1. Software and Hardware Circular Buffer Operations
M. R. Smith, ECE University of Calgary Canada

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?
11/14/2018
Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

3. DCRemoval( ) Not as complex as FIR, but many of the same requirements
Memory intensive Addition intensive
Loops for main code
FIFO implemented as circular buffer
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
11/14/2018
Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

4. Next stage in improving code speed Software circular buffers
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
Was * log2N 6
3 + 6 * N
N Was N + 2 log2N
N = 128 – instructions = 1430
delay cycles = 1730 cycles
11/14/2018
Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

5. DCRemoval( )
FIFO implemented as circular buffer
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
11/14/2018
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
11/14/2018
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
11/14/2018
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?
SLOWER FASTER
11/14/2018
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
Pipeline delay
XR4 = R4 + R5;;
XR4 = R4 + R6;;
Second instruction needs result of first
No Pipeline delay
XR3 = R4 + R6;;
Second instruction DOES NOT need result of first
11/14/2018
Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

10. Generate the tests for the software circular buffer routine
11/14/2018
Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

11. New static pointers needed in Software circular buffer code
11/14/2018
Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

12. New sets of register defines Now using many of TigerSHARC registers
11/14/2018
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
11/14/2018
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
11/14/2018
Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

15. Next stage in improving code speed Software circular buffers
Set up pointers to buffers
Insert values into buffers
SUM LOOP
SHIFT LOOP
Update outgoing parameters
Update FIFO
Function return 2
Was 4
4 + N * 5
Was * log2N 6
Was * N
N Was N
N = 128 – instructions = 677 cycles
delay cycles = 1011 cycles
Was
delay cycles = 1730 cycles
11/14/2018
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 – 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 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
11/14/2018
Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

17. Setting up the circular buffer functions Remember all the tests to start with
11/14/2018
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
11/14/2018
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
11/14/2018
Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

20. Update the static variables Further special CB instructions
A few cycles saved here
11/14/2018
Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

21. Next stage in improving code speed Hardware circular buffers2
Was 4
3 + N * Was 4 + N * 5
Was * log2N 6
Was * N
N Was N
N = 128 – instructions = 549 cycles
delay cycle = 879 cycles Delays are now >50% of useful time
Was
delay cycles = 1011 cycle
Set up pointers to buffers
Insert values into buffers
SUM LOOP
SHIFT LOOP
Update outgoing parameters
Update FIFO
Function return
11/14/2018
Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

22. Tackle the summation part of FIR Exercise in using CB (Assignment 2)
11/14/2018
Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada

23. Place assembly code here
11/14/2018
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
11/14/2018
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?
11/14/2018
Software Circular Buffer Issues, M. Smith, ECE, University of Calgary, Canada