Very Long Instruction Word Microprocessors

RISC and CISC type microprocessors are not the only game in town. VLIW microprocessors where once believed to be the future but not anymore. What happened?

Transmeta Crusoe, a famous VLIW Microprocessor. Image: Wikimedia

Doing Work in Parallel

Scalar vs Superscalar Processors

How Scalar Processors Work

y = a*b + c*d
load r1, a         ; r1 ← a Load contents of a into register r1
load r2, b
load r3, c
load r4, d
multiply r1, r2 ; r1 ← r1 * r2 Multiply r1 and r2. Store in r1
multiply r3, r4
add r1, r3 ; r1 ← r1 + r3
store r1, y ; r1 → y Store register r1 at location y
A Scalar Microprocessor. The data and address bus are like pipes pushing numbers in electrical form around microprocessor. The control lines are used to open “values” on each unit so data can flow in or out of them.

How Superscalar Processors Work

Parallel Instruction Execution with VLIW

add      r1, r2  : ALU1
add r3, r4 : ALU2
multiply r5, r2 : Multiplier1
multiply r6, r4 : Multiplier2
y = a*b + c*d

Out of Order Execution (OoOE)

Why OoOE Beats VLIW

Geek dad, living in Oslo, Norway with passion for UX, Julia programming, science, teaching, reading and writing.

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