1. Very Long Instruction Word (VLIW) Architecture

2. VLIW Machine It consists of many functional units connected to a large central register file Each functional unit have two read ports and one write port Register file would have enough memory bandwidth to balance the operand usage rate of functional units

3. Ideal VLIW Machine

4. VLIW characteristics VLIW contains multiple primitive instructions that can be executed in parallel The compiler packs a number of primitive, independent instructions into a very long instruction word The compiler must guarantee that multiple primitive instructions which group together are independent so they can be executed in parallel.

5. 5 Example of a single VLIW instruction: F=a+b; c=e/g; d=x&y; w=z*h;

6. VLIW Principles 1.The compiler analyzes dependence of all instructions among sequential code and extracts as much parallelism as possible. 2.Based on analysis, the compiler re-codes the sequential code in VLIW instruction words.(One VLIW instruction word contains maximum 8 primitive instructions) 3.Finally VLIW hardware – Fetch the VLIWs from cache, – Decode them, – Dispatch the independent primitive instructions to corresponding functional units and – Execute

7. Advantages of VLIW architecture Reduces Complexity: – Due to parallelism among their primitive instructions Higher possible clock rate because of reduced complexity

8. Drawbacks of VLIW Architecture Compiler has to be aware of technology dependent parameters-like latencies and repetition rate. This restricts the use of same compiler for a family of VLIW processors. Wasted memory space and bandwidth when some EUs are not used Performance is dependent on how the compiler produces VLIW words.

9. Comparison

10. Data Flow Computers

11. They are based on the concept of data driven computation Conventional computers is under program flow control.

12. Features of Control Flow Model Data is passed between instructions via shared memory Flow of control is implicitly sequential Program counters are used to sequence the execution of instruction

13. Features of Data Flow Model Intermediate or final results are passed directly as data token between instructions There is no concept of shared data storage Program sequencing is constrained only by data dependency among instructions