Simple Addition Program

This program adds two numbers entered by the user and outputs the result.

INP       // 901 - Input first number into the accumulator
STA 10    // 310 - Store the first number at address 10
INP       // 901 - Input second number into the accumulator
ADD 10    // 110 - Add the number at address 10 to the accumulator
OUT       // 902 - Output the result
HLT       // 000 - Halt the program

Explanation:

• The first input value is stored in mailbox 10.
• The second input is added to the value in mailbox 10, with the result displayed.

Simple Subtraction Program

This program subtracts one user-entered number from another.

INP       // 901 - Input the first number
STA 10    // 310 - Store the first number in address 10
INP       // 901 - Input the second number
SUB 10    // 210 - Subtract the number at address 10 from the accumulator
OUT       // 902 - Output the result
HLT       // 000 - Halt the program

Explanation:

• The first number is stored, and then the second input is subtracted from it, with the result displayed.

Loop Example: Count Down to Zero

This program accepts a positive integer and counts down to zero, displaying each number.

INP       // 901 - Input a number
STA 10    // 310 - Store the input in address 10
LDA 10    // 510 - Load the value at address 10 into the accumulator
OUT       // 902 - Output the current number
SUB 12    // 212 - Subtract 1 (at address 12) from the accumulator
STA 10    // 310 - Store the updated value back in address 10
BRP 06    // 806 - Branch to address 06 if the accumulator is positive
HLT       // 000 - Halt the program

// Data
12 DAT 1  // Stores the value 1 to decrement the counter

Explanation:

• The input number is repeatedly decremented and displayed until it reaches zero, using the BRP instruction to check for a positive number.

Common Mistakes

• Incorrect Addresses: Using incorrect addresses in LMC can result in unexpected behaviour, as it could reference or modify data.
• Forgetting to Halt: If the HLT command is missing, the program may continue running, leading to errors.
• Accumulator Overflow: Since LMC typically simulates limited memory, very large numbers or unanticipated results can cause overflow errors.

Key Takeaways

• Assembly Language is a low-level language used for efficient hardware control, suitable for systems programming and performance-critical applications.
• Little Man Computer (LMC) provides a simplified way to learn the principles of assembly language, focusing on a limited set of instructions.
• LMC Programs use basic commands like ADD, SUB, STA, and LDA to manipulate values in memory and control the flow of execution.
• Tracing and debugging assembly language code requires careful attention to each instruction and the changes it makes to memory and registers.