     
                       TABLE 2                                                     
______________________________________                                    
Bit #             ASSIGNMENT                                              
______________________________________                                    
0                 IRQ0                                                    
1                 IRQ1                                                    
2                 IRQ2                                                    
3                 IRQ3                                                    
4                 IRQ4                                                    
5                 Reserved                                                
6                 Reserved                                                
7                 Reserved                                                
8                 IRQ0                                                    
9                 IRQ1                                                    
10                IRQ2                                                    
11                IRQ3                                                    
12                IRQ4                                                    
13                Reserved                                                
14                Reserved                                                
15                Reserved                                                
16                IRQ0                                                    
17                IRQ1                                                    
18                IRQ2                                                    
19                IRQ3                                                    
20                IRQ4                                                    
21                Reserved                                                
22                Reserved                                                
23                Reserved                                                
24                IRQ0                                                    
25                IRQ1                                                    
26                IRQ2                                                    
27                IRQ3                                                    
28                IRQ4                                                    
29                Reserved                                                
30                Reserved                                                
31                Reserved                                                
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     The interrupt requests, are interrupts requests from such devices as the parallel port, game port, RTC, floppy controller and General I/O. 
     When register reads are taking place, the IRQ data 717 is provided on Register Data In during the address window as shown in FIG. 7. That is, when address bits are being received, the data in line is still idle even though it is a read operation. The address window for IRQs can be any predetermined time between start bit 701 and the read data 713. If a register read starts in the middle of the receipt of a frame of IRQ data (which is synchronous to frame sync and not the start bit), the PEC ceases transmission of the frame sync IRQ data and starts the register read cycle including the transmission of a field of IRQ data 717 as shown in FIG. 7. 
     In addition to providing IRQ requests, direct memory access (DMA) requests are also generated in functional logic blocks in the PEC such as the floppy disk controller and the parallel port. In order to accommodate these DMA requests, a separate DMA request signal line 413, DRQ In, is provided as shown in FIG. 4. The DMA request signal line 413 provides frames of DMA requests in a manner similar to the IRQ frame data. The DRQs are sent over the DRQ In line synchronous to the frame sync signal. Table 3 shows an exemplary DRQ frame with bit 0 being synchronous to the rising edge of frame sync. 
     
                       TABLE 3                                                     
______________________________________                                    
Bit #             ASSIGNMENT                                              
______________________________________                                    
0                 Floppy DRQ                                              
1                     Parallel port DRQ                                   
2                     Reserved                                            
3                     Reserved                                            
4                     Floppy DRQ                                          
5                     Parallel Port DRQ                                   
6                     Reserved                                            
7                     Reserved                                            
8                     Floppy DRQ                                          
9                     Parallel Port DRQ                                   
10                   Reserved                                             
11                   Reserved                                             
12                   Floppy DRQ                                           
13                   Parallel Port DRQ                                    
14                   Reserved                                             
15                   Reserved                                             
16                   Floppy DRQ                                           
17                   Parallel Port DRQ                                    
18                   Reserved                                             
19                   Reserved                                             
20                   Floppy DRQ                                           
21                   Parallel Port DRQ                                    
22                   Reserved                                             
23                   Reserved                                             
24                   Floppy DRQ                                           
25                  Parallel Port DRQ                                     
26                  Reserved                                              
27                  Reserved                                              
28                  Reserved                                              
29                  Reserved                                              
30                  Reserved                                              
31                  Reserved                                              
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     The parallel port and floppy disk controller DRQ fields are provided seven times each 32 bit frame. The number of fields provided in each frame depends upon the latency acceptable for the DRQ which is affected by such factors as the device issuing the DRQ and the speed of the bit clock. For the embodiment described herein a bit slot in the DRQ frame is determined by both the rising and falling edge of the bit clock provided on the clock line 407. In other words, each clock period contains two data bits. DRQ data is sent synchronous to both the rising and falling edges of frame sync as shown in FIG. 8. 
     The IRQs and DRQs provided by the PEC are routed through respective mapping units to the interrupt controllers and DMA controllers in the South Bridge as described further herein. 
     Referring to FIGS. 3 and 4, the port expansion host controller 326 provides the interface between the various signal lines on the port expansion bus and the rest of the South Bridge integrated circuit. Referring to FIG. 9, the host controller 326 is shown in block diagram form. Block 901 receives the time slot data in and provides the time slot data out signal lines. Block 901 also receives the frame sync and the bit clock. Block 901 provides a multiplexing-demultiplexing function to multiplex data bits from the PIT 304, keyboard/mouse controller 314, MIDI controller 310, and the serial port(s) 316 into their appropriate time slot on the time slot bus. Block 901 also management serial port data, MIDI data, keyboard and mouse data and various power management and general I/O related signals and demultiplexes those signals at their appropriate time slot and provides the signals to the appropriate logic function in the South Bridge. 
     PEC Register access unit 902 receives the bit clock and frame sync from the South Bridge side and provides the register data out signal and receives the register data in signal from the PEC. In addition, the PEC register access unit 902 is coupled to the internal system bus 905, shown as ISA bus 324 in FIG. 3. Writes to an ISA address space on the PEC will be serialized and provided as described previously. For reads from the PEC registers, the data read is provided to the internal South Bridge ISA bus and across the ISA/PCI bridge to the appropriate destination, e.g., the CPU. 
     The serial IRQ extractor and Plug and Play (P &amp; P) mapping table 903, extracts the IRQs from the stream of data on the register data in line and maps the IRQs according to a programmable IRQ mapping table. 
     The serial DRQ extractor and Plug and Play (P &amp; P) mapping table 904, extracts the DRQs from the stream of data on the DRQ data line and maps the DRQs according to a programmable DRQ mapping table. 
     The block diagram of FIG. 9 is shown in greater detail in FIG. 10. The time slot bus unit 901 is mapped into PCI configuration address space. Various functions on the PCI bus are mapped into configuration space as is known in the art. Block 901 provides for bus activation from an inactive state as described further herein, as well as the multiplexing-demultiplexing function. Signals to and from UARTs 316, the mouse port and keyboard controller 314, and the PIT (PC speaker output) 304 are reflected across the PEB. The time slot bus unit multiplexes the outgoing signals onto the time slot data out pin and demultiplexes the incoming signals from the time slot data in pin. 
     The PEB also provides for an activation function which is recognized by logic in the time slot bus unit 901. The PEB activation function provides for a PEC initiated restart from a power down condition. Assume for example that the PC is in a reduced power state with the clocks stopped. The PEC would want to wake up the South Bridge if there were activity on e.g., the control pins of a COM port, say the Ring Indicate pin. Such activity should wake up the South Bridge from its sleep mode. The wakeup can be accomplished by the port expansion circuit requesting that the South Bridge start up the bus clocks (Bit Clock and Frame Sync). Those clocks are preferably stopped when the computer system enters sleep state to reduce power consumption. 
     Referring to FIG. 11, when the bus is stopped to reduce power consumption as shown at region 1101, all of the lines on the port expansion bus are kept at a high voltage level in this embodiment. The port expansion circuit can request that the bus be restarted by pulling the time slot data in signal low as shown at 1102. The time slot data in signal is an output from the port expansion circuit to the South Bridge. The data in pin should be held low until frame sync goes low. Frame sync must go low for at least one bit time before it can go high (signifying the start of a frame). Note that while the protocol for waking up the South Bridge has been described with respect to specific polarities, other polarities can also be used depending upon the choice for indicating assertion of a signal. 
     The PEC register access unit 902 is also shown in more detail in FIG. 10. The PEC register access unit 902 includes the programmable base address table 1003. The table is mapped into the PCI configuration space. The table 1003 is a programmable table of registers with the 16-bit base address for each of the functions located with the PEC. These base addresses are matched (decoded) in address space decoder 1004 with the ISA I/O address from ISA bus 324 to create block selects 1010. The block selects 1010 cause the correct base address (which can be hardwired in the PEC) which is stored in the PEC block base address map 1007, to be presented to the PEB serial register access unit 1005 where the actual serial register access cycle is started in accordance with the timing diagram shown in FIG. 7. Note that the ISA data bus 1009 and ISA bus address 1008 and control signals BHE/, IOR and IOW are provided to the PEB serial register access unit 1005. Thus, for instance, if a device in the PEC has an address space of 16 bytes, e.g., the floppy controller, some of the high order address bits of the 10 address bits provided from the ISA bus to the PEB serial access unit 1005 may be replaced to indicate the correct address as hardwired in the PEC. 
     The serial IRQ extractor and P&amp;P mapping table 903 is shown in more detail in FIG. 10. The serial IRQ extractor gathers IRQ data from the register data in pin during the address portion of each read cycle (717 in FIG. 7) and synchronous with frame sync during both a write cycle and when the bus is idle, based on the time slot assignments (723 in FIG. 7), relative to the start bit or the frame sync as appropriate. Read cycle indication 1011 is provided from register access unit 1005. Frame sync is also provided to the serial IRQ extractor 1013. Once the IRQ data has been gathered, it is provided to IRQ P&amp;P mapper 1015. IRQ P&amp;P mapper 1015 routes each of the five IRQ signals to software programmable destinations on the programmable interrupt controller 308. Note the mapper function may be located physically as part of interrupt controller 308. For legacy reasons, certain fixed interrupt mappings may be preferred. Specifically, the real time clock (RTC) 367 is preferably assigned IRQ8 and the floppy disk controller 357 is preferably assigned IRQ6. In the disclosed embodiment, one IRQ is provided from each of the floppy disk controller, parallel port, RTC, game port and general I/O blocks. Since the parallel port 353 can be assigned as either port 1 or port 2, using IRQ7 and IRQ5, respectively, the parallel port IRQ is software mappable. The game port and general purpose I/O IRQs are also software mapped. The programmed and/or fixed interrupts 1017 are then provided to the programmable interrupt controller 308. 
     Referring to FIG. 12, the IRQ mapper 1015 and the programmable interrupt controller (PIC) 308 (shown as 1203) are shown in a more detailed block diagram. The PIC 1203 in the embodiment shown includes two 8259A compatible PICs. The five IRQs from PEB host controller 316 are provided to the IRQ mapper 1015 which in turn are mapped according to the IRQ programmable table and provided to PIC 1203. The interrupt sequence using 8259A PICs is known in the art and is only briefly described herein. When an asserted interrupt is received over the register data in pin and extracted by the serial IRQ extractor 1013, it is mapped as according to the programmable mapping table and provided to the PIC 1203. Of course, unasserted interrupts are provided in a similar manner to indicate their state. The asserted interrupt causes a bit corresponding to the interrupt in the PIC 1203 interrupt request register (IRR) to be set which in turns results in the CPU 110 being notified in a manner known in the art. The CPU acknowledges the interrupt with an interrupt acknowledge (INTA) command via PCI bus 125 which is provided as signal 1205 to PIC 1203. The INTA command resets the IRR bit and sets the corresponding bit in the interrupt service register (ISR) indicating that that particular interrupt is currently being serviced by the CPU. When the CPU finishes servicing the interrupt, the CPU sends an end of interrupt (EOI) command which clears the ISR bit, thereby indicating that the processor has completed servicing that interrupt. 
     However, the processor is typically operating significantly faster than the relatively slow PEB supplying the IRQs. The maximum latency between updated IRQs (approximately 25 bit times) occurs during register read operations and is approximately 379 nanoseconds. The minimum latency is eight bit times (for five IRQs) when the IRQs are being provided as shown in Table 2. Because of the potential for a relatively long latency, the IRQ signals may not have been updated before the PIC 1203 receives the EOI command. Thus, the PIC may erroneously recognize the same interrupt twice. 
     In order to prevent an erroneous level triggered interrupt due to latency, the interrupt controller 1203 does not reevaluate its IRQ inputs until the IRQ transmission following the End Of Interrupt command (EOI) from the CPU to the PIC. That can be accomplished in several ways. In one embodiment, receipt of the EOI command coming across PCI/ISA bridge 322 is blocked in the Southbridge before it is provided to PIC 1203. The EOI command is then provided to the PIC after an updated IRQ is sent by serial IRQ extractor 1013 after the EOI command has been transmitted through the PCI/ISA bridge 322. That approach has the advantage of avoiding modifying the internal PIC logic but stalls the CPU since the bus transaction did not complete. The EOI command comes across address and data lines 1207 and 1209. Alternatively, instead of blocking the EOI command externally to the PIC, the EOI command is blocked internally to the PIC until after the updated IRQs are received. 
     Referring again to FIG. 10, the serial DRQ extractor and Plug &amp; Play mapping table 904 is shown in more detail. Serial DRQ extractor 1019 receives data from the DRQ in pin, and extracts the floppy and parallel port DRQs from the bit stream based on the time slot assignments. The state of the two DRQ signals is being updated in the illustrated embodiment at a rate of seven times per frame. Referring to FIG. 13, the extracted DRQ signals are sent to the DRQ Plug &amp; Play mapper 1021 where they are programmably routed to the desired DRQ input on the DMA controllers. For legacy reasons, the floppy DRQ is preferably assigned to DRQ2. Thus, for this particular embodiment, only the parallel port DRQ is programmably mapped. The DMA mapper 1021 also receives a SOUND BLASTER DRQ and provides a SOUND BLASTER DAK/ (DMA acknowledge) signal. The mapped DMAs are provided to DMA controller 302 which in the embodiment shown is two 8237A compatible DMA controllers. The DMA controller interfaces with the PCI/ISA bridge 322. 
     The details of a DMA controller such as an 8237A compatible interrupt controller, shown in FIG. 13, are known in the art and are described in, e.g., pages 598-621 of &#34;The Indispensable PC Hardware Book&#34; by Hans-Peter Messmer, Second Edition, 1995; the data sheet entitled &#34;CMOS High Performance Programmable DMA Controller, pages 4-192-4-214, available from Harris Semiconductor, dated 1997; &#34;ISA System Architecture, by Tom Shanley and Don Anderson, pages 367-388, which are incorporated herein by reference. Relevant details of the 8237A interrupt controller are discussed herein where required for a full appreciation of the invention. 
     Note that the mapper function 1021 may be located with the DMA controller 302 as part of an overall DMA P &amp; P mapper. 
     Because of the latency between the start of a DMA generated register access to one of the ISA resident memory locations in the PEC and the update of the DRQ signals (up to approximately 530 ns for a 16 bit transfer (assuming 35 clocks for the transfer)), the DMA controller should not reevaluate its DRQ inputs until after the DRQ transmission after a DMA cycle is complete. That can be implemented by holding the data ready line 1025 inactive. 
     A timing diagram of a typical DMA request (I/O write) is shown in FIG. 14. The timing is known in the art and not described in detail. The DMA Request (DRQ) signal is sent across DRQ In signal line to the Serial DRQ extractor 1019 which extracts the appropriate DRQ from the DRQ In signal line based on the time slot relative to the frame sync signal. The DMA request is sent through the mapper 1021 to DMA controller 302. 
     Assuming the DMA request (an asserted DRQ) from a requesting device is received by DMA controller 302 when it is in the idle state (SI), the DMA controller requests control of the ISA bus with HRQ in SO. Once the bus is granted to the DMA controller, the DMA controller controls the transfer of data by making the address information available, (S1-S4 and SI cycles) and asserting the memory read control line, indicating that data is being read from memory. In response, a memory device will place data onto the data bus. The DMA controller also asserts a write signal for the target device in S4 (S3 for extended write). The assertion of the DMA acknowledge (DAK) by the DMA controller functions as a select for the requesting device. Thus, when the requesting device sees the DAK, the device knows that the information on the data lines is for it. The DMA controller asserts the IOW/ signal to tell the requesting device to latch data available from the source device. ISA Bus cycle generator 1513 rebuilds the timing of data, address, DAK and read/write, which is shown generally in FIG. 14, in the PEC. In FIG. 15, DAKs are provided to the functional blocks 1515 (parallel port and floppy) from the address decoder 1511. The address decoder determines from the address bits (ADDR (9:0)) which DAK to assert Thus, the DAK signals provided to PEB Host Controller 326 are mapped to unique addresses in the port expansion chip. 
     In the illustrated embodiment, the requesting device may be a floppy disk controller in the port expansion chip. Referring to FIG. 15, the floppy DRQ is sent as a request to the Serial DRQ time slot combiner 1509 which places the DRQs from the floppy and parallel port in their appropriate time slot, consistent with Table 3. 
     Once the address, data and DAK signal have been provided to the host controller 326 by DMA controller, the DMA controllers would typically be expected to see that particular DMA cycle as being complete and return to the idle state SI and look for another asserted DRQ. However, in the invention described herein, because the address and data from the source device has to be serialized across the port expansion bus 207, there is a time lag between when the DMA controller would consider the DMA transfer as complete and when the requesting device in the PEC sees that it has been serviced. Note that for single transfer mode timing shown in FIG. 14, DRQ should stay asserted at least until DAK is asserted according to conventional 8237A DMA controller protocol. 
     As described, there is a latency between when the DMA controller sees the DMA transfer as complete and when the requesting device (e.g., the floppy disk controller) sees the DMA transfer as complete. The requesting device, when it sees see the DAK with associated data and/or address information, responds with an updated DRQ signal which is serialized and sent across the DRQ-In signal line. The DMA controller should not evaluate its DRQ inputs until after the updated DRQ signal is received. Thus, to avoid the DMA controller from responding to an asserted DRQ which is erroneous due to the latency, the host controller, in one embodiment of the invention delays the start of the next idle cycle for the DMA controller by extending DMA read and write cycles. 
     The DMA read and write cycles are extended by deasserting the data ready signal 1025. The data ready signal should be deasserted until after the port expansion bus operation which transfers the address and data to the requesting device on the port expansion circuit has been completed and an updated DRQ value that reflects the completed DMA operation has been provided over the DRQ In signal line from the requesting device on the port expansion circuit. 
     That has the effect of inserting wait states (SW) into the DMA cycle. That serves to synchronize the return of the DMA controller to the idle state (SI), when new DRQs are evaluated, to the time when updated DRQ data is available. 
     FIG. 16 shows the sequence of actions the host controller takes during a DMA operation involving the port expansion circuit in order to synchronize the DMA controller evaluating its DRQ input signal to the time when an updated DRQ signal is available from the port expansion circuit. The logic for the sequence of actions is located in DMA data ready circuit 1023 (FIG. 10). The block may be physically located elsewhere in the host controller. The block has to be alerted to the start of a DMA transfer. Note that DAK/ signals are provided to block 1005 to map the DAK/ to the appropriate port expansion circuit address. The host controller detects a DAK/ for the parallel port or floppy disk controller in the port expansion circuit in 1601. The DAK/ is mapped to the appropriate unique port expansion circuit address at 1603 so that address decoder 1511 properly decodes the DAK/ signal. Then the host controller at 1605 deasserts the data ready signal 1025 to the DMA controller. The deassertion of the data ready signal has the effect of inserting wait states (SW) in the DMA cycles as shown in FIG. 14. Note that the wait states shown in FIG. 14 are preferably not used on the port expansion bus side of the DMA operation. That is, neither the parallel port or the floppy disk controller deassert data ready during transfers. However, the DMA controller on the south bridge sees a deasserted data ready signal until an updated DRQ signal is available from the port expansion circuit after the transfer has completed. Thus, the host controller waits in 1607 for the updated DRQ. Then, in 1609 the ready signal line is released. 
     The number of cycles of deasserted ready that should be inserted is dependent upon the speed of the port expansion bus and to some extent on the response time of the requesting device. The length of the deasserted ready has to account for serializing data and address information across the port expansion bus (e.g., 35 cycles), giving the floppy disk controller or parallel port time to deassert DRQ in response to DAK/ (e.g., 1 cycle) and to serialize the deasserted DRQ back across the port expansion bus and get to the DMA controller (e.g., 7 cycles). Thus the host controller, in one embodiment, waits for a fixed number of clock cycles before it releases the data ready signal ready in order to ensure that a valid DRQ is available. 
     Referring to FIG. 15, a more detailed block diagram of the PEC is shown. Time slot bus unit 1501 interfaces to the time slot bus, receiving data from the time slot data out pin, the bit clock, the frame sync, and provides time slot data in. Block 1503 provides a multiplexing-demultiplexing function to demultiplex data bits received from the PIT 304, keyboard/mouse controller 314, MIDI controller 310, and the serial port(s) 316 and provide the received signals to their respective pins. Block 1501 also receives serial port data (UART0 and UART1), MIDI data, keyboard and mouse data and multiplexes those signals along with power buttons, and wakeup and general purpose I/O pins onto the time slot data in signal line. The time slot bus unit also provides the bus activation logic resulting in the assertion of the time slot data in signal as shown in FIG. 11, on receipt of a signal that causes a wakeup condition. 
     The PEC also includes PEB serial register access unit 1505 which receives the bit clock and the register data in signal and provides the register data out signal. The register access unit 1505, responds to the start bit as shown in FIG. 7. The register access unit determines whether the access is a read or write according to the read/write bit 703 and the size of the access according to the data size bit 705 indicating an 8 or 16 bit access. That information, along with the address (and data if the operation is a write operation) is provided to ISA bus cycle generator 1513 which generates an internal PEC ISA bus cycle to recreate a representation of the ISA bus in the PEC. Note that it may be not the same ISA representation as in the South Bridge. For example, the PEC ISA representation in the embodiment shown has fewer address lines. The 10 bits of address information is also provided to address decoder 1511 which decodes the 1 k address space into block selects for the various functional blocks 1515. As described, address decoder 1511 asserts the appropriate DAK/ to the parallel port or floppy disk controller according the address received. The functional blocks may include the floppy controller, RTC, parallel port, PEC control register(s), GP I/O and the game port. 
     The ISA bus cycle generator 1013 provides the IOW (write) and IOR (read) signals along with the data bits and 10 bits of address. The ISA bus cycle receives the I/O16 signal line from the ISA resident logic blocks which indicates the size of the data being provided. The ISA cycle is built in accordance with FIG. 7. IOW/ is asserted at 710 with the leading edge of the bit clock synchronous with bit 0 of the data. IOW/ is deasserted at 712 at the falling edge of the bit clock starting the tenth bit for an 8 bit transfer and at 714 at the falling edge of the clock one bit time after the last data bit is transferred for a sixteen bit transfer as shown in FIG. 7. 
     For read operations, the functional blocks provide 8 or 16 bit data over the data lines to the ISA bus cycle generator 1513 to be transferred on the register data in line to the South Bridge. Read timing is shown in FIG. 7. IOR/ is asserted at 716 and deasserted at 718 following the receipt of the 10 bits of address data. Read data 713 and 715 is provided from one of the selected function blocks 1515 as shown in FIG. 7. Although the number of address bits shown in the embodiment is 10 bits, more or less bits may be provided depending upon the address range desired in the PEC. If the data cannot be provided during a read operation, then the start bit 731 is delayed until data is available. 
     The floppy controller, RTC, parallel port and game port are well known logic functions that will not be described further herein. The PEC control registers are mapped to the ISA to allow for read/write access to PEC control registers. 
     The PEC also includes serial IRQ time slot combiner 1507 which receives a read cycle indication form the PEB serial resister access unit 1505, the frame sync and the bit clock. The serial IRQ time slot combiner 1507 receives the RTC, floppy controller, parallel port, game port and GP I/O interrupt requests, and provides those interrupt requests to the PEB serial register access unit 1505 for multiplexing onto the register data in pin as described in relation to FIG. 7. 
     The PEC also includes serial DRQ time slot combiner 1509 which receives a the frame sync, bit clock and DMA requests from the floppy controller and parallel port. The serial DRQ time slot combiner 1509 provides those DMA interrupt requests to the South Bridge over the DRQ Out pin as described previously herein. 
     The port expansion circuit is pad-limited (meaning that the minimum size of the integrated circuit, as determined by the number of pins located around the outside edge of a square, is larger than the area necessary to contain the chip&#39;s logic). Thus, adding additional the function to the PEC produces little marginal cost increase and integration reduces motherboard chip count, board space, and cost. 
     Providing I/O capability for all the legacy devices as well as performing the bus interface and other functions on the South Bridge would require an integrated circuit with more I/O capability and would therefore could be a more expensive part. It is preferable to build an integrated circuit which can be more inexpensively packaged in, e.g., quad flat packs and avoid more expensive packaging such as ball grid arrays which can provide higher pin density but can also be more costly to produce. In many instances, reducing pin count significantly can reduce the package cost of the product. 
     Placing the legacy I/O terminals as well as some of the of the functional logic blocks (e.g., the game port, parallel port and floppy disk controller) in the PEC as described herein provides further advantages. As discussed, as process geometries shrink for faster and denser integrated circuits, e.g. channel lengths of 0.35 microns and below, supply voltages drop, e.g., to 3.3. volts or less, leading to problems driving the various devices on the I/O interface pins which are typically 5 volts. The legacy I/O interfaces tend to be 5 volts interfaces which is typical of the legacy devices. Since high speed is desired for the South Bridge integrated circuit, it is preferable to build the South Bridge chip with the smaller process geometries to provide a faster and denser chip. However, the port expansion circuit chip can be built with slow technologies, e.g., where the transistors have channel lengths of e.g., 0.5 microns or above. That allows the port expansion circuit to be built with older and less expensive process technology and also allows the port expansion circuit to easily interconnect to the 5 volts interfaces typical of the legacy devices. Additionally, slower logic functions such as the parallel port, floppy controller, and game port can be placed on the PEC, which is pad limited, meaning the size of the chip is determined by the number of I/O pins, not the amount of logic placed on the chip. Additionally, analog functions, such as those required for the game port can be more readily incorporated into older and more stable process technologies. At the same time, the more speed sensitive South Bridge can be built with the most up to date process technologies providing channel lengths for the transistors of the internal logic of, e.g., 0.35 microns, without the problem of interconnecting to a large number of 5 volts interfaces. 
     The description of the invention set forth herein is illustrative, and is not intended to limit the scope of the invention as set forth in the following claims. Variations and modifications of the embodiments disclosed herein, may be made based on the description set forth herein, without departing from the scope and spirit of the invention as set forth in the following claims.