Assembly Language & Little Man Computer (OCR A Level Computer Science)

Assembly Language & Little Man Computer

What is the Purpose of Assembly Language?

• Assembly language sits between high-level languages (like Python, Java) and machine code (binary code executed by the computer hardware)

• Allows developers to write more efficient, albeit more complex, code when compared to high-level languages

• Direct access and manipulation of hardware components, e.g., registers, memory addresses

• Each type of computer CPU has its specific assembly language

Levels of Abstraction of Programming Languages

Little Man Computer

• The Little Man Computer (LMC) is a hypothetical computer model used for understanding the fundamental operations and mechanics of a computer

• The LMC is a simplified version of a computer

• It has key elements like memory, a calculator, an accumulator, and an instruction set

Little Man Computer Instruction set

• The following mnemonics represent different actions that can take place in an LMC program

Example 1: Add two numbers

INP;     // Input the first number
STA 90;  // Store the first number in memory location 90
INP;     // Input the second number
ADD 90;  // Add the number in memory location 90 to the accumulator
OUT;     // Output the result
HLT;     // End the program
DAT;     // Memory location 90 for storing data

Example 2: Find the smallest of three numbers

This program inputs three numbers and determines the smallest of the three, outputting the result.

INP                             // Input the first number
STA 91                          // Store the first number in memory location 91
INP                             // Input the second number
STA 92                          // Store the second number in memory location 92
INP                             // Input the third number
STA 93                          // Store the third number in memory location 93

LDA 91                          // Load the first number
SUB 92                          // Subtract the second number
BRP CHECK_THIRD_FROM_FIRST      // If result is positive, then first number > second number

LDA 92                          // Load the second number
SUB 93                          // Subtract the third number
BRP OUTPUT_SECOND               // If result is positive, then second number > third number
LDA 93
OUT                             // Output the third number
HLT

CHECK_THIRD_FROM_FIRST:
    LDA 91
    SUB 93
    BRP OUTPUT_FIRST
    LDA 93
    OUT
    HLT

OUTPUT_FIRST:
    LDA 91
    OUT
    HLT

OUTPUT_SECOND:
    LDA 92
    OUT
    HLT

DAT                             // Memory locations for data storage
DAT
DAT