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
Logical Operations
Examples:
- COMP
Compare 2 values
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. Used 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. Examples are 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
Instruction Cycle
A program is executed as a series of steps, which are called sub-cycles.
Instruction Fetch
An instruction is loaded into the CPU from memory. A memory read cycle is executed. Common for all instruction.
Decode
Handled by CU. Binary pattern of the instruction and what action to take is decided. Common for all instruction.
Operand Fetch
If the operation requires operand(s), they are read from memory. Depends on the operation. Optional for some operations.
Execution Cycle
CU executes the instruction. CPU generates the signal for its internal circuits to carry out the operation. Depends on the operation.