Abstract:
The present invention relates to the design of highly reliable high performance microprocessors, and more specifically to designs that use cache memory protection schemes such as, for example, a 1-hot plus valid bit scheme and a 2-hot vector cache scheme. These protection schemes protect the 1-hot vectors used in the tag array in the cache and are designed to provide hardware savings, operate at higher speeds and be simple to implement. In accordance with an embodiment of the present invention, a tag array memory including an input conversion circuit to receive a 1-hot vector and to convert the 1-hot vector to a 2-hot vector. The tag array memory also including a memory array coupled to the input conversion circuit, the memory array to store the 2-hot vector; and an output conversion circuit coupled to the memory array, the output conversion circuit to receive the 2-hot vector and to convert the 2-hot vector back to the 1-hot vector.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of application Ser. No. 10/726,492 filed Dec. 4, 2003 now U.S. Pat. No. 6,839,814, which is divisional of application Ser. No. 09/750,094 filed Dec. 29, 2000, now U.S. Pat. No. 6,675,266, and a divisional of application Ser. No. 10/435,386 filed May 12, 2003, now U.S. Pat. No. 6,775,746, which is also a divisional of application Ser. No. 09/750,094, filed Dec. 29, 2000 now U.S. Pat. No. 6,675,266; this application is also a continuation of application Ser. No. 10/743,069 filed Dec. 23, 2003 now U.S. Pat. No. 6,904,502, which is a continuation of application Ser. No. 10/435,386 filed May 12, 2003 now U.S. Pat. No. 6,775,746, which is a divisional of application Ser. No. 09/750,094 filed Dec. 29, 2000 now U.S. Pat. No. 6,675,266, all of which are hereby incorporated herein in their entireties by reference thereto. 

   FIELD OF THE INVENTION 
   The present invention relates to the design of highly reliable high performance microprocessors, and more specifically to designs using a 2-hot vector tag protection scheme in high speed memories. 
   BACKGROUND 
   Modern high-performance processors, for example, Intel® Architecture 32-bit (IA-32) processors, include on-chip memory buffers, called caches, to speed up memory accesses. IA-32 processors are manufactured by Intel Corporation of Santa Clara, Calif. These caches generally consist of a tag array and a data array. The data array generally stores the data that is needed during the execution of the program. The tag array generally stores either a physical address or a virtual address of the data as tags. For reliability reasons, these stored tags are often protected for error detection by associating a separate parity bit with each tag. In even higher performance processors, for example, Intel® Architecture 64-bit (IA-64) processors, each tag is generally stored as a 1-hot vector in a 1-hot cache, which is derived during a Translation Look-aside Buffer (TLB) lookup for an address translation. IA-64 processors are manufactured by Intel Corporation of Santa Clara, Calif. A “1-hot vector” is an n-bit, binary address in which a single bit is set to specify a matching address translation entry in the TLB. The advantage of using a 1-hot vector as a tag is that it improves the operating frequency of a cache. Unfortunately, the protection of these 1-hot vectors presents a great challenge since the conventional parity bit protection scheme used to protect the standard tag in the conventional cache does not work well for the 1-hot vectors. For example, when an entry in the TLB is replaced, all of the tags with the corresponding 1-hot vectors in the 1-hot cache must be invalidated. This invalidation can be performed using a blind invalidate operation, in which all 1-hot vectors in the cache with the “1” bit matching the selected TLB entry will be invalidated. However, since the blind invalidate operation only overwrites the 1-hot vector and not the associated parity bit, the associated parity bit is no longer valid for the new value in the 1-hot vector. In addition, in the 1-hot cache, since all of the cleared bits are now zero, if any of the bits are changed by a soft error to a 1, then, the cleared entry becomes a 1-hot vector, which is indistinguishable from a real, valid 1-hot vector that also may be stored in the 1-hot cache. A “soft” error is an error that occurs when a bit value that is set to a particular value in the processor is changed to an opposite value by, for example, an alpha particle bombardment and/or gamma-ray irradiation of the bit. 
   A straight forward protection scheme for the 1-hot tag cache that does work for the 1-hot vectors involves having a second tag array to maintain a duplicate copy of the 1-hot vectors in the tag array. However, although this duplicate tag array scheme works, it requires a larger chip area and a high timing impact to implement. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a logic block diagram of a 1-hot tag cache, in accordance with an embodiment of the present invention. 
       FIG. 2  is a circuit schematic diagram of a known 1-hot tag memory cell, illustrating how the 1-hot tag cache operates with no interaction between the memory bit circuits in the 1-hot tag memory cell. 
       FIG. 3  is a circuit schematic diagram of a 1-hot tag plus valid bit memory cell, illustrating the interaction between the memory bit circuits in the 1-hot tag plus valid bit memory cell, in accordance with an embodiment of the present invention. 
       FIG. 4  is a logic block diagram of a 2-hot tag cache based on the 1-hot tag cache in  FIG. 1 , in accordance with an embodiment of the present invention. 
       FIG. 5  is a circuit schematic diagram of a 2-hot tag memory cell, illustrating the interaction between the memory bit circuits in the 2-hot memory cell, in accordance with an embodiment of the present invention. 
       FIG. 6  is a circuit schematic diagram of a known alternative 1-hot tag memory cell, which also illustrates how the 1-hot tag cache operates with no interaction between the memory bit circuits in the 1-hot tag memory cell. 
       FIG. 7  is a circuit schematic diagram of an alternative 2-hot tag memory cell, implemented from the 1-hot tag memory cell in  FIG. 6 , which illustrates the interaction between the memory bit circuits in the 2-hot memory cell, in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   In accordance with embodiments of the present invention, circuits and methods to protect the 1-hot vectors used in the tag cache are described herein. As a way of illustration only, two embodiments of the present invention are described: a 1-hot plus valid bit and a 2-hot vector scheme, however, these two embodiments should not be taken to limit any alternative embodiments, which fall within the spirit and scope of the appended claims. 
   In general, a cache that stores 1-hot vectors as tags is referred to as a 1-hot tag cache and a cache that stores 2-hot vectors as tags is referred to as a 2-hot tag cache. A 1-hot vector is an n-bit string that contains a single “1” and n- 1  “0&#39;s”, for example, “00001000” is an eight-bit 1-hot vector. Similarly, a 2-hot vector is an n-bit string that contains two consecutive “1&#39;s” and n- 2  “0&#39;s”, for example, “00011000” is an eight-bit 2-hot vector. The right most “1” bit in a 2-hot vector is called a primary bit and a left neighbor “1” bit of the primary bit is called an aux (auxiliary) bit. 
     FIG. 1  is a logic block diagram of a known implementation of a 1-hot tag cache  119 . The 1-hot tag cache  119  shown in  FIG. 1  is a 4-way set associative cache, which means that four tags are stored for any given set (row) in the cache. In  FIG. 1 , the 1-hot tag cache  119  is coupled to a TLB  109 , which includes a TLB virtual address array  110 . The 1-hot tag cache  119  includes a 1-hot tag array  120 , a cache data array  125 , comparators  130 - 133 , a first multiplexer  140 , and a second multiplexer  150 . 
   In  FIG. 1 , during a read request, the TLB virtual address array  110 , receives a tag  102  from an incoming tag cache access address request  100  that specifies the desired tag in the TLB virtual address array  110  and, based on the virtual address stored in the specified tag, outputs an n-bit 1-hot vector  112 , where the number of bits, n, in the 1-hot vector is equal to the size of the TLB, that is, the number of tags in the TLB. At generally about the same time that the TLB virtual address array  110  receives the tag  102 , the 1-hot tag array  120  receives an index address  104  that specifies which set, that is, row, in the 1-hot tag array  120  to read out and, then, the 1-hot tag array  120  reads out the tags from the memory cells in the specified set. The comparators  130 - 133  each receive one of the tags read out from the 1-hot tag array  120  and the 1-hot vector  112  from the TLB. Each of the comparators  130 - 133 , then compares the 1-hot vector  112  with the tag it received from the 1-hot tag array  120  to determine if the received tag is the desired tag from the set. Each of the comparators  130 - 133 , outputs a value representing whether the desired tag was located in that specific comparator to a first multiplexer  140 . The first multiplexer  140  also receives four-way data from the data cache array  125  as specified in the index address  104  and, then, based on the values of the comparators  130 - 133 , determines which one way of the four-way data to read out. If there is a match between the desired tag value and one of the four-way data values, the way that matched is read out of the first multiplexer  140 . The second multiplexer  150  receives the read-out data and a byte select value  106  from the incoming tag cache access address request  100  and, then, based on the byte select value  106 , the second multiplexer  150  outputs the desired data. 
     FIG. 2  is a circuit schematic diagram of a known 1-hot tag memory cell architecture, illustrating how the 1-hot tag cache can operate with no interaction between the memory bit circuits in the 1-hot tag memory cell. In  FIG. 2 , the 1-hot tag memory cell is shown to include word lines wl 0 , wl 1  and wl 2  that are coupled to memory bit circuits  210 ,  220  and  230 . The memory bit circuits  210 ,  220  and  230  are coupled together with a plurality of other memory bit circuits to form an n-bit memory cell. In  FIG. 2 , for ease of illustration, only the memory bit circuits  210 ,  220  and  230  are shown, the remainder being generally indicated by the dotting to the left of memory bit circuit  210  and to the right of memory bit circuit  230 . Each of the memory bit circuits  210 ,  220  and  230  include bit lines bl 0 , bl 1  and bl 2 . The bit lines bl 0  and bl 1  can be used to read out the content of the memory bit circuits and bl 2  can be used to write data to the memory bit circuits  210 ,  220  and  230 . 
   Operation of the 1-hot tag array. In  FIG. 2 , the 1-hot tag array has two read ports. For a read operation in the 1-hot tag array, either wl 0  or wl 1  can be asserted to read out a bit from each memory bit circuit  210 ,  220  and  230  on the memory bit circuit&#39;s bl 0  or bl 1 , respectively. 
   In  FIG. 2 , performing a write operation in the 1-hot tag memory cell requires two phases. In the first phase, in each memory bit circuit  210 ,  220  and  230 , one or both of the bit lines bl 0  and bl 1  can be grounded to “0” and one or both of the word lines wl 0  and wl 1  can be asserted, to write a “0” into each memory bit circuit  210 ,  220  and  230 . In the second phase, wl 2  can be asserted and the data indicated on the bl 2  line is a blind clear (bc) signal in an inverted form, which is the inverse of the data to be written to the 1-hot array. That is, in the inverted form of the bc signal, in all of the memory bit circuits where a “1” is to be written the bl 2  will have a value equal to “0” and in all memory bit circuits where a “0” is to be written the bl 2  will have a value equal to “1”. In this way, the inverse of the 1-hot vector is written into the memory cell, for example, if an 8-bit 1-hot vector value is “00010000” then an inverse 8-bit bc signal, which will be written into the memory cell, is “11101111”. The bit values will be inverted when they are read out of the array, thus, producing the desired 1-hot vector. 
   In  FIG. 2 , to perform a blind invalidate in the 1-hot tag memory cell, the wl 2  line of all rows are asserted and each bl 2  contains the non-inverted version of the 1-hot vector bit, which clears the content of all of the memory bit circuits in the 1-hot memory cell indicated by the 1-hot vector. 
   In accordance with an embodiment of the present invention, a 1-hot plus valid bit scheme involves adding one bit to each 1-hot vector to serve as a valid identification (V id ) bit. In the 1-hot plus valid bit scheme, while conceptually simple, a multi-cycle read-modify operation can be used to update the valid bit to avoid the timing impact. In addition, in accordance with an embodiment of the present invention, in the 1-hot plus valid bit scheme an additional word line is used to read out the content of the 1-hot column. Therefore, in accordance with an embodiment of the present invention, in this scheme, a single bit is appended at the end of each 1-hot vector to serve as the V id  bit. 
     FIG. 3  is a circuit schematic diagram of a 1-hot tag plus valid bit memory cell, illustrating the interaction between the memory bit circuits in the 1-hot tag plus valid bit memory cell, in accordance with an embodiment of the present invention. In the circuit illustrated in  FIG. 3 , the V id  bit memory bit circuit  340  is shown as an extra bit circuit coupled at the end of the plurality of memory bit circuits that make up the 1-hot memory cell of  FIG. 2 . For the sake of clarity, an analogous memory bit circuit for the memory bit circuit  210  of  FIG. 2  has been omitted from  FIG. 3 . In  FIG. 3 , the structure of the V id  bit memory bit circuit  340  is different than the memory bit circuits  320  and  330  in the 1-hot memory cell in that the V id  bit memory bit circuit  340  does not have the bl 2  bit line. In place of the bl 2  line is the output of a latch  344 . Furthermore, the gate of transistor  300 , which, when turned on, can cause the value at the output of the latch  344  to affect the value stored in the V id  bit memory bit circuit  340 , which is coupled to a bit enable line  348 . The embodiment in  FIG. 3  also has an additional word line w 13 , which is the input to the latch  344 . The w 13  word line also is coupled to transistors  322  and  332 , each of which is coupled to the bl 2  bit line in each of the memory bit circuits  320  and  330 , respectively. Furthermore, each of memory bit circuits  320 ,  330 , etc. have an additional transistor  302 , which is coupled to the bl 2  bit line in the respective memory bit circuits  320  and  330 . The latch  344  is also coupled to a clock output  346 . 
   In accordance with embodiments of the present invention, on a read operation in the 1-hot plus valid bit scheme, the V id  bit is accessed at the same time as the 1-hot vector and, if the V id  bit is set, the 1-hot vector is considered valid, otherwise, the 1-hot vector is considered invalid by external processor logic (not shown). The V id  bit is cleared on a blind invalidate just as for the 1-hot tag array. The detailed operation of the 1-hot plus V id  bit is described below. It should be noted that the 1-hot plus V id  bit scheme is somewhat slower than the 1-hot tag memory cell due to the added read port via w 13  being slower than wl 0  and wl 1 . 
   Operation of the 1-hot plus valid bit. In  FIG. 3 , in accordance with an embodiment of the present invention, for a read operation in the 1-hot tag plus valid bit memory cell, either wl 0  or wl 1  is asserted to read out the content of the bits in the array on bl 0  or bl 1 , respectively. Similarly, the valid bit is read at the same time as the 1-hot vector bits. A 1-hot vector that does not have the valid bit set is considered an error, which causes the processor to vector into the error recovery firmware (FW) code. This FW code will flush the entire cache to correct the error. 
   In accordance with an embodiment of the present invention, in  FIG. 3 , a write operation in the 1-hot tag plus valid bit memory cell is performed in two phases in the same manner as described above for the write operation in the 1-hot tag memory cell. In the first phase of a clock cycle (each clock has a high phase and a low phase), in each memory bit circuit  320  and  330 , one or both of the bit lines bl 0  and bl 1  are grounded to “0” and one or both of the word lines wl 0  and wl 1  are asserted, to write a “0” into each memory cell  320  and  330 . In the second phase, wl 2  is asserted and the data indicated on the bl 2  lines is a blind clear (bc) signal in an inverted form, which is the inverse of the data to be written to the 1-hot array. That is, in the inverted form of the bc signal, in all of the memory bit circuits where a “1” is to be written the bl 2  will have a value equal to “0” and in all of the memory bit circuits where a “0” is to be written the bl 2  will have a value equal to “1”. In this way, the inverse of the 1-hot vector is written into the memory cell, for example, if the 8-bit 1-hot vector value is “00010000” then the inverse 8-bit bc signal, which will be written into the cell, is “11101111”. The bit values will be inverted when they are read out of the array, thus, producing the desired 1-hot vector. 
   In accordance with an embodiment of the present invention, in  FIG. 3 , a blind invalidate is performed in 2 clock cycles in the 1-hot tag array. In the first clock cycle, the 1-hot vector bit values can be indicated by the n bl 2  bit lines and wl 2  word lines of all rows are asserted. As a result, all rows that are indicated by the 1-hot vector will be cleared, that is, invalidated. In addition, if any of the bits of a cleared cell in the rows contain a “1”, then the latch  344  can be set via w 13 . In the second clock cycle the enable bit line  348  can be asserted and the valid bit can be cleared as well. 
   2-hot vector protection scheme. In accordance with an embodiment of the present invention, in the 2-hot vector scheme, the 1-hot vector is converted to a 2 hot vector. This is accomplished by local logic prior to the cache tag during the write operation of the 1-hot vector into the tag. During the read out, the 2-hot vector is automatically converted back to a 1-hot vector by local logic subsequent to the cache tag. In this way, the accesses of the cache work identically to the 1-hot tag cache described above. 
   In accordance with an embodiment of the present invention, while the 2-hot vector scheme is more complicated, it does not require the multi-cycle operation of the 1-hot plus valid bit scheme. In addition, in accordance with an embodiment of the present invention, the 2-hot scheme does not require additional bit lines or word lines. 
     FIG. 4  is a logic block diagram of a 2-hot tag cache  419  based on the 1-hot tag cache in  FIG. 1 , in accordance with an embodiment of the present invention. In  FIG. 4 , the 2-hot tag cache  419  works in a similar way as the 1-hot cache  119  in  FIG. 1  except that, in  FIG. 4 , the 1-hot vector tag is converted to a 2-hot vector and then stored in the 2-hot tag array  420 . In  FIG. 4 , the numbering convention used in  FIG. 1  has been continued in  FIG. 4  for those elements that remain unchanged from  FIG. 1 . In  FIG. 4 , a convert to 2-hot vector block  418  is coupled to the write data path of the 2-hot tag array  420  and the convert to 2-hot vector block  418  receives the incoming 1-hot vector data and then converts the 1-hot vector to the 2-hot vector. The 2-hot vector is then stored in the 2-hot tag array  420 . An output of the 2-hot tag array  420  is coupled to a convert to 1-hot vector block  422 , which converts the 2-hot vectors from the 2-hot tag array  420  back to 1-hot vectors, which are then input into the comparators  130 - 133  and the operation continues as described above for the 1-hot tag cache of  FIG. 1 . 
     FIG. 5  is a circuit schematic diagram of a 2-hot tag memory cell, illustrating the interaction between the memory bit circuits in the 2-hot tag memory cell, in accordance with an embodiment of the present invention. In  FIG. 5 , the 1-hot tag memory cell shown in  FIG. 2 , is illustrated with modifications that convert the 1-hot tag memory cell to a 2-hot tag memory cell, in accordance with an embodiment of the present invention. In  FIG. 5 , each memory bit circuit  510 ,  520  and  530  in the 2-hot tag memory cell is implemented with a primary clear bit line b 13  that is coupled to a primary clear circuit  519 , which is coupled to each memory bit circuit  510 ,  520  and  530  to clear the bit in that memory bit circuit. In accordance with an embodiment of the present invention, an auxiliary clear circuit  517  is coupled to a primary clear circuit  519  in memory bit circuit  510  and clears the aux bit in the memory cell to the right of the memory cell that contains the primary bit. Similar auxiliary clear and primary clear circuits are implemented in each of the memory bit circuits in the 2-hot tag memory cell. 
   Operation of the 2-hot tag cache. In  FIG. 5 , in accordance with an embodiment of the present invention, the read operation is the same as the read operation in the 1-hot tag memory cell in  FIG. 2 , in which a 1-hot vector is read out. Specifically, for the read operation either wl 0  or wl 1  can be asserted to read out the content of the bits in the memory cell on bl 0  or bl 1 , respectively. When this occurs, the 2-hot vector data stored in the 2-hot tag array can be read out and converted back to a 1-hot vector by the convert to 1-hot vector block  422  of  FIG. 4  (not shown in  FIG. 5 ). Before the conversion from a 2-hot to a 1-hot vector, the 1-hot vector coming from the 2-hot cache tag can be considered an error and can cause the processor to vector to the FW code for proper error recovery. 
   In accordance with an embodiment of the present invention, in  FIG. 5 , for a write operation in the 2-hot tag memory cell, the write operation is performed in the same manner as described above for the 1-hot tag memory cell in  FIG. 2 , except that the data is stored as a 2-hot vector. 
   In accordance with an embodiment of the present invention, in  FIG. 5 , a blind invalidate is performed by asserting the b 13  bit lines to cause each memory bit circuit to look at the memory bit circuit&#39;s right neighbor blind clear signal (bc) and the memory bit circuit&#39;s left and right neighboring memory bit circuits. Specifically, the bits to be invalidated will be indicated by the b 13  lines in a 1-bit format. The aux bit is cleared if and only if the aux bit&#39;s primary b 13  bit line is asserted and its left neighbor bit is a “0”. The primary bit is cleared if the primary bit&#39;s b 13  bit line is asserted and the left neighbor of the primary bit is a “0”. 
   While the aux bit has been described located in the bit just to the right of the primary bit, in an alternate embodiment of the present invention, the aux bit can be located in any bit position within the 2-hot vector. However, embodiments in which the aux bit is located closer to the primary bit, in general, perform better than those embodiments in which the aux bit is located farther away from the primary bit. 
     FIG. 6  is a circuit schematic diagram of a known alternative 1-hot tag memory cell, which also illustrates how the 1-hot tag cache operates with no interaction between the memory bit circuits in the alternative 1-hot tag memory cell. In  FIG. 6 , the read operation is performed in the same manner as described above for the read operation in the 1-hot tag memory cell in  FIG. 2 . Specifically, in  FIG. 6 , for the read operation either wl 0  or wl 1  can be asserted to read out the content of the bits in the 1-hot tag memory cell on bl 0  or bl 1 , respectively. 
   In accordance with an embodiment of the present invention, in  FIG. 6 , to perform a write operation, in the 1-hot tag memory cell, wl 0  and wl 1  can be selected. The data can be indicated on bit lines bl 0  and bl 1 . The data on the bl 1  bit line can be the inverted version of the data on the bl 0  bit line in each memory bit circuit. In this way, differential writes can be implemented. 
   In accordance with an embodiment of the present invention, in  FIG. 6 , to perform a blind invalidate in the 1-hot tag memory cell the bl 2  line can be asserted, which causes each of the bit circuits to be discharged and a “0” to be written into each of the bit circuits. 
     FIG. 7  is a circuit schematic diagram of an alternative 2-hot tag memory cell, implemented from the 1-hot tag array cell in  FIG. 6 , illustrating the interaction between the memory bit circuits in the 1-hot tag memory cell, in accordance with an embodiment of the present invention. In  FIG. 7 , the interaction between the primary bit and the left and right neighbor bits of the primary bit are illustrated. In  FIG. 7 , in accordance with an embodiment of the present invention, the read operation is performed in the same manner as described above for the read operation in the 1-hot tag memory cell in  FIG. 6 . Specifically, for the read operation either wl 0  or wl 1  can be asserted to read out the content of the bits in the 2-hot tag memory cell on bl 0  or bl 1 , respectively. 
   In accordance with an embodiment of the present invention, in  FIG. 7 , for a write operation in the 2-hot tag memory cell, the write operation can be performed in the same manner as described above for the 1-hot tag memory cell in  FIG. 6 , except that the data to be stored is a 2-hot vector. 
   In accordance with an embodiment of the present invention, in  FIG. 7 , a blind invalidate can be performed by asserting the bl 2  bit lines to cause each bit to look at the bit&#39;s right neighbor blind clear signal (bc) and the bit&#39;s left and right neighboring bits. Specifically, the bits to be invalidated can be indicated by the bl 2  lines in a 1-bit format. The aux bit can be cleared if and only if the aux bit&#39;s primary bl 2  bit line is asserted and its left neighbor bit is a “0”. The primary bit can be cleared if the primary bit&#39;s bl 2  bit line is asserted and the left neighbor of the primary bit is a “0”. In the blind invalidate the data can be a 1-hot vector and the aux and primary bits can be invalidated in the same cycle. 
   While the embodiments described above relate to the 1-hot plus valid bit and 2-hot vector embodiments, they are not intended to limit the scope or coverage of the present invention. In fact, for example, the 2-hot scheme described above can be extended to a 3-hot vector to protect errors in 2 consecutive bits or to a 4-hot or higher vector to protect errors in 3 and higher consecutive bits, respectively. Similarly, other bit patterns other than the 2-hot scheme may be used depending on the type of the errors, such as, for example, double bit errors, that a designer is trying to protect against. 
   In addition, the 1-hot plus valid bit scheme is, generally, good for microprocessor designs that are not wire congested in the physical layout and, thus, have available area for the additional read line. Likewise, the 2-hot scheme is good for microprocessor designs that are, generally, wire congested in the physical layout and, thus, do not have much available area for the additional hardware that is associated with the 1-hot plus valid bit scheme. 
   The 2-hot scheme described above minimizes global routing at the expense of local interconnect and transistors. Other 2-hot schemes can use a multiple clock blind invalidation scheme by using a different signal for invalidating the aux bit. 
   Both the 1-hot plus valid bit and 2-hot vector protection schemes can be implemented in high performance microprocessors and high performance multi-processors on a single chip. 
   It should, of course, be understood that while the present invention has been described mainly in terms of microprocessor- and multi-processor-based personal computer systems, those skilled in the art will recognize that the principles of the invention may be used advantageously with alternative embodiments involving other integrated processor chips and computer systems. Accordingly, all such implementations which fall within the spirit and the broad scope of the appended claims will be embraced by the principles of the present invention.