Instruction Types

Introduction

A processor's instruction set contains every operation the hardware can perform. These instructions fall into distinct categories based on what they do. Understanding instruction types helps you see the full vocabulary available to a machine-language programmer (or compiler). Every program — no matter how complex — is ultimately built from these basic instruction categories.

Data Transfer Instructions

Move data between registers and memory without modifying it:

Key insight: In RISC architectures (load/store), ONLY load and store access memory. All other operations work on registers. This simplifies hardware design enormously.

Arithmetic Instructions

Perform mathematical operations:

CMP (compare) is special — it performs subtraction but only updates flags, not the destination. This sets up condition codes for subsequent branch instructions.

Logical Instructions

Perform bitwise Boolean operations:

Common uses:

• AND with mask to clear bits
• OR with mask to set bits
• XOR to toggle or check equality
• TEST to check if specific bits are set

Shift and Rotate Instructions

Control Flow Instructions

Change the sequence of instruction execution:

Unconditional Branch

Conditional Branch

Function Call/Return

Loop Control (CISC)

I/O Instructions

Transfer data between processor and peripheral devices:

Note: In memory-mapped I/O systems (ARM, MIPS), regular LOAD/STORE serve as I/O instructions — no separate I/O instructions exist.

System/Privileged Instructions

Control processor state and operating system functions:

These are typically restricted to kernel/supervisor mode for security.

Instruction Mix

Not all instruction types are used equally. Studies of real programs show:

This means data movement dominates — which is why register count and cache design are so important.

RISC vs CISC Instruction Philosophies

RISC (Reduced Instruction Set)

• Fewer, simpler instructions
• All instructions same length, same execution time
• Only LOAD/STORE access memory
• Compiler does the heavy lifting of instruction selection

CISC (Complex Instruction Set)

• Many specialized instructions (string copy, polynomial evaluate, etc.)
• Variable-length instructions
• Memory operands in arithmetic instructions
• Hardware does more, compiler does less

Key Takeaways

1. Instructions fall into categories: data transfer, arithmetic, logical, shift, control flow, I/O, system
2. Data transfer instructions are the most frequently executed (~40% of all instructions)
3. Control flow instructions direct program execution — conditional branches enable decisions and loops
4. CMP and TEST set flags without saving results, preparing for conditional branches
5. RISC architectures keep instructions simple; CISC adds complex specialized instructions
6. Arithmetic and logical instructions modify data; transfer instructions move data unchanged
7. Understanding instruction frequency guides hardware optimization priorities