Instruction Set Architecture The portion of the machine visible to the programmer Issues: Internal storage model Addressing modes Operations Operands Encoding Compilation Issues

Internal Storage Model Memory- Memory ( Add C, A, B )

Internal Storage Model Most post 1980 architectures use a load store architecture Main reasons registers faster than memory Compilers can utilise registers more effectively than other models Variables can be held in registers Instruction set can use 2 or 3 operand format

Memory Addressing How memory addresses interpreted memory addresses are specified Interpretation Size (byte, half word, word) Ordering (Big Endian, Little Endian) Alignment (Word)

Memory Addressing (Addressing modes) How an address is specified Modes included on machines: Register (e.g. Add R4, R3 ) Immediate (e.g. Add R4, #3 ) Displacement (e.g. Add R4, 100(R1) ) Absolute ( Add R4, (1000) ) Memory Indirect ( Add ) Autoincrement ( Add R4, (R1)+ ) Autodecrement ( Add R4, -(R1) )

Memory Addressing Use of addressing Modes: register accounts for 50% (not counted below) 1%

Displacement What is the range of displacements used

Immmediate

Immediate

Operations in the Instruction Set Can be classified into the following types Arithmetic and logical Data transfer Control System

Top 10 instructions on 8086 Averaged over 5 SPECin92 programs

Instructions for Control Flow Four types Conditional Branches Jumps Procedure Calls Procedure returns Destination address must always be supplied. Can be: specified explicitly indirect jumps (procedure return)

Branch Distance

PC relative Branching Target often close to the branch instruction Destination is obtained by adding a displacement to the PC Requires fewer bits to encode instruction helps make code position independent

Specifying Branch Condition Most branches are equality|inequality tests or comparisons with zero

Type and Size of Operands Two main alternatives operand types specified in opcode (e.g. add, addb, addf) data annotated with tags (very rare)

Encoding the Instruction Set Representation affects size of compiled program implementation of CPU Architect must balance the desire to have as many registers and addressing modes as possible the impact of the size of register and addressing mode fields

VIARABLE (e.g. VAX, 68000) Better code density FIXED (e.g. MIPS and PowerPC) Simpler hardware Variations in Instruction decoding OpcodeResultOperand 1Operand 2 OpcodeAdd spec 1Add field 1Add spec nAdd field n...

Example 1: A Registerless processor L1 Cache and registers about same speed no need for registers An fixed length instruction set with 3 operand format is proposed Simulations show that the number of instructions execution is about half of a load store architecture