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

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

How does branching work in LMC?

• A branch instruction makes the program jump to a different line instead of running the next line in sequence

• The line to jump to is marked with a label, e.g. loop or end; the assembler replaces each label with the memory address of that line

• BRA always branches, without checking the accumulator

• BRZ branches only if the value in the accumulator is zero

• BRP branches only if the value in the accumulator is zero or positive

• If the condition is not met, the program carries on with the next line

• To compare two values, subtract one from the other and then branch on the result:

  • LDA a, SUB b, BRZ same jumps if a and b are equal

  • LDA a, SUB b, BRP bigger jumps if a is greater than or equal to b

• To create a loop, use a branch that jumps back to a label earlier in the program

Tracing a loop

• This program counts down from a number that is input:

        INP
        STA count
loop    LDA count
        OUT
        SUB one
        STA count
        BRP loop
        HLT
one     DAT 1
count   DAT 0

• If 2 is input, the trace is:

• The outputs are 2, 1, 0

• A common mistake is to forget that BRP branches on zero as well as on positive values

Example 1: Add two numbers

        INP          // Input the first number
        STA first    // Store it in the memory location labelled first
        INP          // Input the second number
        ADD first    // Add the value stored in first to the accumulator
        OUT          // Output the result
        HLT          // End the program
first   DAT          // Reserves a memory location (address 6) labelled first

Example 2: Find the largest of three numbers

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

          INP              // Input the first number
          STA num1
          INP              // Input the second number
          STA num2
          INP              // Input the third number
          STA num3
          LDA num1
          SUB num2
          BRP compare13    // If num1 >= num2, compare num1 with num3
          LDA num2
          SUB num3
          BRP output2      // If num2 >= num3, output num2
          LDA num3
          OUT
          HLT
compare13 LDA num1
          SUB num3
          BRP output1      // If num1 >= num3, output num1
          LDA num3
          OUT
          HLT
output1   LDA num1
          OUT
          HLT
output2   LDA num2
          OUT
          HLT
num1      DAT
num2      DAT
num3      DAT

How it works:

• This worked example demonstrates branching logic and how comparisons are chained to identify the correct result using SUB and BRP

  • Inputs are stored in the memory locations labelled num1, num2 and num3

  • The program first checks if the first number ≥ second number

  • If so, it then checks the first vs third

  • If not, it checks if the second number ≥ third number

  • The value that is greatest is loaded into the accumulator and output