CPU Instructions

Instruction set

The set of operations a CPU can perform. Varies with each CPU. Most operations fall into 4 categories.

Data movement operations

Moves data from one place to another.

Examples:

• LOAD: Read data into CPU from memory or IO
• STORE: Write data from CPU to memory or IO

Mathematical operations

Examples:

• ADD: Adds 2 values in CPU

Logical operations

Examples:

• COMP: Compare 2 values in CPU

CPU control instructions

Examples:

• JUMP: Jump to a program address
• JUMPZ: Jump to a program address only if the last operation had a zero value
• JUMPNZ: Jump to a program address only if the last operation had a non-zero value
• NOP: Do nothing but spend some CPU time

Note

NOP operation is required to keep the CPU idle while waiting for some data.

Composition of instructions

OpCode

A coded value which tells what the instruction is supposed to do. OpCodes are the examples mentioned above (such as ADD and COMP).

Operands

Number and type of operations depend on the OpCode. Sometimes optional.

Can either be a:

• Value
• Memory address

Note

Single-instruction-single-data architecture (SISD) means that CPU takes one instruction and data associated with the instruction, executes it and goes to the next instruction.

Execution of a program (Instruction cycle)

A program is executed as a series of steps.

Instruction Fetch

Next instruction is loaded into the CPU from memory. A memory read cycle is executed. Similar for any instruction.

Decode

CPU decides the instruction binary pattern and decides on what action to take. Similar for any instruction.

Operand Fetch

If the operation requires additional operands in memory, ready cycles are executed to fetch them. Depends on the operation. Somtimes optional.

Execution cycle

Instruction is executed by the CU. CPU generates the signal for its internal circuits to carry out the operation. Depends on the operation.