Instruction Set
Architecture
Stephen Murphy
What is ISA?
Aspects of the computer visible to the
programmer:
 Data Types
 Registers
 Instructions
 Addressing
Data Types
Unsigned Integers
 Signed Integers
 Characters
 Floating-point Numbers

Unsigned Integers

Stores the integer in standard binary
format
00101011 = 1 + 2 + 8 + 32 = 43
Signed Integers

Stores the integer in two’s complement
format to denote sign
Steps to convert signed integer to base 10:
 Store leading bit, 1 = negative, 0 = positive
 Invert all bits
 Add 1
Characters

Represented by integer values
Many different character encodings exist:
 ASCII
 UNICODE
 UTF-8
 UTF-16
Floating-point Numbers
Four components in a floating-point number:
 Sign
 Mantissa
 Radix
 Exponent
Registers
Registers provide a variety of services:
 Instruction Counter
 Register-to-Register Operations
 Memory-to-Register Operations
 Memory-to-Memory Operations
 Processor Status
Instruction Counter Register

All von Neumann machines have an
instruction counter which is a register
Register-to-Register Registers
Operate on one register using another
register (ADD)
 Supported by all ISA’s

Memory-to-Register Registers
Operate on memory using a register
(LOAD, STORE)
 Not supported by all ISA’s

Memory-to-Memory Registers
Operate on one memory location using
another memory location (STRCMP)
 Not supported by all ISA’s

Processor Status Registers
A collection of registers that indicate the
current state of the processor (Carry,
Interrupt, Zero)
 Different ISA’s use different status
registers

Instructions
Categories of instructions:
 Operate
 Memory Access
 Control
 Miscellaneous
Operate Instructions
Typical operations include:
 Arithmetic (ADD, SUB, MUL, DIV)
 Logical (AND, OR, NOT, XOR)
 Shift (ROL, ROR)
 Character (STRCMP)
 Stack (PUSH, POP)
Memory Access Instructions
Typical operations include:
 Load (LODSB)
 Store (STOSB)
Control Instructions
Conditional Branch (JZ, JC, JO, JS)
 Unconditional Branch (JMP)

Miscellaneous Instructions
Input/Output (IN, OUT)
 Interrupts (CLI, INT, IRET)
 Halt
 Privileged

Addressing
Register Addressing
 Memory Addressing

Register Addressing

When an operand is taken from a register
it is called register addressing
Memory Addressing
Logical Address Space (sequential
memory locations)
 Physical Address Space (many types
exists: absolute addressing, indirect
addressing, etc.)

ISA Examples
MIPS
 Motorola 6800
 ARM
 SPARC
 PowerPC
 x86
 IA-64

ISA Humor
BOB – Branch On Bug
 ADN – Add Nauseam
 BPDI – Be Polite, Don’t Interrupt
 BPE – Bypass Program Error
 DMNS – Do what I Mean, Not what I Say
 DC – Divide and Conquer
 CDHI – Crash Disk Head Immediately

References
http://webster.cs.ucr.edu/AoA/Windows/HT
ML/ISA.html
 http://www.wordiq.com/definition/Instructio
n_set
 http://www.cs.panam.edu/~meng/Course/
CS4335/Notes/master/node22.html
