Instruction Set Design I

Tom Kelliher, CS 240

Feb. 4, 2004

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Assignment

Read 3.5.

From Last Time

Calculating performance.

Outline

1. Design principles.

2. Classes of instructions.

3. Operands.

4. MIPS registers.

5. Memory addressing.

Coming Up

Conditional instructions in MIPS.

Instruction Set Design

Things to consider:

1. General design principles. E. g., simplicity.

2. Operations.

3. Operands: number, memory addressing modes.

Design Principles

1. Simplicity favors regularity. Example: the layout of a WalMart store.

2. Smaller is faster. Example: L1 vs. L2 caches.

Classes of Instructions

Consider your favorite HLL. What classes of instructions (operations) are required?

Now, consider a multi-user OS such as Unix? What additional classes of instructions are required?

Any general purpose architecture must support these classes.

Arithmetic Instructions

Consider the two simplest:

add
sub

f = (g + h) - (i + j);

1. Should an instruction set directly support complex arithmetic statements?

2. How about directly supporting a variable number of operands?

3. How many operands should arithmetic instruction take? 3? 2? 1? 0? Tradeoffs.

Instruction Semantics

add a, b, c        # This, BTW, is a comment.
sub a, a, b

add a, b, c
add a, a, d
add a, a, e

Compile each of the following:

a = b + c;
d = a - e;
f = (g + h) - (i + j);

Instruction Operands

Consider operands within an HLL:

#include <stdio.h>

int main()
{
   int foo = 1234;

   printf("%d,   %p\n", foo, &foo);

   return 0;
}

RISC instruction sets don't ordinarily support memory operands. Why not?

Where are the operands? Registers.