Lesson 6. Assembly Language (LMC)

Lesson Objective

Explain the Von Neumann architecture, with reference to the Little Man Computer and assembly language.
Use basic assembly language code to demonstrates how a processor fetches, decodes and executes data.
Create complex assembly language code to implement your own ideas/programs in the Little Man Computer.

Assembly language uses mnemonics to represent the operation codes.
Typically, 2, 3 or 4 character codes are used to represent all the machine code instructions.
There are different assembly languages for each different type of processor.
The assembler translates the assembly language program into machine code for execution.

This is an imaginary computer with a very limited instruction set, created by Dr Stuart Madnick in 1965.

For the purpose of the simulation, the table shows some instruction that can be used.

Note: There is no 4! This code is unused and gives an error.

The LMC has only 11 instructions, and the imaginary computer on which it runs has only 100 memory locations.

On a real computer, there will be many more instructions.

An instruction register to hold the first digit of the instruction read from memory - this is opcode.

An address register to hold the second and third digit of the instruction read from memory - this is operand.

LMC Instruction Set

Code	Mnemonic	Instruction	Code+	Description
1	ADD	ADD	1xx	Add the contents of the memory address to the Accumulator.
2	SUB	SUBTRACT	2xx	Subtract the contents of the memory address from the Accumulator.
3	STA	STORE	3xx	Store the value in the Accumulator in the memory address given.
5	LDA	LOAD	5xx	Load the Accumulator with the contents of the memory address given.
6	BRA	BRANCH (unconditional)	6xx	Branch - use the address given as the address of the next instruction.
7	BRZ	BRANCH IF ZERO (conditional)	7xx	Branch to the address given if the Accumulator is zero.
8	BRP	BRANCH IF POSITIVE (conditional)	8xx	Branch to the address given if the Accumulator is zero or positive.
9	INP	INPUT	901	Input into the accumulator.
9	OUT	OUTPUT	902	Output contents of accumulator.
0	HLT	Halt	0	Stops the execution of the program.
	DAT	DATA		Used to indicate a location that contains data.