just the once the instruction has been fetched and is accumulated, the next step is to decode the instruction so as to work out what actions should be performed to execute it. This occupies examining the opcode to observe which of the machine codes in the CPU’s instruction set it communicates to, and also verifying which addressing mode needs to be used to get any required data.
Once the opcode is known, the execution cycle can take place. Different actions require to be carried out dependant on the opcode, without two opcodes requiring the same actions to occur. Though, there are usually four groups of different actions that can occur:
• Move of data between the CPU and memory.
• move of data between the CPU and an input or output devices.
• Processing of data, perhaps involving the use of the arithmetic and logic unit.
• A control operation, with the intention of change the sequence of subsequent operations. These can probably be conditional, based on the values stored at that point within the flag register.
For greater ease, and as describing all the possible instructions:
Mnemonic Description
MOV Moves a data value from one location to another
ADD Adds to data values using the ALU, and returns the result to the accumulator
STO Stores the contents of the accumulator in the specified location
END Marks the end of the program in memory