The Little Man Computer (LMC)

Gap-fill exercise

Fill in all the gaps, then press "Check" to check your answers. Use the "Hint" button to get a free letter if an answer is giving you trouble. Note that you will lose points if you ask for hints!
The Little Man Computer (LMC)
This is a simplified model of a Central Unit and can you to understand how one works.
A program is written in language and when this is the LMC program converts each instruction into a three-digit code. These codes are placed in memory addresses, along with any data that makes up the program.
Each memory slot has an address between 00 and 99. Each slot can therefore contain either one 3-digit instruction OR some data.
The processor works by an instruction from memory, it and then fetching the next instruction etc. This is called the fetch-execute cycle.
The accumulator box displays the contents of the accumulator, either loaded data or the results of a .
The LMC has one input, where data is entered, and one outout (from the ).
The counter stores the memory address of the next instruction that is going to be .
The LMC Instruction Set
The machine code for the LMC is very simple. Each 3 digit instruction has 1 digit, the OP-code, which tells the LMC which instruction to execute, and 2 extra digits.
For the 7 instructions below, the 2 extra digits show the memory address where the data is stored or where the program counter is to branch to.
  • LOAD
  • STORE
  • ADD
  • SUBTRACT
  • BRANCH IF ZERO
  • BRANCH IF ZERO OR POSITIVE
  • BRANCH ALWAYS
While the program is running these 7 instructions the MEM address box displays the address the instruction is referring to and the MEM data box displays the data stored at that address.

For the 2 instructions below the 2 extra digits define if the instruction is an (01) to the accumulator or an output (02) from it.
  • INPUT
  • OUTPUT
While the program is running these 2 instructions or the END instruction, the MEM address box and the MEM data box displays can therefore be ignored.
Details of the full instruction set


mnemonic - LDA
machine code - 5
Action - Load the contents of the given memory slot onto the .

STORE
mnemonic -
machine code - 3
Action - Store the contents of the accumulator to the memory slot of the given address.


mnemonic - ADD
machine code - 1
Action - Add the contents of the given memory slot onto the accumulator.

SUBTRACT
mnemonic -
machine code - 2
Action - Subtract the contents of the given memory slot from the accumulator.


mnemonic - INP
machine code - 901
Action - Copy the value from the "in box" onto the accumulator.

OUTPUT
mnemonic -
machine code - 902
Action - Copy the value from the accumulator to the "out box".

END
mnemonic -
machine code - 000
Action - Causes the Little Man Computer to stop executing the program.

BRANCH IF ZERO
mnemonic -
machine code - 7
Action - If the contents of the accumulator are 000, the PC (program counter) will be set to the given address.

BRANCH IF ZERO OR
mnemonic - BRP
machine code - 8
Action - If the contents of the accumulator are or positive, the PC (program counter) will be set to the given address.

BRANCH ALWAYS
mnemonic -
machine code - 6
Action - Set the contents of the PC (program counter) to the given address.

DATA LOCATION
mnemonic -
machine code - (the data)
Action - none, instead this instruction will reserve the next memory slot for storage.