Assembling

lmc file format

An lmc file is composed of lines, with each line in the following format:

LABEL MNEMONIC ADDRESS

The ADDRESS is only needed for instructions which need an address to function. The LABEL is optional, and help with variables and loops.

For example, the following are legal lines:

• INP
• ONE DAT 1
• DAT - The value of DAT defaults to 0
• ADD ONE

The following are not legal lines:

• INP 1
• ADD
• HLT 42

Comments begin with # (python-style) and can be on the end of a line or take up a whole line:

• ADD X # Add the user value to the loaded constant
• # Main loop below

Instructions

x denotes an address.

Numeric Code	Mnemonic Code	Instruction	Description
000	HLT	Halt	Stop the processor. The program should always end with this.
1xx	ADD	Add	Add the contents of address xx to the accumulator
2xx	SUB	Subtract	Subtract the contents of address xx from the accumulator
3xx	STA	Store	Store the value of the accumulator to address xx
5xx	LDA	Load	Load the value at address xx to the accumulator
6xx	BRA	Branch	Unconditionally change the instruction counter to xx
7xx	BRZ	Branch if zero	Change the instruction counter to xx if the accumulator is 0
8xx	BRP	Branch if positive	Change the instruction counter to xx if the accumulator is 0 or greater [1]
901	INP	Input	Sets the accumulator to the value given by the user
902	OUT	Output	Outputs the value of the accumulator
xxx	DAT	Data declaration	Sets the value at its address to xxx. Defaults to 0 [2]

1. Since the LMC uses decimalised twos-complement, the comparison is accumulator < 500

2. DAT is not a real instruction, as it never makes it to the processor. Instead, it simply places its value instead of an instruction. E.g. DAT 123 results in the code 123. It can be used with labels to provide variables. E.g.:

           INP
           ADD ONE
           OUT
           HLT
   ONE     DAT 1
   

Assembly

yaplmc assembles a program in the following way:

• First read the program, remove comments, get the mnemonic and address (if any) and make a table of label to line
• The list of mnemonics is then converted into a list of numeric codes, with the address (if any) resolved (if it is a label) and combined with the numeric code

Assembly Errors

• Duplicate label 'X' on lines Y and Z - A label has been used twice. Change one of them to a different name.
• Invalid mnemonic at line X: 'Y' - Y is not a valid mnemonic. Only the ones above are accepted (no Easter eggs yet!).
• Invalid line X: 'Y' - Line X could not be parsed.
• Bad argument on line X: 'Y' - Y is not on integer or is too large.
• Instruction on line X takes an argument: 'Y' - Y takes an argument.
• Instruction on line X has no argument: 'Y' - Y does not take an argument.

Example Program

# Add two numbers

        INP
        STA FIRST
        INP
        ADD FIRST
        OUT
        HLT
FIRST   DAT

This program will assemble into the following numeric codes:

901
306
901
106
902
0
0

Other examples can be found in the examples folder.

Next page: Execution

Also see: [LMC Style Guide](./LMC Style Guide)