Little Man Computer Extended User Guide

Branching

Branching is the process of executing a different sequence of instructions.

Instructions

The following are instructions relevant to branching.

Instruction	Description
BRA	Branches to the given memory address.
BRZ	Branches to the given memory address, if the value in the Accumulator is 0.

Example (improved_calc)

Continuing with the trend of a calculator, let's build a dual-purpose calculator using the skills learnt above.

START    INP          # Waits for input and stores it in the accumulator
         STA MODE     # Stores the contents of the accumulator into the memory location of the label: MODE
         INP          # Waits for input and stores it in the accumulator
         STA VALUE_O  # Stores the contents of the accumulator into the memory location of the label: VALUE_O
         INP          # Waits for input and stores it in the accumulator
         STA VALUE_T  # Stores the contents of the accumulator into the memory location of the label: VALUE_T
         LDA MODE     # Loads the contents of the memory location of the label: MODE to the accumulator
         BRZ ADDITION # Branches to the memory location of the label: ADDITION is the contents of the accumulator are 0
         BRA SUBTRACT # Branches to the memory location of the label: SUBTRACT
ADDITION LDA VALUE_O  # Loads the contents of the memory location of the label: VALUE_O to the accumulator
         ADD VALUE_T  # Add the contents of memory address of the label: NUM to the accumulator
         BRA END      # Branches to the memory location of the label: END
SUBTRACT LDA VALUE_O  # Loads the contents of the memory location of the label: VALUE_O to the accumulator
         SUB VALUE_T  # Subtract the contents of memory address of the label: NUM to the accumulator
         BRA END      # Branches to the memory location of the label: END
END      OUT          # Outputs the contents of the accumulator
         HLT          # Signify the end of the program
MODE     DAT 1        # Defines the label: MODE as containing data, and sets it to a preset of 1
VALUE_O  DAT 1        # Defines the label: VALUE_O as containing data, and sets it to a preset of 1
VALUE_T  DAT 2        # Defines the label: VALUE_T as containing data, and sets it to a preset of 2