Above is a Little Man Computer (LMC) simulator. The original LMC uses denary instructions, but I created this one to use binary instead to more closely model what happens in a CPU. It has a 5bit address bus and an 8bit data bus. All instructions are 8bits with direct addressing as the only addressing mode. Memory is limited to 32 x 8bit slots for 32bytes total memory.
Binary LMC instruction set
| 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) |
| 00001xxx | INP | Input from user into accumulator (xxx not used) |
| 00010xxx | OUT | Output value of accumulator to the output box (xxx not used) |
| 00000xxx | HLT | Terminates execution of the program |
| - | DAT | A memory location that contains data |
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'
Data is 8bit integers in 2s-complement, between -128 and +127, but negative integers not properly supported yet.
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