LMC Branch Instructions (for implementing loops)

The following program will demonstrate how to use the branching instructions of the LMC to implement looping.  Note: The Branch Always instruction should only be used to connect paths indicated by your flowchart. 

Flowchart

The following is the flowchart for a program that will output the values from 1 to 10.

Program

LDA ONE
STA COUNT
OUT
LOOPTOP LDA COUNT
ADD ONE
OUT
STA COUNT
SUB TEN
BRP ENDLOOP
BRA LOOPTOP
ENDLOOP HLT
ONE DAT 001
TEN DAT 010
COUNT DAT

What you should do

1. Click on the "LMC Simulator Applet" link to start the LMC simulator.
2. Clear the Message Box and all of the LMC mailboxes -- click the "Clear Messages" button and the "Clear" button if necessary.
3. Copy the fourteen line program above and paste it into the Message Box
4. Click on the "Compile Program" button.
5. Click on the "Run" button.
6. 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 9 the instructions 511, 313, 902, 513, 111, 902, 313, 212, 810, 603.  The Program Counter should start at 0 (click on "Reset" if necessary).
• DAT reserves mailbox 11 for ONE, mailbox 12 for TEN, and mailbox 13 for COUNT.  A three-digit number after DAT represents the initial value that the compiler should store in the mailbox (e.g. 001 for ONE and 010 for TEN). 
• The LOOPTOP identifier is the first instruction in the loop.  When the code in the loop has been executed, a BRANCH always instruction (e.g. BRA LOOPTOP) causes the LMC to "jump" back to the start of the loop so that the code section can be executed again. 
• When you click on "Run" or "Step", the Message Box will describe the actions of each instruction.
• The first three instructions initialize the COUNT mailbox to 1 and output this value to the Out-Box.
• In the body of the loop, COUNT is loaded onto the Accumlator, one is added to it, the new value is output, and then the new value of COUNT is stored back to its mailbox.  The value of COUNT must be repeatedly loaded and stored because the value of COUNT is erased from the Accumulator when the loop termination condition is checked.
• The SUB instruction is used as part of checking the loop termination condition.  The loop is to execute ten times.  The SUB TEN instruction subtracts ten from the value of the Accumulator.
• If the value of the Accumulator is smaller, subtracting TEN from it will cause the Accumulator to become negative.  The BRP instruction will then do nothing, and the program will continue to mailbox 9.  Mailbox 9 represent the completion of the loop, so its instruction (BRA LOOPTOP) causes the LMC to return to the top of the loop so that it can be executed again.
• When the value of the Accumulator/COUNT becomes ten, subtracting TEN from it will cause the Accumulator to become zero.  The BRP instruction will now update the Program Counter to ENDLOOP (mailbox 10).