Computer instructions are a set of machine language instructions that a particular processor understands and executes. A computer performs tasks on the basis of the instruction provided.
An instruction comprises of groups called fields. These fields include:
A basic computer has three instruction code formats which are:
Memory - reference instruction
In Memory-reference instruction, 12 bits of memory is used to specify an address and one bit to specify the addressing mode 'I'.
Register - reference instruction
The Register-reference instructions are represented by the Opcode 111 with a 0 in the leftmost bit (bit 15) of the instruction.
Note: The Operation code (Opcode) of an instruction refers to a group of bits that define arithmetic and logic operations such as add, subtract, multiply, shift, and compliment.
A Register-reference instruction specifies an operation on or a test of the AC (Accumulator) register.
Just like the Register-reference instruction, an Input-Output instruction does not need a reference to memory and is recognized by the operation code 111 with a 1 in the leftmost bit of the instruction. The remaining 12 bits are used to specify the type of the input-output operation or test performed.
Instruction Set Completeness
A set of instructions is said to be complete if the computer includes a sufficient number of instructions in each of the following categories:
Arithmetic, logic and shift instructions provide computational capabilities for processing the type of data the user may wish to employ.
A huge amount of binary information is stored in the memory unit, but all computations are done in processor registers. Therefore, one must possess the capability of moving information between these two units.
Program control instructions such as branch instructions are used change the sequence in which the program is executed.
Input and Output instructions act as an interface between the computer and the user. Programs and data must be transferred into memory, and the results of computations must be transferred back to the user.