CPU Instruction Sets

1. What is an Instruction Set?

An instruction set (also called an Instruction Set Architecture / ISA) is the complete collection of machine code instructions that a CPU is designed to understand and execute.

Every CPU has its own instruction set built into its hardware.

Programs must be translated into the CPU's specific instruction set before they can run.

Different processors (e.g., Intel x86, ARM) have different instruction sets.

2. Machine Code & Assembly Language

Machine Code

The only language a CPU can directly execute.

Consists entirely of binary digits (0s and 1s).

Extremely difficult for humans to read or write.

Assembly Language

A low-level language that uses mnemonics (short text codes) to represent machine code instructions.

Each mnemonic maps directly to one machine code instruction.

Converted to machine code by a program called an Assembler.

Assembly Mnemonic	Meaning
`LDD`	Load (direct addressing)
`STO`	Store a value in memory
`ADD`	Add a value
`SUB`	Subtract a value
`INC`	Increment (add 1)
`DEC`	Decrement (subtract 1)
`JMP`	Jump to an address (unconditional)
`JPE`	Jump if equal
`JPN`	Jump if not equal
`CMI`	Compare (immediate)
`END`	End of program

3. Structure of an Instruction

Each instruction in an instruction set is made up of two parts:

[ OPCODE ] [ OPERAND ]

Part	Description
Opcode	Tells the CPU what operation to perform (e.g., ADD, LOAD, JUMP)
Operand	Tells the CPU what data or address to use

Example:

ADD 005

Opcode → ADD (perform addition)

Operand → 005 (the memory address or value to add)

4. Types of Instructions

An instruction set typically includes instructions from these categories:

🔵 Data Transfer

Move data between memory and registers.

LDD 050 — Load the value at memory address 050 into the Accumulator

STO 050 — Store the Accumulator's value into memory address 050

🔵 Arithmetic Operations

Perform mathematical calculations.

ADD 010 — Add the value at address 010 to the Accumulator

SUB 010 — Subtract the value at address 010 from the Accumulator

INC ACC — Increase the Accumulator by 1

DEC ACC — Decrease the Accumulator by 1

🔵 Comparison & Branching (Jump Instructions)

Control the flow of a program.

CMI 5 — Compare the Accumulator with the immediate value 5

JMP 020 — Jump unconditionally to address 020

JPE 030 — Jump to address 030 if the comparison result was equal

JPN 040 — Jump to address 040 if the comparison result was not equal

🔵 I/O Instructions

Handle input and output.

IN — Input a value (store in Accumulator)

OUT — Output the value in the Accumulator

🔵 Stop

END — Terminates the program

5. Addressing Modes

Direct Addressing

The operand is the memory address where the data is stored.

LDD 050 → Load the value stored AT address 050

Immediate Addressing

The operand is the actual data value (not an address).

LDM #25 → Load the actual value 25 into the Accumulator

Tip: A # symbol before the operand usually indicates immediate addressing.

6. The Accumulator (ACC)

A special register inside the CPU used to hold the result of arithmetic/logic operations.

Most instructions in the CIE instruction set read from or write to the Accumulator.

7. Worked Example — Simple Assembly Program

Task: Add two numbers stored in memory and save the result.

Address	Instruction	Comment
000	`LDD 050`	Load value at address 050 into ACC
001	`ADD 051`	Add value at address 051 to ACC
002	`STO 052`	Store result into address 052
003	`END`	End the program

Memory:

Address	Value
050	12
051	8
052	(result = 20)

8. Key Vocabulary

Term	Definition
Instruction Set	The full set of binary instructions a CPU can execute
Machine Code	Binary instructions directly executed by the CPU
Assembly Language	Low-level language using mnemonics; converted by an Assembler
Assembler	A program that translates assembly language into machine code
Opcode	The part of an instruction specifying the operation to perform
Operand	The data or address the opcode acts upon
Accumulator (ACC)	A CPU register used to store results of operations
Mnemonic	A short, memorable code representing a machine code instruction

9. Exam Tips ✅

Know the difference between opcode and operand.

Be able to trace a short assembly program and state the value in the Accumulator at each step.

Remember that each CPU has its own unique instruction set — programs must be compiled/assembled for the target CPU.

# before a number = immediate (the number itself); no # = direct (a memory address).

Machine code is in binary; assembly uses mnemonics — do not confuse them.

CPU Instruction Sets

CPU Instruction Sets#

1. What is an Instruction Set?#

2. Machine Code & Assembly Language#

Machine Code#

Assembly Language#

3. Structure of an Instruction#

Example:#

4. Types of Instructions#

🔵 Data Transfer#

🔵 Arithmetic Operations#

🔵 Comparison & Branching (Jump Instructions)#

🔵 I/O Instructions#

🔵 Stop#

5. Addressing Modes#

Direct Addressing#

Immediate Addressing#

6. The Accumulator (ACC)#

7. Worked Example — Simple Assembly Program#

8. Key Vocabulary#

9. Exam Tips ✅#

CPU Instruction Sets

1. What is an Instruction Set?

2. Machine Code & Assembly Language

Machine Code

Assembly Language

3. Structure of an Instruction

Example:

4. Types of Instructions

🔵 Data Transfer

🔵 Arithmetic Operations

🔵 Comparison & Branching (Jump Instructions)

🔵 I/O Instructions

🔵 Stop

5. Addressing Modes

Direct Addressing

Immediate Addressing

6. The Accumulator (ACC)

7. Worked Example — Simple Assembly Program

8. Key Vocabulary

9. Exam Tips ✅