LMC Add and Subtract Instructions

The following program will demonstrate the add and subtract instructions of the LMC. Note: The Accumulator (calculator) in the LMC is only designed to work with non-negative three-digit numbers. Mathematical operations which produce values greater than 999 or less than 000 can cause undefined effects to occur.

Program

INP
STA FIRST
INP
ADD FIRST
OUT
INP
SUB FIRST
OUT
HLT
FIRST DAT

What you should do

Click on the "LMC Simulator Applet" link to start the LMC simulator.
Clear the Message Box and all of the LMC mailboxes -- click the "Clear Messages" button and the "Clear" button if necessary.
Copy the ten line program above and paste it into the Message Box
Click on the "Compile Program" button.
Click on the "Run" button.
When prompted, enter three-digit numbers in the "In-Box", and press the "Enter" button.

What you should see

After the program is compiled, you should see from mailbox 0 to 7 the instructions 901, 309, 901, 109, 902, 901, 209, 902. The Program Counter should start at 0 (click on "Reset" if necessary).
DAT is the tenth instruction of your program, so it refers to mailbox 9 (0-indexed counting). FIRST is the identifier that has been declared to represent this mailbox in the assembly language program.
When you click on "Run" or "Step", the Message Box will describe the actions of each instruction.
After the first INP instruction, the Accumulator has a copy of the first value entered in the In Box.
After the STA instruction, the input value is copied from the Accumulator to mailbox 9.
After the second INP instruction, the Accumulator has a copy of the second value entered into the In Box (try values that will lead to three and four digit sums).
After the ADD instruction, the Accumulator value represents the sum of the two input values -- 1 means ADD and 09 refers to the mailbox where the value to be added to the Accumulator is stored.
After the third INP instruction, the Accumulator has a copy of the third value entered into the In Box (try values that will lead to positive and negative results).
After the SUB instruction, the Accumulator value represents the difference between the two input values -- 2 means SUBTRACT and 09 refers to the mailbox where the value to be subtracted from the Accumulator is stored.