LMC Branching – Instructions To Implement Decisions

The following program will demonstrate the use of the three branch instructions: BRZ, BRP and BRA.

Running the program: INPUT a number and the program will OUTPUT a sequence of numbers, starting at 1 and finishing with the INPUT number. The program will continue until a 0 is entered.

start LDA zero STA count INP BRZ finish STA maxNumber loopTop LDA count ADD step STA count OUT SUB maxNumber BRP endLoop BRA loopTop endLoop BRA start finish HLT step DAT 1 maxNumber DAT count DAT zero DAT

0 LDA 17 1 STA 16 2 INP 3 BRZ 13 4 STA 15 5 LDA 16 6 ADD 14 7 STA 16 8 OUT 9 SUB 15 10 BRP 12 11 BRA 5 12 BRA 0 13 HLT 14 DAT 1 15 DAT 0 16 DAT 0 17 DAT 0

Instructions

• Copy the 18 line program above and paste it into the Program box.
• Click on the “Assemble Program” button.
• After the program is assembled you should see RAM addresses 0 to 13 contain the machine code instructions shown in the image.
• Click on the RUN button.
• If you have difficulty following what is happening, read the explanation below and use STEP instead of RUN so you can follow each step.

Explanation

step, maxNumber, count and zero are used to label DAT instructions. DAT identifies the 15th, 16th, 17th and 18th instructions as data. The labels therefore refer to RAM addresses 14, 15, 16 and 17 (0-indexed counting).

0 start LDA step //load the contents of memory address 17 (labelled zero) 
 into the accumulator.
1 STA count //store the accumulator contents in RAM address 16
 (labelled count).
2 INP //INPUT a number and store in the accumulator.
3 BRZ finish //if the accumulator contains zero then branch to the
 instruction at memory location 13 (labelled finish)
4 STA maxNumber //store the accumulator contents in RAM address 15 
 (labelled maxNumber).
5 loopTop LDA count //load the contents of memory address 16 (labelled count)
 into the accumulator. This instruction is the start
 of the counting loop.
6 ADD step //add the contents of RAM address 14 (labelled step, value = 1)
 to the accumulator and store the result in the accumulator.
7 STA count //store the accumulator contents in RAM address 16 (labelled 
 count).
8 OUT //OUTPUT the value in the accumulator.
9 SUB maxNumber //subtract the contents of RAM address 15 (labelled maxNumber)
 from the acccumulator and store the results in the 
 accumulator.
10 BRP endLoop //if the accumulator contains zero or a positive number then
 count has reached maxNumber so branch to the instruction at
 memory location 12 (labelled endLoop).
11 BRA loopTop //branch to the instruction at memory location 5 (labelled
 loopTop) to continue the counting loop.
12 endLoop BRA start //branch to the instruction at memory location 0 (labelled
 start) to zero the counter and INPUT another number.
13 finish HLT //halt the program.

Example results

INPUT = 5
OUTPUT = 1, 2, 3, 4, 5

Further Readings:

• Little man computer