CS5222 Adv. Comp. Arch. Part 3 Page.1 Chi C.H. Fall 2004 NUS

CS5222Advanced Computer Architecture

Part 3: VLIW Architecture

Fall Term, 2004/2005

Chi Chi Hung (email: chich@comp.nus.edu.sg)

Building S/17, Rm 5-13

Phone: 6874-2832

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Basic Working Principles of VLIW

° Aim at speeding up computation by exploiting instruction-level parallelism.

° Same hardware core as superscalar processors, having multiple execution units (EUs) working in parallel.

° An instruction is consisted of multiple operations; typical word length from 52 bits to 1 Kbits.

° All operations in an instruction are executed in a lock-step mode.

° One or multiple register files for FX and FP data.

° Rely on compiler to find parallelism and schedule dependency free program code.

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Basic VLIW Approach

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Register File Structure for VLIW

° What is the challenge to register file in VLIW? R/W ports

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Differences Between VLIW & Superscalar Architecture (I)

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° Instruction formulation:

• Superscalar:

- Receive conventional instructions conceived for seq. processors.

• VLIW:

- Receive (very) long instruction words, each comprising a field (or opcode) for each execution unit.

- Instruction word length depends (a) number of execution units, and (b) code length to control each unit (such as opcode length, register names, …).

- Typical word length is 64 – 1024 bits, much longer than conventional machine word length.

Differences Between VLIW & Superscalar Architecture (II)

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° Instruction scheduling:• Superscalar:

- Done dynamically at run-time by the hardware.

- Data dependency is checked and resolved in hardware.

- Need a lookahead hardware window for instruction fetch.

Differences Between VLIW & Superscalar Architecture (III)

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° Instruction scheduling (cont’d):• VLIW:

- Static scheduling done at compile-time by the compiler.

- Advantages:

– Reduce hardware complexity.– Tasks such as decoding, data dependency detection,

instruction issue, …, etc. becoming simple.– Potentially higher clock rate.– Higher degree of parallelism with global program

information.

Differences Between VLIW & Superscalar Architecture (IV)

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° Instruction scheduling (cont’d):

• VLIW:

- Disadvantages

– Higher complexity of the compiler.

– Compiler optimization needs to consider technology dependent parameters such as latencies and load-use time of cache.

(Question: What happens to the software if the hardware is updated?)

– Non-deterministic problem of cache misses, resulting in worst case assumption for code scheduling.

– In case of un-filled opcodes in a (V)LIW, memory space and instruction bandwidth are wasted.

Differences Between VLIW & Superscalar Architecture (V)

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Development history of Proposed/Commercial VLIWs

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Case Study of VLIW: Trace 200 Family (I)

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Case Study of VLIW: Trace 200 Family (II)

° Only two branches might be used in Trace 7/2000

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° It is found that code in VLIW is expanded roughly by a factor of three.

° For “long” VLIW, more opcode fields will be emptied. This will result in wasting bandwidth and storage space. Can you propose a solution for it?

Code Expansion in VLIW

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