1. Instruction Cycle
Instruction cycle is the time that is needed by a computer for reading and processing
instruction from memory. Each CPU has different cycle because it has different instruction set.
The processing of instruction can be classified to be 4, they are:
a. Read the instruction from main memory.
CPU takes the value in PC(Program Counter),and then CPU takes instruction from main
memory passed bus to Memory Data Register(MDR). The value in MDR will be stored in
IR (Instruction Register). Decoder interpretes instruction and executes it.
b. Read data from main memory
Data that will be processed is taken with the instruction and will be put into register.
c. Execute the instruction.
From IR data are decoded by Control Unit. Actions that can be performed are: read the
value in register, process it in ALU and store the result in register.
d. Store the result of instruction.
Read the result again in memory (main memory or output media).
Instruction cycle is devided by two,they are:
a. Fetch cycle ( a and b). This step is always same for all of instruction. Instruction consists of
opcode and operand.
b. Execution cycle(c and d). this step is always changed for each instruction. The first step is
memory processing and then data is transferred between CPU and I/O module. Finally,
data are processed used mathematic or logic processing.
2. Instruction Register
It is part of CPU that store the instruction that is being executed. In simple processor, each
instruction that will be executed , is put into IR and will be stored during process of decode
run, prepared and finally it is executed.
3. Program Counter(PC)
PC is register that it used for keeping the sequence of instruction. PC also stores the address of
instruction that is being executed or will be executed. PC will do iteration automatically for
each instruction cycle. Special instruction, for example: subprogram and ramification are
processed with do interrupt to PC to change the value in PC.
4. Instruction Set
Instruction Set is a horde of instruction that can be executed by processor. Instruction set is
classified by three kinds , they are: arithmetic instruction (add and substract ) and logic
instruction(AND ,OR, and NOT). Data instruction(move,input,output,store). Control
The format of instruction is:[opcode l operand]