LMC Branch Instructions (for making decisions)

The following program will demonstrate how to use the branching instructions of the LMC to implement decision making.  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 largest of two input values.  Note: if the values are equal, both numbers represent the largest value, so either can be output.

Program

INP
STA FIRST
INP
STA SECOND
SUB FIRST
BRP SECONDBIG
LDA FIRST
OUT
BRA PROGRAMEND
SECONDBIG LDA SECOND
OUT
PROGRAMEND HLT
FIRST DAT
SECOND 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 10 the instructions 901, 312, 901, 313, 212, 809, 512, 902, 611, 513, 902.  The Program Counter should start at 0 (click on "Reset" if necessary).
• DAT reserves mailbox 12 for FIRST and mailbox 13 for SECOND.
• SECONDBIG and PROGRAMEND are identifiers like FIRST and SECOND that the assembly language program can use to label certain mailboxes.  When an identifier appears before an LMC instruction (e.g. FIRST DAT or PROGRAMEND HLT), the identifier is declared to the compiler, and the compiler records which mailbox the instruction is stored in (e.g. mailbox 12 for DAT and mailbox 11 for HLT).  When an identifier appears after an LMC instruction (e.g. STA FIRST or BRP SECONDBIG), the compiler converts that identifier into a mailbox to complete the machine code instruction (e.g. STA is machine code 3 and FIRST refers to mailbox 12, so STA FIRST becomes machine code 312).
• When you click on "Run" or "Step", the Message Box will describe the actions of each instruction.
• The first four instructions store your two input values.
• The SUB instruction is used to determine which of the two values is larger. 
• If the FIRST value is larger, subtracting it from the SECOND value will cause the Accumulator to become negative.  The BRP instruction will then do nothing, and the program will continue to mailbox 6.  After the LOAD and OUTPUT instructions to output the FIRST value to the Out-Box, the program will use the BRA instruction to skip the set of instruction that output the SECOND value and go straight to the next/last instruction of the program -- HLT.
• If the FIRST value is smaller (or equal), subtracting it from the SECOND value will leave the Accumulator with a positive (or zero) value.  The BRP instruction will then update the Program Counter -- 8 means BRANCH on zero or positive and 09 refers to the new value of the Program Counter (i.e. the mailbox which holds the first instruction of the sequence that will output the SECOND value).  This BRANCH instructions causes the LMC to skip the part of the program that outputs the FIRST value.  After outputing the SECOND value, the program continues on with the next/last instruction of the program -- HLT.