Representing computer instructions

Parts of a computer instruction

The following information must be encoding inside a computer instruction:

Operation, such as:
What does the operation use as source operands
Where does the operation store the result of the operation

A computer instruction typically consists of 4 to 32 bytes

The computer instruction has a instruction format that is different for each type of computer

But all computer instructions can be sub-divided into 3 parts:

A opcode part that encodes (= represents) the computer operation
A source part that encode one or two source operands
A destination part that encode the destination

Let me show you a concrete example: suppose a computer instruction consist of 4 bytes (= 32 bits):

1 computer instruction: 4 bytes (= 32 bits) <-------------------------------------------------------------> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Then:

Some of the bits will be used to encode the operation
Some of the bits will be used to encode the source operands
Some of the bits will be used to encode the destination operands

For example:

In fact, the bits that encode the various part need not be contiguous.

But there is always a logical scheme to encode the various parts.

The logical scheme to encode the opcode, source operands and destinations varies from computer to computer.

Sample encoding scheme for the computer operation

To encode (=represent) the computer instruction, each computer instruction is represented by a unqiue operation code (it's called opcode for short)

For example:

Operation Opcode ------------- -------- Move 0000 Add 0001 Subtract 0010 Multiply 0011 Divide 0100 ... ... (and so on)

Again, each computer has its own encoding scheme and it is not important to memorize the encoding.

It's enough that you know there is a systematic way to encode the operation code.

Encoding the source operands and destination
- To encode (= represent) the source operand(s) and the destination, the location information is encoded
  Again, each computer CPU has its own way to encode the operand information
  The encoding technique is not part of the CS 255 material - all you need to know is each computer CPU has its own systematic way to encode where the operands are located
  Also, the same encoding technique is used to encode the destination
- A few webpages later, I will show you how the ARM and the M68000 CPUs encode their source and destination operands.
  And I only do it to satisfy your curiosity - and not to discuss encoding technique in general.