Little Man Computer

Python version of: LMC by Peter Higginson

Modern computers contain a processor which executes instructions and memory which stores both the instructions and also the data that is to be used. In addition there are various means both to give data to the computer and to receive information from it. Although The Little Man Computer is a simulation of how a computer works. It is a heavily simplified version of a computer from the one that sits on your desk but nonetheless it is an excellent tool to learn how a computer works. from You will recall that a computer has various components including means to input data, output information, carry out calculations and store results.

Instruction	Mnemonic	Machine Code	Info	Example
End	HLT	000	Ends the program	HLT
Add	ADD	1xx	Adds the value at address xx to the accumulator	ADD 50
Subtract	SUB	2xx	Subtracts the value at address xx from accumulator	SUB 50
Store	STA	3xx	Storecontents of accumulator to address xx	STA 50
Load	LDA	5xx	Loads contents of address xx to the accumulator	LDA 50
Branch Always	BRA	6xx	Jumps to the instruction at address xx	BRA 05
Branch If Zero	BRZ	7xx	Jumps to the instruction at address xx, given the value in the accumulator is 0	BRZ 05
Branch If Positive	BRP	8xx	Jumps to the instruction at address xx, given the value in the accumulator is 0, or greater	BRP 05
Input	INP	901	Prompts user for input, store in accumulator	INP
Output	OUT	902	Outputs value currently in accumulator	OUT
Data	DAT	-	Reserves a place in memory for a variable, followed by the initial value of said var	VarName DAT 50

How to use LMC

1. The 100 memory addresses in the computer memory are numbered 0 to 99 and can each contain a 'machine code' instruction or data.
2. Each assembly language instruction is made up of a 3 letter mnemonic (which represents the operation code), usually followed by the memory address of the data the CPU is to act on (this is called absolute memory addressing).
3. Pressing the Assemble Program button translates the assembly language instructions into 'machine code' and loads them into RAM. it also resets the Program Counter to zero.
4. The Input box allows the user to enter numerical data (-999 to 999) while a program is running and load it into the accumulator.
5. The Output box can output the contents of the accumulator while a program is running.
6. A RAM memory address that is used to store data can be given a meaningful label. Data can also be stored in these named address locations.
7. The results of any ADD or SUBTRACT instructions are stored in the accumulator .
8. The Program Counter stores the memory address of the instruction being carried out. It will automatically increment by 1 after each instruction is completed.
9. If the CPU receives an non-sequential instruction to branch (BRP, BRP or BRZ) then the Program Counter is set to the memory address of that instruction.
10. Branch instructions are set to branch to a labelled memory location.
11. To restart a program, the Program Counter is reset to 0.

Reference