Abstract:
A multi-phase, multi-access pipeline memory system includes a number, n, of processors; a pipeline memory including a latch; and a bus for interconnecting the processors and pipeline memory; a clock circuit responsive to a system clock signal divides the system clock signal into n phases for providing multiple clock signals corresponding to the n phases of the system clock signal for application to each processor to allow data and address to be transferred only during its assigned phase thereby enabling the memory and each processor to operate at the system clock rate while allowing n accesses to the memory during each system clock signal period, one access for each processor.

Description:
RELATED INVENTIONS 
     This application is a continuation-in-part application of U.S. Ser. No. 08/052,073, now U.S. Pat. No. 5,471,607 entitled “Multi-phase Multi-Access Pipeline Memory System” by the same inventor, filed Apr. 23, 1993. This is a continuation of Ser. No. 08/417,660, filed Apr. 5, 1995, now abandoned. Which is a division, of application Ser. No. 08/215,508, now abandoned, filed March 22, 1994, which is a Continuation-in-Part of application Ser. No. 08/052,073, now U.S. Pat. No. 5,471,607, filed Apr. 23, 1993. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of the Invention 
     This invention relates to a multi-phase multi-access pipeline memory system. 
     2. Description of the Prior Art 
     In conventional computers and microcomputers there is a constant contention between the microprocessor and the I/O processor for memory access. This is a particularly serious problem in signal processing applications requiring high speed processing of massive amounts of data and high I/O rates. There are four approaches commonly used to enable fair sharing of the memory. One is to use dual port memories: these are generally complex and expensive and have small capacity. A second approach uses “cycle stealing” wherein one of the microprocessor and I/O processor has priority over the other and “steals” memory access when it needs it, thereby interrupting the operation of the other. This slows down the response of the interrupted processor and of the whole system. The third approach uses a number of separate memory banks so that most of the time each of the microprocessor and I/O processor can be accessing a different bank. In order to effect his, however, the user/programmer must superimpose on the programming the goal of minimizing overlap in access demand for the memory banks by the microprocessor and I/O processor. This requires careful scheduling of the I/O and computing tasks so that simultaneous demand by both processors for the same memory bank is avoided or at least reduced. That imposes a burdensome ancillary constraint on the user. The fourth approach is to simply run the memory at twice the normal cycle speed. But this is difficult, especially in signal processing systems where memories are already operating at near capacity as a rule. 
     Also, even in a single processor design, there may be the need to issue two instructions and receive two data valves per system clock cycle or, one or more processors could be operating in this way while other processors in the system may issue only one instruction and receive one data value per clock cycle. In any case, the same shortcomings in the prior art are present with these designs. 
     SUMMARY OF INVENTION 
     It is therefore an object of this invention to provide an improved, multi-phase multi-access pipeline memory system. 
     It is a further object of this invention to provide such a multi-phase multi-access pipeline memory system which does not increase, expand or limit memory capacity. 
     It is a further object of this invention to provide such a multi-phase, multi-access pipeline memory system which does not interrupt other processors&#39; access to the memory and does not slow down the system operation as a whole. 
     It is a further object of this invention to provide such a multi-phase, multi-access pipeline memory system which requires no special programming or scheduling of processor-memory interaction. 
     It is a further object of this invention to provide such a multi-phase, multi-access pipeline memory system which does not require operating the memory above normal speeds. 
     It is a further object of this invention to provide such a multi-phase, multi-access pipeline memory system for implementing instructions or data caches. 
     The invention results from the realization that a truly effective multi-access memory system can be achieved in which each processor has access to the memory once in each system clock period without interfering with the access of the other processors by phase shifting the memory access of the processors and pipelining the memory so that each processor accesses memory during a different phase of the system clock period while maintaining the memory operation at its normal speed. Underlying this approach is the fundamental realization that in most cases there is no reason why all the processors, be they microprocessors or I/O processors, must access memory in phase so that their accesses start and end simultaneously: an overlapped sequence of access is acceptable and therefore pipelined memory in combination with phase overlapped sequencing can be used to full advantage. Also, it was realized that such a dual access, dual phase memory system may be used to implement instructions or data caches. 
     This invention features a dual phase, dual access pipeline memory system having first and second processors, a pipeline memory including latch means, and a bus means for interconnecting the processor to the pipeline memory. There is a clock circuit responsive to a system clock signal for providing a first clock signal in phase with the system clock signal for operating the access of the first processor, a second clock signal out of phase with the system clock signal for operating the access of the second processor out of phase with the first processor, and a third clock signal at twice the system clock signal rate for clocking the pipeline memory through the latch means to allow an address to be supplied to the pipeline memory by the first processor while accessing data from the address supplied in the previous cycle during one phase. Conversely, it allows an address to be supplied to the pipeline memory by the second processor while accessing data from the address supplied in the previous cycle during the other phase. 
     In the preferred embodiment the processors may include an I/O processor and a microprocessor, or the processors may be both microprocessors. The processors may also include subprocessors in the same microprocessor. The subprocessors may include an instruction fetch unit and a data fetch unit. The pipeline memory may include a plurality of memory banks and the bus means may include a plurality of data address bus pairs and there may be third and fourth processors. The first and third processors may include data fetch units which access different memory banks from each other in the same phase with each other and the second and fourth processors may include an instruction fetch unit and an I/O processor which access different memory banks from each other in the same phase with each other but out of phase with the first and third processors. 
     In a more comprehensive sense, the invention features a multi-phase, multi-access pipeline memory system which includes a number, n, of processors, a pipeline memory including latch means, and bus means interconnecting the processors and pipeline memory. There is a clock circuit responsive to a system clock signal for dividing the system clock signal into n phases for providing multiple clock signals corresponding to the n phases of the system clock signal for operating the access of each processor to allow data and addresses to be transferred only during the assigned phase, thereby enabling the memory and each processor to operate at the system clock rate while allowing n accesses to the memory during each system clock signal period, one access for each processor. 
     In a preferred embodiment at least one of the processors may be an I/O processor and one may be a microprocessor, or the processors may both be microprocessors. The processors may include subprocessors in the same microprocessor. The subprocessors may include a data fetch unit and an instruction fetch unit. The latch means may include a plurality of latches to enable pipelining of n accesses of the pipeline memory during each system clock signal period. 
     SUMMARY OF INVENTION 
     This invention also features a dual-phase, dual-access pipeline memory system comprising processor means, a pipeline memory including latch means bus means for interconnecting the processor means and the pipeline memory. A clock circuit, responsive to a system clock cycle, provides the first clock signal in phase with the system clock signal, and a second clock signal out of phase with the system clock signal, and the third clock signal with twice the system clock signal rate for clocking the pipeline memory through the latch means to allow an address to be supplied to the pipeline memory by the processor means during one phase while the processor means is accessing data from the previous cycle in the same phase, and conversely to allow another address to be supplied to the pipeline memory by the processor means during the other phase while the processor means is accessing data from the previous cycle in the same phase. The processor means may include a single processor issuing first and second instructions to the memory and receiving first and second data values from the memory for a system clock cycle. Alternatively, the processor means may include first and second processors and the first clock signal accesses the first processor and the second clock signal accesses the second processor out of phase with the first processor. 
     In a more comprehensive sense, the invention features a multi-phase, multi-access pipeline memory system comprising a number of processors which issue two instructions and receive two data values per system clock cycle and a number of different types of processors which only issue one instruction and receive one data value per system clock cycle. This divides the system clock signal into two phases per each first type of processor, and one phase for each second type of processor and one phase for each second type of processor. Instead of dividing the clock cycle into more phases, other processor can share the memory by cycle stealing. 
     This invention also features a method of operating two processors which access the pipeline memory by dividing the clock cycle into two phases, operating the first processor to issue instructions to the memory during one phase and to receive data in the same phase, and operating the second processor to issue an instruction to the memory during the second phase. For the single processor embodiment, the method includes dividing the clock cycle into two phases and operating the processor to issue an instruction during the first phase and to issue an instruction during the second phase, and operating the processor to receive data from the memory during the first phase and to receive data from the memory in the second phase. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 simplified block diagram of a multi-phase, multi-access pipeline memory system according to this invention; 
     FIG. 2 is an illustration of the clock signals that occur at various points in FIG. 1; 
     FIG. 3A is a more detailed block diagram of the system of FIG. 1 showing an additional holding latch which expedites the operation of the pipeline memory; 
     FIG. 3B is a more detailed block diagram of the system of FIG. 1 but only having one processor according to another embodiment of the subject invention in which the single processor uses two phases of the clock cycle; 
     FIG. 3C is a block diagram of another embodiment of the subject invention including one processor using two phases of a clock cycle and another processor using a third phase of the clock cycle. 
     FIG. 3D is a block diagram of another embodiment of the subject invention in which multiple processors access memory by cycle stealing; 
     FIG. 4 is a timing diagram showing the sequence of occurrences with respect to the operation of the pipeline memory in FIGS. 1 and 3; 
     FIG. 5 is a more detailed block diagram of the pipeline memory of FIGS. 1 and 3A; 
     FIGS. 6A and B are a detailed block diagram of a specific implementation of the invention using a four-bank pipeline memory and a number of subprocessors in a microprocessor sharing dual data and program buses; and 
     FIG. 7 is a illustration of yet a further improvement according to this invention in the operation of the system of FIGS.  6 A and  6 B. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings. 
     There is shown in FIG. 1 a multi-phase, multi-access pipeline memory system  10  according to this invention including a pipeline memory  12  and two processors  14  and  16 , along with clock circuit  18  which is driven by a system clock signal that it receives on line  20 . Clock circuit  18  divides the system clock signal into two phases: φ 1  delivered on line  22  to processor  14 , and φ 2  delivered on line  24  to processor  16 . Clock circuit  18  also creates a signal at twice the frequency of the incoming system clock signal on line  20  and delivers it on line  26  to latch  28  in pipeline memory  12 . During phase one, processor  14  presents an address and provides control signals over line  30  to pipeline memory  12 . During phase two, memory  12  provides the data on line  32  back to processor  14  (read), or the processor  14  supplies data (write). Also during phase two, processor  16  on line  34  requests an address and provides control signals and the data is returned (read) or supplied (write) to it in response thereto over line  36  during phase one of the next period of the system clock signal. Operating clocks  50  and  52  which may be independent of the access are provided to each processor  14  and  16  respectively. 
     This can be seen more readily with respect to the wave form shown in FIG.  2 . There the system clock  40  which appears on line  20  is shown as having two phases: φ 1  and φ 2 . In clock circuit  18  there are two clock signals developed from that: phase one signal  42  and the phase two signal  44 , which is the inverse of the phase one signal  42 . Signal  42  is actually a duplication of the system clock signal, in this case where the pipeline memory system is operating as a dual access system and there are but two processors  14  and  16  to deal with. The third clock signal delivered to pipeline memory  12  on line  26  appears as shown at  46  as occurring at twice the rate of the system clock signal  40 . However, it should be understood that this is not operating the memory at twice the normal memory speed; rather, it is simply operating latch  28  to perform the pipelining function of pipeline memory  12 . As the number of processors increases, the number of latches must also increase in order to adequately pipeline the requests and data inputs and outputs. The third clock signal need not be a separately created signal; it can be derived from the rising and falling edges of the original clock. 
     In order to expedite processing of the request by pipeline memory  12 , the addresses from processors  1  and  2  are driven onto a common address bus. The address from processor  1  is driven in φ 2  and is latched in  50 , FIG. 3A, at the start of φ 1 . The address from processor  2  is driven in and is latched in  50  at the start of φ 2 . Thus the address decoder can immediately begin decoding the addresses at the start of φ 1 , and φ 1 . Latch  50  is clocked by the same 2× clock that drives the pipeline latch  28 . 
     FIG. 3B shows a single processor  15  to replace the two processors  14  and  10  of FIG.  3 A. In this application single processor  15  issues two instructions and receives two data values per systemdock cycle. The processor is pipelined so that it is issuing a new address φ 1 , while it is receiving data from the address previously supplied in φ 1 . Similarly it issues an address in φ 2 while it is receiving data from the address previously supplied in φ 2 . 
     This allows a single processor, with a pipelined access to memory  12  to access the pipelined memory  12  twice per cycle, thus achieving an overall performance doubling above a normal processor and memory without pipelines. 
     FIG. 3C shows an extensions of the idea of FIG. 3A wherein the system clock cycle is divided into three phases φ, φ 2 , and φ 3 , to allow three accesses to the memory on each system clock cycle. 
     Processor means  15  including processor  17  and processor  21  operating such that processor  19  may use φ 1  and φ 2  in the manner described in FIG. 3A to access memory twice per cycle while processor  21  accesses the memory in the remaining phase, φ 3 . Note that processor  1  will probably, but not necessarily operate from a ×2 clock to allow it to process two memory accesses per cycle while processor  2  will probably but not necessarily operate from a ×1 clock to allow it to process one memory access per cycle. 
     It is not always necessary to divide the memory period into more than two phases if another processor needs to access memory. FIG. 3D shows processor  23  that accesses the pipeline memory by pre-empting processor  25 . This method is also known as “cycle stealing”. Which processor may access the memory is determined by a priority selector  27 . 
     Priority selector  27  determines which processor can have access to memory by asserting a grant signal. The access determination may be either “fairness” or strict priority. 
     FIG. 4 is a detailed timing diagram showing the signals that occur at various points in FIG.  3 A. As can be seen, system clock signal  40  has two phases, φ 1  and φ 2 , which makeup the total clock period  41 . The next clock period  41 ′ includes phase φ 11  and φ 12 , and so on. The double rate clock signal  46  driving latch  28  triggers latch  50  to store the incoming address request from processor  14  at time  60 . During the first phase, φ 1 , ending at time  62 , this address is decoded. In the second phase, ending at time  64 , the decoded address is used to retrieve the data and return it on line  32  to processor  14 . Simultaneously during phase φ 2 , beginning at time  62 , latch  50  secures the address for processor  16  during the rest of phase φ 2 : The use of latch  50  forces the processors to have the address valid at the beginning of each phase, but this address can be removed (by the processors) immediately after the phase begins. Latch  50  in essence shifts the timing of address and data such that each processor supplies its address and gets its previous data in the same phase. (See FIG. 4.) This is completed at time  64  at the end of phase φ 2  and the close of clock period  41 . During phase φ 11 , of the next clock period  41 ′ ending at time  70 , the data is retrieved in accordance with the decoded address and returned to processor  16 . Although in this particular explanation latch  28  is right in the middle of pipeline memory  12 , dividing the address decode and the data retrieval functions, this is not a necessary limitation of the invention as one or more latches may be used to implement latch  28  and these may be used at various locations in the pipeline memory  12 . Note that at time  62  latch  28  latches the decoded address of processor  14  in response to the address in latch  50 . Thus latch  28  affects the pipelining function of memory  12  by permitting the memory to decode the address for one processor request held in latch  50  while simultaneously accessing the memory array with the decoded address held in latch  28 . 
     Included in the address request on lines  30  and  34  from processors  14  and  16  are control signals which indicate whether there is information that has to be written into or read out of, or neither, of the decoded address. The read data waveform  80 , FIG. 4, indicates that the data to be read or written is valid toward the end of φ 2  for processor  14 . If the data is to be read the waveform  80  indicates that the data to be read will be valid when time  64  is reached as indicated at read data  82 . If the data is to be written into memory then waveform  84  indicates that the data is valid to write at  86 . Similarly, the data valid indications  88  and  90  for microprocessor  16  indicate that the data is valid at time  70 . 
     A more detailed implementation of pipeline memory  12 , FIG. 5, shows that it includes an address decoder  100 , latch  28 , memory array  102 , column decoder  104 , and latches  106 . There is a MUX  108  at the output. Assuming the two processors in this dual-access pipeline memory system include one microprocessor and one I/O processor, the address of each will be fed in on line  114  during each phase of the clock: drive processor  1  during φ 2  and drive processor  2  during φ 1 . Latch  112  is a holding latch similar to latch  50  which holds the previous processor address request so that the new one is available for address decoder  100  immediately upon the beginning of the period. Latch  28  operates in conjunction with address decoder  100  and memory array  102  in the normal manner of pipeline memories. Column decode  104  receives the output data from memory array  102  and delivers it to latches  106  which are also anticipatory latches that maintain the output data at the ready condition for delivery to the requesting processor and the data to be written for delivery to the memory array. The data lines  118  and  120  return to the microprocessor and I/O processor, respectively, the data which they have requested. Data lines  118  and  120  may be time-shared in the same way as are the address lines. Latch  106  functions to maintain the data at the ready for delivery to the processor. It is this function that produces the overlapping sections  120 ,  122 ,  124  and  126  of waveform sections  82 ,  86 ,  88  and  90 , respectively, indicating that the data is there and ready at the moment the processor is ready to receive or write the data. The control signals previously referred to which are delivered by the processor to the pipeline memory and part of the address are delivered over another latch  122  to column decode  104 . Latch  122  operates to synchronize the operation of column decode  104  with the pipeline latch  28 . 
     In one application, memory  12 , FIGS. 6A and 6B, includes four memory banks  150 ,  152 ,  154  and  156 , and processor  14  is a microprocessor which includes a data fetch unit data address generators  158  and  160 , cache memory  162 , and an instruction fetch unit, program sequencer  164 . Microprocessor  14  also includes conventional flags  166 , timers  168  and interrupts  170 . There is also a register file  172 , floating and fixed point multiplier and a fixed point accumulator  174 , barrel shifter  176 , and a floating point and fixed point arithmetic logic unit  178 . The other processor, processor  16 , is actually an I/O processor which includes I/O processor register  180 , serial port  182 , nibble port  184 , and data memory address control  186 . The external port  190  includes a pair of MUXes  192 ,  194  and an external port control  196 . In this application data address generator  158  and  160  and program sequencer  164  each are considered a processor, although in this case they are actually subprocessors of microprocessor  14 , so that there are four processors in the eyes of memory  12 : data address generators  158  and  160 , program sequencer  164 , and I/O processor  16 . In this application there are two bus pairs: the program address and the program memory data buses  200  and  202  are one pair, and the data memory address and data memory data buses  206  and  208  are the other pair. 
     In accordance with this invention, during phase one, data address generator  160  or program sequencer  164  can access the program memory bus pair  200  and  202 . Also during phase one the data address generator  158  can access the data memory address pair  206 ,  208 . During phase two, only I/O processor  16  accesses the memory banks of memory  12  via program memory address pair  200  and  202 . If both the program sequencer  164  and data address generator  160  need to address the program memory address bus  200 ,  202 , then one must access it in phase one of a first clock period and the other obtain its access during phase one of the next clock period. 
     An even more advantageous application of the invention to the system of FIGS. 6A and 6B can be seen with respect to FIG. 7, where the competition between program sequencer  164  and data address generator  160  for the program memory address pair  200  and  202  can be avoided by having data address generator  158  and  160  deliver their addresses to the two different buses  202  and  206  during the first phase, and then have the program sequencer  164  and I/O processor  16  deliver their address requests during the second phase to buses  200  and  206 . This is achievable because program sequencer  164  is only loosely coupled to the data address generators  158  and  160 . This underlies all applications of this invention as the phase distribution of the requests depends upon the ability of the various processors to work synchronously but out of phase with one another in order to obtain the multiple access to the pipeline memory without interrupting one another or requiring the memory to run at double or multiple speeds. With this implementation, as shown in FIG. 7, the bus pairs  200 ,  202  and  206 ,  208  are more efficiently used, data address generator  160  and program sequencer  164  are no longer in competition, and cache memory  162  can be eliminated. While thus far the illustrative examples have all been with respect to a dual access pipeline memory system, this is not a necessary limitation of the invention as the invention applies to any multi-access pipeline memory system as shown in phantom in FIG. 7 where an additional pair of processors such as I/O processor  220  and microprocessor  224  are shown utilizing program memory address bus  200  and data memory address bus  206  during a third phase, φ 3 , where of course the system clock signal has been divided into three phases or more, depending upon the number of processors to be accommodated and the number of buses available in conjunction with the pipeline memory. 
     Another application of this invention is for implementing instructions or data caches. In a cache memory system, when a cache miss occurs, a new line (8 or 16 words) must be loaded into the cache. Typically, the processor stops while this cache line is loaded. A dual phase memory may remove this dead time for sequential code as described below. 
     In a dual phase cache memory, one phase would be used for an access in the current cache line while the other phase would be used to load the next sequential cache line in anticipation of bringing in this next line . This is likely to happen since most code is sequential. Fetching the next line early has no impact on the performance at the current line. 
     Without the dual phase memory, the next line fill would have no benefit because the processor would be shut out while the line fill is occurring. 
     In FIG. 7, the I/O device  16 , may in fact be doing the anticipatory line fill for the cache of either memory  1  or  2 . 
     In the case of a non-sequential access (branches), the anticipated line fill should be aborted and the line fill should start again at the branch location and continue, model fashion for that line. 
     Therefore, although specific features of this invention are shown in some drawings and not others, this is for convenience only as some feature may be combined with any or all of the other features in accordance with the invention.