checking dependencies between instructions to determine which instructions can be grouped together for parallel execution; assigning instructions to the functional units on the hardware; determining when instructions are initiated placed together into a single word.

1. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA
Superscalar and VLIW
Architectures
VLSI ARCHITECTURES

2. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA
Outline
• Types of architectures
• Superscalar
• Differences between CISC, RISC and VLIW
• VLIW ( very long instruction word )

3. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA
Parallel processing
Processing instructions in parallel requires three
major tasks:
1. checking dependencies between instructions to
determine which instructions can be grouped
together for parallel execution;
2. assigning instructions to the functional units on
the hardware;
3. determining when instructions are initiated placed
together into a single word.

4. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA
Major categories
VLIW – Very Long Instruction Word
EPIC – Explicitly Parallel Instruction Computing

5. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA
Superscalar Processors
• Superscalar processors are designed to exploit more
instruction-level parallelism in user programs.
• Only independent instructions can be executed in
parallel without causing a wait state.
• The amount of instruction-level parallelism varies
widely depending on the type of code being executed.

6. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA
Pipelining in Superscalar Processors
• In order to fully utilise a superscalar processor of
degree m, m instructions must be executable in
parallel. This situation may not be true in all clock
cycles. In that case, some of the pipelines may be
stalling in a wait state.
• In a superscalar processor, the simple operation
latency should require only one cycle, as in the base
scalar processor.

7. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA

8. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA
Superscalar Execution

9. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA
Superscalar Implementation
• Simultaneously fetch multiple instructions
• Logic to determine true dependencies involving
register values
• Mechanisms to communicate these values
• Mechanisms to initiate multiple instructions in
parallel
• Resources for parallel execution of multiple
instructions
• Mechanisms for committing process state in
correct order

10. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA
VLIW History
The term coined by J.A. Fisher (Yale) in 1983
ELI S12 (prototype)
Trace (Commercial)
Origin lies in horizontal microcode optimization
Another pioneering work by B. Ramakrishna Rau in
1982
Poly cyclic (Prototype)
Cydra-5 (Commercial)
Recent developments
Trimedia – Philips
TMS320C6X – Texas Instruments

11. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA
The VLIW Architecture
• A typical VLIW (very long instruction word) machine
has instruction words hundreds of bits in length.
• Multiple functional units are used concurrently in a
VLIW processor.
• All functional units share the use of a common large
register file.

12. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA
Why Superscalar Processors are
commercially more popular as
compared to VLIW processor ?
Binary code compatibility among scalar &
superscalar processors of same family
Same compiler works for all processors (scalars
and superscalars) of same family
Assembly programming of VLIWs is tedious
Code density in VLIWs is very poor
- Instruction encoding schemes
Area Performance

13. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA
Data path : A simple VLIW Architecture
FU FU FU
Register file
Scalability ?
Access time, area, power consumption sharply increase with
number of register ports

14. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA
Data path : Clustered VLIW Architecture
(distributed register file)
FU FU
Register file
FU FU
Register file
FU FU
Register file
Interconnection Network

15. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA
Coarse grain Fus with
VLIW core
MULT RAM ALU
Coarse grain
FU
Reg2
Reg1
Reg1
Reg1
Reg2
Reg2
Multiplexer network
Micro
Code
IR
Prg. Counter
Logic
Embedded (co)-processors as Fus in a VLIW architecture

16. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA
Application Specific FUs
FUfunctionality
number of inputs
number of outputs
latency initiation interval I/O time shape
Functional Units

17. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA
Comparison: CISC, RISC, VLIW

18. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA

19. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA
Advantages of VLIW
Compiler prepares fixed packets of multiple
operations that give the full "plan of execution"
– dependencies are determined by compiler and used to
schedule according to function unit latencies
– function units are assigned by compiler and
correspond to the position within the instruction
packet ("slotting")
– compiler produces fully-scheduled, hazard-free code
=> hardware doesn't have to "rediscover"
dependencies or schedule

20. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA
Disadvantages of VLIW
Compatibility across implementations is a major
problem
– VLIW code won't run properly with different number
of function units or different latencies
– unscheduled events (e.g., cache miss) stall entire
processor
Code density is another problem
– low slot utilization (mostly nops)
– reduce nops by compression ("flexible VLIW",
"variable-length VLIW")

21. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA

22. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA

23. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA
References
1. Advanced Computer Architectures, Parallelism, Scalability,
Programmability, K. Hwang, 1993.
2. M. Smotherman, "Understanding EPIC Architectures and
Implementations" (pdf)
http://www.cs.clemson.edu/~mark/464/acmse_epic.pdf
3. Lecture notes of Mark Smotherman,
http://www.cs.clemson.edu/~mark/464/hp3e4.html
4. An Introduction To Very-Long Instruction Word (VLIW) Computer
Architecture, Philips Semiconductors,
http://www.semiconductors.philips.com/acrobat_download/other
/vliw-wp.pdf
5. Texas Instruments, Tutorial on TMS320C6000 VelociTI Advanced
VLIW Architecture.
http://www.acm.org/sigs/sigmicro/existing/micro31/pdf/m31_sesha
n.pdf

24. VLSI DESIGN GROUP – METS SCHOOL OF ENGINEERING , MALA
Thanks