A computer accepts only the language of zeros and once. Therefore, the instructions have to be encoded in binary. An example of such encoding is presented below.
|
00000100 10000000 00000000 10000001 00000101 10000000 00001111 00000000 |
The binary encoding above is splitted into two columns for the programmer convenience. Each column represents a byte (an 8-bit string) stored in computer memory. The meaning of the bytes for this specific code is explained below.
The first column of the encoding is an operation code, which the second column represents an argument for that instruction.
| Code | Parameter | Action |
|---|---|---|
| 00000100 | X | Load accumulator with the contents of memory location X |
| 00000000 | X | Add contents of memory location X to the accumulator |
| 00000101 | X | Store the accumulator contents in memory location X |
| 00001111 | X | Halt execution |
With the instruction encoding provided by the above table, our code makes more sense now:
Load 10000000 Add 10000001 Store 10000000 Halt |
| Data flow instructions | |
|---|---|
| Op. Code | Comments |
| LOD X (or #X) | X -> A, place X into accumulator |
| STO Y | A -> Y, store contents of the accumulator in location Y |
| Arithmetic - logic instructions | |
| Op. Code | Comments |
| ADD X (or #X) | A+X -> A, add X to contents of accumulator |
| SUB X (or #X) | A-X -> A |
| MUL X (or #X) | A*X -> A |
| DIV X (or #X) | A/X -> A |
| AND X (or #X) | A and X -> A If contents of A and location X are both nonzero, put 1 into A, otherwise, put 0 into A |
| NOT A | not A -> A If A=0, set A=1, otherwise set A=0 |
| CPZ X | if X=0, set A=1, otherwise set A=0 |
| CPL X | if X<0, set A=1, otherwise set A=0 |
| Control instructions | |
| Op. Code | Comments |
| JMP Y | goto instruction number Y |
| JMZ Y | if A=0, go to instruction number Y if A is not 0, proceed with next instruction |
| NOP | no operation |
| HLT | halt execution |