1. DSPs Vs General Purpose Microprocessors
AOE 5984 – Real Time Systems

2. Common DSP Applications…
Communications
Audio, Video processing
Graphics, 3-D rendering
Navigation, radars, GPS
Controls – Robotics, guidance, Machine Vision
Filtering
Frequency-Time transformations (FFT-IFFT)

3. Common DSP Tasks… Modulation-Demodulation, Error correction
Noise reduction, equalization, echo
cancellation
Audio compression
Vector and Matrix calculations
Control algorithms

4. DSPs Need to Do… Efficient repetitive numerical calculations
Maintain numeric fidelity
Provide high memory bandwidth
Streaming data
Real Time processing

5. DSPs Need to Minimize… Real Time execution unpredictability Memory use
Power consumption
Cost
Development time

6. What Do DSPs Have? Specialized memory architecture (Harvard)
Specialized parallel execution units
Specialized addressing modes
Specialized instruction sets for parallel
execution
Specialized peripherals

7. FIR Filtering… x D y h0 h1 hn Two data fetches, Multiply operation,
DSP algorithms are the basis for most of the DSP special hardware units.
Two data fetches,
Multiply operation,
Accumulate Operation,
Input vector shifting

8. Multiply-Accumulate (MAC)
Multiplication in single cycle
Execution time ~ 200 ns
Register
Multiplier
ALU
Accumulator

9. Special Hardware Units…
Hardware shifter.
Hardware circular buffers.
Special h/w for zero overhead looping.
Special address generation units.

10. Address Generation Units…
Work in parallel with DSP core execution unit.
Access new addresses without pausing to calculate new addresses.
Take advantage of predictability in the pattern of data access in DSP algorithms, using special addressing modes.
e.g. register-indirect with post increment addressing, circular (modulo) addressing, bit-reverse addressing in hardware.

11. Von Neumann Architecture…
Fetch MAC instruction
Read value of ‘x’
Read value of ‘h’
Multiply x, h and accumulate
Write result to memory
Processor Core
Address bus
Data bus
4 memory access operations
One multiplication
Memory
(Code+Data)

12. Harvard Architecture…
Data and Code in separate memory segments
Multiple address and data buses
Double memory bandwidth
Simultaneous code and data fetch
Processor Core
AB1
DB1
AB2
DB2
Memory A
Memory B

13. Caches in DSP and GPP…
GPPs normally contain two on-chip caches – one for data and the other for instructions.
Allows full speed retrieval of instructions and data without accessing slower off-chip memory.
DSPs contain a very small instruction cache and no data cache.
GPPs use control logic to determine what code and data goes into cache, while in DSPs it is programmer’s job to make a decision.

14. Fixed-Point Arithmetic…
Most DSPs use fixed point arithmetic than floating point.
Faster.
Cheaper.
Hardware support for saturation arithmetic, rounding and shifting.

15. Special Instructions Why special instructions?
Multiple operations per instruction cycle.
Minimize program memory space.
Specify several parallel operations in a single instruction.
These instructions permit restricted access to registers and do not allow arbitrary operation combinations.

16. Special Instructions…
MAC X0, Y0, A, X: (R0)+,X0, Y:(R4)+N4, Y0
Multiply contents of X0 and Y0
Add result to accumulator A
Load register X0 from X memory location pointed to by R0
Load register Y0 from Y memory location pointed to by R4
Post-increment R0 by 1
Post-increment R4 by the contents of register R4
This instruction calculates one tap of the FIR filter in one clock cycle

17. Execution Time Predictability…
Non-DSP applications have a maximum average response time (firm real time).
DSP applications are hard real time.
Important to be able to calculate exactly the processing time required, or at least the worst time scenario.
GPPs do not have a good execution time predictability.
Lack of execution time predictability affect code optimization.

18. Execution Time Predictability…
GPPs – complicated algorithms for branch prediction and caching.
Speculative code execution depending on branch prediction.
Programmer does not know which instructions and data will go into cache and when.
Worst case execution time may be a order of magnitude greater than the actual execution time.

19. Execution Time Predictability…
DSPs do not use branch prediction algorithms.
Programmer decides which instruction go into cache.
No data cache in most DSPs.

20. Other features of DSPs and GPPs
VLIW (Very Long Instruction Word).
Combines a number of different instructions in a long instruction word.
e.g. 256 bytes word – 8 instructions.
More MACs, ALUs and other execution units.
GPPs use SIMD.