Little Man Computer - CPU simulator - help page

The Little Man Computer (LMC) is an instructional model of a computer, created by Dr. Stuart Madnick in 1965. It models the architecture of a simple computer and has all the basic features. See en.wikipedia.org/wiki/Little_man_computer for more information.

The original concept was that inside the CPU is a tiny man who runs around executing instructions encoded in a list. Inside the CPU there are:

100 Memory locations (numbered 0 to 99), which can each hold a number between -999 and 999
An Accumulator to store the result of the last operation or calculation.
A Program Counter to store the address (in memory) of the next instruction to do.
An Instruction Register to hold the top digit of the instruction read from memory.
An Address Register to hold the bottom two digits of the instruction read from memory.
An Input tray into which a number can be typed when needed.
An Output area where any numbers output are printed.

The Program Counter holds only 2 digits (0 to 99) and the Accumulator holds 3 digits and a sign (-999 to 999). The Output area holds a certain amount of information in two columns and then scrolls (and old information disappears).

When an instruction is read from memory the top digit is used to decide what to do. To help us write programs more easily each instruction type is given a name which the ASSEMBLE function converts to the corresponding code. The bottom two digits are used as an address associated with the instruction. The instructions are:

Code	Name	Description
0	HLT	Stop (Little Man has a rest).
1	ADD	Add the contents of the memory address to the Accumulator
2	SUB	Subtract the contents of the memory address from the Accumulator
3	STA	Store the value in the Accumulator in the memory address given.
4		This code is unused and gives an error.
5	LDA	Load the Accumulator with the contents of the memory address given
6	BRA	Branch - use the address given as the address of the next instruction
7	BRZ	Branch to the address given if the Accumulator is zero
8	BRP	Branch to the address given if the Accumulator is zero or positive
9	INP	Input. Put the value from Input into the Accumulator.
9	OUT	Output. Put the value from the Accumulator into Output.
	DAT	Used to indicate a location that contains data.

Note that the LMC computer cannot tell whether memory locations contain instructions or data. Most modern computers can segment their memory but there has to be a way for the output from one program the become the instructions of another. DAT is a fake instruction to tell the assembler that this location contains data.

This webpage was modified from https://peterhigginson.co.uk/lmc/help_new.html