Processor instruction sets

An instruction set is all the operations a processor can perform and their opcodes. Each processor has its own instruction set. Typically an instruction set will include instructions for:
Mathematical calculations, Bit shifts, Comparisons, Logical operators, Branching, Data transfer

Parts of an instruction

An instruction is made up of three parts:
An opcode that itself is made up of two parts:
An operation that specifies the operation to be carried out
An addressing mode that lets you specify either direct or immediate addressing
An operand that specifies an actual value or register or memory location depending on the operation

Addressing modes

Although there are more addressing modes in actual computers you only need to be familiar with two:
Immediate where the operand represents an actual value in this case 27
Direct where the operand directs you to the location where the value is stored in this case memory location 27

Number of bits used

The number of bits used for the operation, address mode and operand determine how many operations can be requested, how many addressing modes can be used and how many storage locations can be accessed or how big the largest number that can be entered directly is. The number of operations, address modes and memory locations that can be addressed is 2n where n is the number of bits for the section of the instruction. The largest immediate number that can be entered is 2n-1

Knowledge check

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