Software and Hardware Circular Buffer Operations

1. Software and Hardware Circular Buffer Operations M. R. Smith, ECEUniversity of CalgaryCanada

2. Tackled today • Have moved the DCremoval( ) over to the X Compute block • Circular Buffer Issues • DCRemoval( ) • FIR( ) • Coding a software circular buffer in C++ and TigerSHARC assembly code • Coding a hardware circular buffer • Where to next? Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

3. DCRemoval( ) MemoryintensiveAdditionintensive Loops formain code FIFO implementedas circularbuffer • Not as complex as FIR, but many of the same requirements • Easier to handle • You use same ideas in optimizing FIR over Labs 2 and 3 • Two issues – speed and accuracy. Develop suitable tests for CPP code and check that various assembly language versions satisfy the same tests Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

4. Set up pointers to buffers Insert values into buffers SUM LOOP SHIFT LOOP Update outgoing parameters Update FIFO Function return 2 4 4 + N * 5 1 Was 1 + 2 * log2N 6 3 + 6 * N 2 --------------------------- 23 + 11 N Was 22 + 11 N + 2 log2N N = 128 – instructions = 1430 1430 + 300 delay cycles = 1730 cycles Next stage in improving code speedSoftware circular buffers Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

5. DCRemoval( ) FIFO implementedas circularbuffer • If there are N points in the circular buffer, then this approach of moving the data from memory to memory location requires • N Memory read / N Memory write (possible data bus conflicts) • 2N memory address calculations Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

6. Alternative approach • Move pointers rather than memory values • In principle – 1 memory read, 1 memory write, pointer addition, conditional equate Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

7. Note: Software circular buffer is NOT necessarily more efficient than data moves • Watch out – my version of FIR uses a different sort of circular buffer • FIR FIFO – newest element earliest in array (matching FIR equation) • DCremoval FIFO – newest element latest in array – because that is the way I thought of it Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

8. Note: Software circular buffer is NOT necessarily more efficient than data moves • Now spending more time on moving / checking the software circular buffer pointers than moving the data? SLOWERFASTER Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

9. On TigerSHARC • Since we can have multiply instructions on one line, then “perhaps” if we can avoid pipeline delays then software circular buffer is faster than memory moves Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

10. Generate the tests for the software circular buffer routine Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

11. New static pointers needed in Software circular buffer code Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

12. New sets of register definesNow using many of TigerSHARC registers Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

13. Code for storing new value into FIFO requires knowledge of “next-empty” location • First you must get the address of where the static variable – saved_next_pointer • Second you must access that address to get the actual pointer • Third you must use the pointer value • Will be problem in labs and exams with static variables stored in memory Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

14. Adjustment of software circular buffer pointer must be done carefully Get and update pointer Check the pointer Save corrected pointer Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

15. Set up pointers to buffers Insert values into buffers SUM LOOP SHIFT LOOP Update outgoing parameters Update FIFO Function return 2 8 Was 4 4 + N * 5 1 Was 1 + 2 * log2N 6 14 Was 3 + 6 * N 2 --------------------------- 37 + 5 N Was 23 + 11 N N = 128 – instructions = 677 cycles 677 + 360 delay cycles = 1011 cycles Was 1430 + 300 delay cycles = 1730 cycles Next stage in improving code speedSoftware circular buffers Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

16. Next step – Hardware circular buffer • Do exactly the same pointer calculations as with software circular buffers, but now the calculations are done behind the scenes – high speed – using specialized pointer features • Only available with J0, J1, J2 and J3 registers (On older ADSP-21061 – all pointer registers) • Jx -- The pointer register • JBx – The BASE register – set to start of the FIFO array • JLx – The length register – set to length of the FIFO array • VERY BIG WARNING? – Reset to zero. On older ADSP-21061 it was very important that the length register be reset to zero, otherwise all the other functions using this register would suddenly start using circular buffer by mistake. • Still advisable – but need special syntax for causing circular buffer operations to occur Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

17. Setting up the circular buffer functionsRemember all the tests to start with Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

18. Store values into hardware FIFO • CB instruction ONLY works on POST-MODIFY operations Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

19. Now perform Math operation using circular buffer operation • MUST NOT DO XR2 = CB [J0 + i_J8]; • Save N cycles as no longer need to increment index Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

20. Update the static variablesFurther special CB instructions A few cycles saved here Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

21. Set up pointers to buffers Insert values into buffers SUM LOOP SHIFT LOOP Update outgoing parameters Update FIFO Function return 2 8 Was 4 3 + N * 4 Was 4 + N * 5 1 Was 1 + 2 * log2N 6 14 Was 3 + 6 * N 2 --------------------------- 37 + 4 N Was 23 + 5 N N = 128 – instructions = 549 cycles 549 + 300 delay cycle = 879 cyclesDelays are now >50% of useful time Was 677 + 360 delay cycles = 1011 cycle Next stage in improving code speedHardware circular buffers Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

22. Tackle the summation part of FIR Exercise in using CB (Assignment 2) Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

23. Place assembly code here Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

24. The code is too slow because we are not taking advantage of the available resources • Bring in up to 128 bits (4 instructions) per cycle • Ability to bring in 4 32-bit values along J data bus (data1) and 4 along K bus (data2) • Perform address calculations in J and K ALU – single cycle hardware circular buffers • Perform math operations on both X and Y compute blocks • Background DMA activity • Off-load some of the processing to the second processor Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

25. Tackled today • Have moved the DCremoval( ) over to the X Compute block • Circular Buffer Issues • DCRemoval( ) • FIR( ) • Coding a software circular buffer in C++ and TigerSHARC assembly code • Coding a hardware circular buffer • Where to next? Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada