LDA and STA Instructions – LMC Load and Save

The following program will demonstrate the use of LDA and STA, the Load and Save instructions used in the LMC.

Running the program: when prompted, INPUT a set of two numbers. The program should then OUTPUT them in the same order that they were entered.

INP STA first INP STA second LDA first OUT LDA second OUT HLT first DAT second DAT

0 INP 1 STA 9 2 INP 3 STA 10 4 LDA 9 5 OUT 6 LDA 10 7 OUT 8 HLT 9 DAT 0 10 DAT 0

Instructions

• Copy the eleven 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 8 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

first and second are used to label a DAT instruction. DAT identifies the 10th and 11th instructions as data. The labels therefore refer to RAM addresses 9 and 10 respectively (0-indexed counting).

0 INP //the first input value is copied into the accumulator.
 1 STA first //the accumulator contents are stored in RAM address 9 (labelled
 first).
 2 INP //the second input value is copied into the accumulator.
 3 STA second //the accumulator contents are stored in RAM address 10 (labelled
 second).
 4 LDA first //the contents of memory address 9 (labelled first) are loaded into
 the accumulator.
 5 OUT //the value in the accumulator is sent to the OUTPUT.
 6 LDA second //the contents of memory address 10 (labelled second) are loaded into
 the accumulator.
 7 OUT //the value in the accumulator is sent to the OUTPUT.
 8 HLT //the program halts.

Example results

INPUT= 2, 3
OUTPUT = 2, 3

Further Readings:

• Little man computer