Abstract:
The level two memory of this invention supports coherency data transfers with level one cache and DMA data transfers. The width of DMA transfers is 16 bytes. The width of level one instruction cache transfers is 32 bytes. The width of level one data transfers is 64 bytes. The width of level two allocates is 128 bytes. DMA transfers are interspersed with CPU traffic and have similar requirements of efficient throughput and reduced latency. An additional challenge is that these two data streams (CPU and DMA) require access to the level two memory at the same time. This invention is a banking technique for the level two memory to facilitate efficient data transfers.

Description:
CLAIM OF PRIORITY 
     This application claims priority under 35 U.S.C. 119(e)(1) to U.S. Provisional Application No. 61/387,283 filed Sep. 28, 2010. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The technical field of this invention is caches for digital data processors. 
     BACKGROUND OF THE INVENTION 
     This invention is applicable to data processing systems with second level (L2) memory used for both unified (data and instructions) level two cache and flat (L2 SRAM) memory used to hold critical data and instructions. The second level memory (L2) directly addressable SRAM memory is accessible by both external and internal direct memory access (DMA) units. 
     In the applicable digital data processor all CPU activity is in multiples of cache lines. The level one instruction cache line size is 32 bytes. The level one data cache line size is 64 bytes. The level two cache line size is 128 bytes. The L2 memory controller should be able to handle this traffic efficiently to ensure high throughput and reduced latency for CPU traffic. 
     SUMMARY OF THE INVENTION 
     The level two memory of this invention supports coherency data transfers with level one cache and DMA data transfers. The width of DMA transfers is 16 bytes. The width of level one instruction cache transfers is 32 bytes. The width of level one data transfers is 64 bytes. The width of level two allocates is 128 bytes. DMA transfers are interspersed with CPU traffic and have similar requirements of efficient throughput and reduced latency. An additional challenge is that these two data streams (CPU and DMA) require access to the level two memory at the same time. This invention is a banking technique for the level two memory to facilitate efficient data transfers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of this invention are illustrated in the drawings, in which: 
         FIG. 1  illustrates the organization of a typical digital signal processor to which this invention is applicable (prior art); 
         FIG. 2  illustrates details of a very long instruction word digital signal processor core suitable for use in  FIG. 1  (prior art); 
         FIG. 3  illustrates the pipeline stages of the very long instruction word digital signal processor core illustrated in  FIG. 2  (prior art); 
         FIG. 4  illustrates the instruction syntax of the very long instruction word digital signal processor core illustrated in  FIG. 2  (prior art); 
         FIG. 5  illustrates a computing system including a local memory arbiter according to an embodiment of the invention; 
         FIG. 6  is a further view of the digital signal processor system of this invention showing various cache controllers; and 
         FIG. 7  illustrates the preferred cache line sizes for level one instruction cache, level one data cache and level two cache; and 
         FIG. 8  illustrates the level two cache banking in accordance with this invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  illustrates the organization of a typical digital signal processor system  100  to which this invention is applicable (prior art). Digital signal processor system  100  includes central processing unit core  110 . Central processing unit core  110  includes the data processing portion of digital signal processor system  100 . Central processing unit core  110  could be constructed as known in the art and would typically includes a register file, an integer arithmetic logic unit, an integer multiplier and program flow control units. An example of an appropriate central processing unit core is described below in conjunction with  FIGS. 2 to 4 . 
     Digital signal processor system  100  includes a number of cache memories.  FIG. 1  illustrates a pair of first level caches. Level one instruction cache (L1I)  121  stores instructions used by central processing unit core  110 . Central processing unit core  110  first attempts to access any instruction from level one instruction cache  121 . Level one data cache (L1D)  123  stores data used by central processing unit core  110 . Central processing unit core  110  first attempts to access any required data from level one data cache  123 . The two level one caches are backed by a level two unified cache (L2)  130 . In the event of a cache miss to level one instruction cache  121  or to level one data cache  123 , the requested instruction or data is sought from level two unified cache  130 . If the requested instruction or data is stored in level two unified cache  130 , then it is supplied to the requesting level one cache for supply to central processing unit core  110 . As is known in the art, the requested instruction or data may be simultaneously supplied to both the requesting cache and central processing unit core  110  to speed use. 
     Level two unified cache  130  is further coupled to higher level memory systems. Digital signal processor system  100  may be a part of a multiprocessor system. The other processors of the multiprocessor system are coupled to level two unified cache  130  via a transfer request bus  141  and a data transfer bus  143 . A direct memory access unit  150  provides the connection of digital signal processor system  100  to external memory  161  and external peripherals  169 . 
       FIG. 1  illustrates several data/instruction movements within the digital signal processor system  100 . These include: (1) instructions move from L2 cache  130  to L1I cache  121  to fill in response to a L1I cache miss; (2) data moves from L2 cache  130  to L1D cache  123  to fill in response to a L1D cache miss; (3) data moves from L1D cache  123  to L2 cache  130  in response to a write miss in L1D cache  123 , in response to a L1D cache  123  victim eviction and in response to a snoop from L2 cache  130 ; (4) data moves from external memory  161  to L2 cache  130  to fill in response to L2 cache miss or a direct memory access (DMA) data transfer into L2 cache  130 ; (5) data moves from L2 cache  130  to external memory  161  in response to a L2 cache victim eviction or writeback and in response to a DMA transfer out of L2 cache  130 ; (6) data moves from peripherals  169  to L2 cache  130  in response to a DMA transfer into L2 cache  130 ; and (7) data moves from L2 cache  130  to peripherals  169  is response to a DMA transfer out of L2 cache  130 . 
       FIG. 2  is a block diagram illustrating details of a digital signal processor integrated circuit  200  suitable but not essential for use in this invention (prior art). The digital signal processor integrated circuit  200  includes central processing unit  1 , which is a 32-bit eight-way VLIW pipelined processor. Central processing unit  1  is coupled to level one instruction cache  121  included in digital signal processor integrated circuit  200 . Digital signal processor integrated circuit  200  also includes level one data cache  123 . Digital signal processor integrated circuit  200  also includes peripherals  4  to  9 . These peripherals preferably include an external memory interface (EMIF)  4  and a direct memory access (DMA) controller  5 . External memory interface (EMIF)  4  preferably supports access to supports synchronous and asynchronous SRAM and synchronous DRAM. Direct memory access (DMA) controller  5  preferably provides 2-channel auto-boot loading direct memory access. These peripherals include power-down logic  6 . Power-down logic  6  preferably can halt central processing unit activity, peripheral activity, and phase lock loop (PLL) clock synchronization activity to reduce power consumption. These peripherals also include host ports  7 , serial ports  8  and programmable timers  9 . 
     Central processing unit  1  has a 32-bit, byte addressable address space. Internal memory on the same integrated circuit is preferably organized in a data space including level one data cache  123  and a program space including level one instruction cache  121 . When off-chip memory is used, preferably these two spaces are unified into a single memory space via the external memory interface (EMIF)  4 . 
     Level one data cache  123  may be internally accessed by central processing unit  1  via two internal ports  3   a  and  3   b . Each internal port  3   a  and  3   b  preferably has 32 bits of data and a 32-bit byte address reach. Level one instruction cache  121  may be internally accessed by central processing unit  1  via a single port  2   a . Port  2   a  of level one instruction cache  121  preferably has an instruction-fetch width of 256 bits and a 30-bit word (four bytes) address, equivalent to a 32-bit byte address. 
     Central processing unit  1  includes program fetch unit  10 , instruction dispatch unit  11 , instruction decode unit  12  and two data paths  20  and  30 . First data path  20  includes four functional units designated L1 unit  22 , S1 unit  23 , M1 unit  24  and D1 unit  25  and 16 32-bit A registers forming register file  21 . Second data path  30  likewise includes four functional units designated L2 unit  32 , S2 unit  33 , M2 unit  34  and D2 unit  35  and 16 32-bit B registers forming register file  31 . The functional units of each data path access the corresponding register file for their operands. There are two cross paths  27  and  37  permitting access to one register in the opposite register file each pipeline stage. Central processing unit  1  includes control registers  13 , control logic  14 , and test logic  15 , emulation logic  16  and interrupt logic  17 . 
     Program fetch unit  10 , instruction dispatch unit  11  and instruction decode unit  12  recall instructions from level one instruction cache  121  and deliver up to eight 32-bit instructions to the functional units every instruction cycle. Processing occurs simultaneously in each of the two data paths  20  and  30 . As previously described each data path has four corresponding functional units (L, S, M and D) and a corresponding register file containing 16 32-bit registers. Each functional unit is controlled by a 32-bit instruction. The data paths are further described below. A control register file  13  provides the means to configure and control various processor operations. 
       FIG. 3  illustrates the pipeline stages  300  of digital signal processor core  110  (prior art). These pipeline stages are divided into three groups: fetch group  310 ; decode group  320 ; and execute group  330 . All instructions in the instruction set flow through the fetch, decode, and execute stages of the pipeline. Fetch group  310  has four phases for all instructions, and decode group  320  has two phases for all instructions. Execute group  330  requires a varying number of phases depending on the type of instruction. 
     The fetch phases of the fetch group  310  are: Program address generate phase  311  (PG); Program address send phase  312  (PS); Program access ready wait stage  313  (PW); and Program fetch packet receive stage  314  (PR). Digital signal processor core  110  uses a fetch packet (FP) of eight instructions. All eight of the instructions proceed through fetch group  310  together. During PG phase  311 , the program address is generated in program fetch unit  10 . During PS phase  312 , this program address is sent to memory. During PW phase  313 , the memory read occurs. Finally during PR phase  314 , the fetch packet is received at CPU  1 . 
     The decode phases of decode group  320  are: Instruction dispatch (DP)  321 ; and Instruction decode (DC)  322 . During the DP phase  321 , the fetch packets are split into execute packets. Execute packets consist of one or more instructions which are coded to execute in parallel. During DP phase  322 , the instructions in an execute packet are assigned to the appropriate functional units. Also during DC phase  322 , the source registers, destination registers and associated paths are decoded for the execution of the instructions in the respective functional units. 
     The execute phases of the execute group  330  are: Execute 1 (E1)  331 ; Execute 2 (E2)  332 ; Execute 3 (E3)  333 ; Execute 4 (E4)  334 ; and Execute 5 (E5)  335 . Different types of instructions require different numbers of these phases to complete. These phases of the pipeline play an important role in understanding the device state at CPU cycle boundaries. 
     During E1 phase  331 , the conditions for the instructions are evaluated and operands are read for all instruction types. For load and store instructions, address generation is performed and address modifications are written to a register file. For branch instructions, branch fetch packet in PG phase  311  is affected. For all single-cycle instructions, the results are written to a register file. All single-cycle instructions complete during the E1 phase  331 . 
     During the E2 phase  332 , for load instructions, the address is sent to memory. For store instructions, the address and data are sent to memory. Single-cycle instructions that saturate results set the SAT bit in the control status register (CSR) if saturation occurs. For single cycle 16 by 16 multiply instructions, the results are written to a register file. For M unit non-multiply instructions, the results are written to a register file. All ordinary multiply unit instructions complete during E2 phase  322 . 
     During E3 phase  333 , data memory accesses are performed. Any multiply instruction that saturates results sets the SAT bit in the control status register (CSR) if saturation occurs. Store instructions complete during the E3 phase  333 . 
     During E4 phase  334 , for load instructions, data is brought to the CPU boundary. For multiply extension instructions, the results are written to a register file. Multiply extension instructions complete during the E4 phase  334 . 
     During E5 phase  335 , load instructions write data into a register. Load instructions complete during the E5 phase  335 . 
       FIG. 4  illustrates an example of the instruction coding of instructions used by digital signal processor core  110  (prior art). Each instruction consists of 32 bits and controls the operation of one of the eight functional units. The bit fields are defined as follows. The creg field (bits  29  to  31 ) is the conditional register field. These bits identify whether the instruction is conditional and identify the predicate register. The z bit (bit  28 ) indicates whether the predication is based upon zero or not zero in the predicate register. If z=1, the test is for equality with zero. If z=0, the test is for nonzero. The case of creg=0 and z=0 is treated as always true to allow unconditional instruction execution. The creg field is encoded in the instruction opcode as shown in Table 1. 
                                                           TABLE 1                       Conditional   creg   z                Register   31   30   29   28                   Unconditional   0   0   0   0           Reserved   0   0   0   1           B0   0   0   1   z           B1   0   1   0   z           B2   0   1   1   z           A1   1   0   0   z           A2   1   0   1   z           A0   1   1   0   z           Reserved   1   1   1   x                    
Note that “z” in the z bit column refers to the zero/not zero comparison selection noted above and “x” is a don&#39;t care state. This coding can only specify a subset of the 32 registers in each register file as predicate registers. This selection was made to preserve bits in the instruction coding.
 
     The dst field (bits  23  to  27 ) specifies one of the 32 registers in the corresponding register file as the destination of the instruction results. 
     The scr2 field (bits  18  to  22 ) specifies one of the 32 registers in the corresponding register file as the second source operand. 
     The scr1/cst field (bits  13  to  17 ) has several meanings depending on the instruction opcode field (bits  3  to  12 ). The first meaning specifies one of the 32 registers of the corresponding register file as the first operand. The second meaning is a 5-bit immediate constant. Depending on the instruction type, this is treated as an unsigned integer and zero extended to 32 bits or is treated as a signed integer and sign extended to 32 bits. Lastly, this field can specify one of the 32 registers in the opposite register file if the instruction invokes one of the register file cross paths  27  or  37 . 
     The opcode field (bits  3  to  12 ) specifies the type of instruction and designates appropriate instruction options. A detailed explanation of this field is beyond the scope of this invention except for the instruction options detailed below. 
     The s bit (bit  1 ) designates the data path  20  or  30 . If s=0, then data path  20  is selected. This limits the functional unit to L1 unit  22 , S1 unit  23 , M1 unit  24  and D1 unit  25  and the corresponding register file A  21 . Similarly, s=1 selects data path  20  limiting the functional unit to L2 unit  32 , S2 unit  33 , M2 unit  34  and D2 unit  35  and the corresponding register file B  31 . 
     The p bit (bit  0 ) marks the execute packets. The p-bit determines whether the instruction executes in parallel with the following instruction. The p-bits are scanned from lower to higher address. If p=1 for the current instruction, then the next instruction executes in parallel with the current instruction. If p=0 for the current instruction, then the next instruction executes in the cycle after the current instruction. All instructions executing in parallel constitute an execute packet. An execute packet can contain up to eight instructions. Each instruction in an execute packet must use a different functional unit. 
       FIG. 5  is a block diagram illustrating a computing system including a local memory arbiter according to an embodiment of the invention.  FIG. 5  illustrates system on a chip (SoC)  500 . SoC  500  includes one or more DSP cores  510 , SRAM/Caches  520  and shared memory  530 . SoC  500  is preferably formed on a common semiconductor substrate. These elements can also be implemented in separate substrates, circuit boards and packages. For example shared memory  530  could be implemented in a separate semiconductor substrate.  FIG. 5  illustrates four DSP cores  510 , but SoC  500  may include fewer or more DSP cores  510 . 
     Each DSP core  510  preferably includes a level one data cache such as L1 SRAM/cache  512 . In the preferred embodiment each L1 SRAM/cache  512  may be configured with selected amounts of memory directly accessible by the corresponding DSP core  510  (SRAM) and data cache. Each DSP core  510  has a corresponding level two combined cache L2 SRAM/cache  520 . As with L1 SRAM/cache  512 , each L2 SRAM/cache  520  is preferably configurable with selected amounts of directly accessible memory (SRAM) and data cache. Each L2 SRAM/cache  520  includes a prefetch unit  522 . Each prefetch unit  522  prefetchs data for the corresponding L2 SRAM/cache  520  based upon anticipating the needs of the corresponding DSP core  510 . Each DSP core  510  is further coupled to shared memory  530 . Shared memory  530  is usually slower and typically less expensive memory than L2 SRAM/cache  520  or L1 SRAM/cache  510 . Shared memory  530  typically stores program and data information shared between the DSP cores  510 . 
     In various embodiments, each DSP core  510  includes a corresponding local memory arbiter  524  for reordering memory commands in accordance with a set of reordering rules. Each local memory arbiter  524  arbitrates and schedules memory requests from differing streams at a local level before sending the memory requests to central memory arbiter  534 . A local memory arbiter  524  may arbitrate between more than one DSP core  510 . Central memory arbiter  534  controls memory accesses for shared memory  530  that are generated by differing DSP cores  510  that do not share a common local memory arbiter  524 . 
       FIG. 6  is a further view of the digital signal processor system  100  of this invention. CPU  110  is bidirectionally connected to L1I cache  121  and L1D cache  123 . L1I cache  121  and L1D cache  123  are shown together because they are at the same level in the memory hierarchy. These level one caches are bidirectionally connected to L2  130 . L2 cache  130  is in turn bidirectionally connected to external memory  161  and peripherals  169 . External memory  161  and peripherals  169  are shown together because they are at the same level in the memory hierarchy. Data transfers into and out of L1D cache  123  are controlled by data memory controller (DMC)  610 . Data transfers into and out of L1I cache  121  are controlled by program memory controller (PMC)  620 . Data transfers into and out of L2  130  including both cache and directly addressable memory (SRAM) are controlled by unified memory controller (UMC)  630 . This application is primarily concerned with level 2 cache and UMC  630 . 
       FIG. 7  illustrates the preferred cache line sizes for L1I cache  121 , L1D cache  123  and L2 cache  130 . In the preferred embodiment of this invention L2 cache line size is 128 bytes, the L1D cache line size is 64 bytes and L1I cache line size is 32 bytes.  FIG. 7  illustrates cache lines line 0  711 , line 1  712 , line 2  713  and line 3  714  of L1I cache  121 . Each cache line  711 ,  712 ,  712  and  714  includes 32 bytes. These four cache lines total 128 bytes.  FIG. 7  illustrates cache lines line 0  721  and line 1  722  of L1D cache  123 . Each cache line  721  and  722  includes 64 bytes. These two cache lines total 128 bytes.  FIG. 7  illustrates cache line  731  of L2 cache  130 . This cache line is 128 bytes. 
     This invention departs from the banking scheme of previous digital signal processors of the TMS320C6000 family. This invention employs a smaller number of wider banks.  FIG. 8  illustrates the banking technique of this invention. 
     UMC  630  employs two physical memory banks  820  and  830 . Each physical bank  820  and  830  includes four virtual banks. Physical bank 0  820  includes virtual banks  821 ,  822 ,  823  and  824 . Physical bank 1  830  includes virtual banks  831 ,  832 ,  833  and  834 . 
       FIG. 8  illustrates the physical storage locations of consecutive data sequences yy00, yy01 . . . to yy17. The first data yy00 is stored in bank 0  820 , virtual bank  821 ; yy01 is stored in bank 0  820 , virtual bank  822 ; yy02 is stored in bank 0  820 , virtual bank  823 ; yy03 is stored in bank 0  820 , virtual bank  824 ; yy04 is stored in bank 1  830 , virtual bank  831 ; yy05 is stored in bank 1  830 , virtual bank  832 ; yy06 is stored in bank 1  830 , virtual bank  833 ; yy07 is stored in bank 1  830 , virtual bank  834 ; yy08 is stored in bank 0  820 , virtual bank  821 ; yy09 is stored in bank 0  820 , virtual bank  822 ; yy0A is stored in bank 0  820 , virtual bank  823 ; yy0B is stored in bank 0  820 , virtual bank  824 ; yy0C is stored in bank 1  830 , virtual bank  831 ; yy0D is stored in bank 1  830 , virtual bank  832 ; yy0E is stored in bank 1  830 , virtual bank  833 ; yy0F is stored in bank 1  830 , virtual bank  834 ; yy10 is stored in bank 0  820 , virtual bank  821 ; yy11 is stored in bank 0  820 , virtual bank  822 ; yy12 is stored in bank 0  820 , virtual bank  823 ; yy13 is stored in bank 0  820 , virtual bank  824 ; yy14 is stored in bank 1  830 , virtual bank  831 ; yy15 is stored in bank 1  830 , virtual bank  832 ; yy16 is stored in bank 1  830 , virtual bank  833 ; and yy17 is stored in bank 1  830 , virtual bank  834 . 
     Many of the variables that constrain system performance are specific to the memory. These variables include speed and latency cycles. These variables cannot be changed by controller hardware. This invention banks the memories so that the memory controller enables optimal performance in latency and throughput for all applications. 
     There are three types of use cases. These are: L1I cache  121  misses; L1D cache  123  misses; and L2 cache  130  allocates and victims. This invention enables pipelined accesses for all these cases without the need to introduce a large number of stalls. 
     This invention employs multi-level banking. Banking is the relationship between the address and the physical location where the corresponding data is stored. Physical banking separates the memory into sets which are least significant (LS) word banked. Each bank is 16 bytes wide. That enables direct memory access (DMA) transfers which are 16-byte accesses to efficiently access the memory. Each physical bank is then divided into four LS-banked virtual banks which are also 16-bytes wide. This permits CPU  110  access which are typically either 32, 64 or 128 bytes to be reads out be pipelined byte accesses. 
     CPU traffic is of the three types noted above. A level one instruction miss has a size of 32 bytes. This requires both physical banks  820  and  830 . A level one data miss has a size of 64 bytes. This requires both physical banks twice. That is enabled by keeping two halves of the level one data cache line in separate virtual banks. Thus two accesses to the two physical banks can be pipelined because these two accesses are to different virtual banks. Level two allocates and victims have a size of 128 bytes. This requires both physical banks be accessed four times. To achieve maximum pipelining, the level two cache line is spread across both physical banks but kept in eight virtual banks using four virtual banks in each of the two physical banks. 
     DMA data transfers are 16 bytes wide. These use just one physical bank in one virtual bank. Since the virtual banks are LS-banked with a distance of 16 bytes and typical DMA data transfers are longer bursts, these will trip across the 8 virtual banks. Only one physical bank and one virtual bank is accessed in each cycle enabling 100% pipelining on bursts of DMA data transfers. Table 2 shows the data widths and L2 cache  130  banking for these access types. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Access 
                 Data Width 
                 Banks 
               
               
                   
               
             
             
               
                   
                 DMA 
                 16 bytes 
                 1 physical bank, 1 virtual bank 
               
               
                   
                 L1I miss 
                 32 bytes 
                 2 physical banks 
               
               
                   
                 L1D miss 
                 64 bytes 
                 2 physical banks, twice 
               
               
                   
                 L2 allocate 
                 128 bytes  
                 2 physical banks, four times 
               
               
                   
                 L2 victim 
                 128 bytes  
                 2 physical banks, four times 
               
               
                   
               
             
          
         
       
     
     Prior art solutions have typically focused on getting specially optimized memories with smaller latencies and faster speeds. This increases the complexity in memory design, becomes application dependent, reduces the configurability of the controller and increases power consumption. Some prior art solutions force the controller to support multiple modes depending on the memory being used. Other prior art solutions also require the application to constrain data/instruction storage locations and DMA traffic to get around the memory limitations. 
     This invention works with any memory as long at it is banked in the way described in the invention. Reduced speed or increased latencies will not have a large effect on the performance of the application. This invention does not require existing applications to be reworked or re-adapted for a change in memory variables.