This is a Little Man Computer simulator. The original LMC uses denary instructions, but this uses binary to more closely model a real CPU. It has a 5 bit address bus and an 8 bit data bus. Instructions are 8 bits with direct addressing mode. Data are 8 bit signed integers in 2's-complement, between -128 and +127. Memory is limited to 32 x 8 bit slots for 32 bytes total memory.
| Opcode | Mnemonic | Description |
|---|---|---|
| 001xxxxx | STA | Store value from accumulator to memory address xxxxx |
| 010xxxxx | LDA | Load value from memory address xxxxx into accumulator |
| 011xxxxx | ADD | Add contents of memory address xxxxx to accumulator |
| 100xxxxx | SUB | Subtract contents of memory address xxxxx from accumulator |
| 101xxxxx | BRA | Branch to memory address xxxxx, unconditionally |
| 110xxxxx | BRZ | Branch to memory address xxxxx if accumulator is zero |
| 111xxxxx | BRP | Branch to memory address xxxxx if accumulator is positive (or zero) |
| 00001--- | INP | Input from user into accumulator (--- not used) |
| 00010--- | OUT | Output value of accumulator to the output box (--- not used) |
| 00000--- | HLT | Terminates execution of the program |
| - | DAT | A memory location that contains data (an integer from -128 to 127) |
The instruction set is the same as for the denary LMC - programs written in LMC assembly should work fine. Lines of assembly start with an optional label, then have one of the 11 mnemonics, then an optional value (which could be a denary integer or the name of a label defined on another line). Example program:
INP # user inputs a value
STA a # which is stored into 'a'
INP # user inputs another value
ADD a # which has 'a' added to it
OUT # and is then output
HLT # program terminates
a DAT # ... location to store 'a'
# Not used in this program...
b DAT 34 # put '34' in memory labelled 'b'
This simulator has a single R/W (read/write) signal for the control bus (real CPUs will have several more signals as part of this bus). The simulator's CPU has the same registers as in the OCR A level specification: