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 far 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, e.g. x86 or ARM

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

Mnemonic	Instruction	Alternative Mnemonic
ADD	Add
SUB	Subtract
STA	Store	STO
LDA	Load	LOAD
BRA	Branch always	BR
BRZ	Branch if zero	BZ
BRP	Branch if positive OR zero	BP
INP	Input	IN, INPUT
OUT	Output
HLT	End program	COB, END
DAT	Data location

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 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 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
SUB 92
BRP compare13     // If the first number is greater than or equal to the second, compare it to the third
LDA 92
SUB 93
BRP output2       // If the second number is greater than or equal to the third, output it
LDA 93
OUT
HLT
 
compare13 LDA 91
SUB 93
BRP output1       // If the first number is greater than or equal to the third, output it
LDA 93
OUT
HLT
 
output1   LDA 91
OUT
HLT
 
output2   LDA 92
OUT
HLT
 
DAT
DAT
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 memory locations 91, 92, and 93
  • 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

Worked Example

A digital thermostat has a CPU that uses the Little Man Computer Instruction Set.

The thermostat allows users to set a desired room temperature. The acceptable range for room temperature settings is between 15 and 25 degrees Celsius, inclusive. If the user sets a temperature within the range, the code outputs a 1 indicating a valid setting. If the temperature setting is outside of the acceptable range, the code outputs a 0 indicating an invalid setting.

The code is as follows:

INP
STA tempSetting
 
LDA tempSetting
SUB minTemp
BRP checkMax      // If tempSetting ≥ 15, check against max
 
LDA invalidVal
BRA end
 
checkMax LDA maxTemp
SUB tempSetting
BRP isValid       // If maxTemp - tempSetting ≥ 0, input is valid
 
LDA invalidVal
BRA end
 
isValid  LDA validVal
 
end      OUT
HLT
 
validVal   DAT 1
invalidVal DAT 0
minTemp    DAT 15
maxTemp    DAT 25
tempSetting DAT

a) What is the purpose of the label checkMax in the code? Describe its role in determining the validity of the temperature setting.

[2]

b) If a user inputs a temperature setting of 14, what will be the output? Justify your answer.

[2]

c) If the acceptable range of temperature settings was expanded to include temperatures up to 30 degrees Celsius, what modification would be needed in the code?

[2]

Answer:

Example answer that gets full marks: a) The label checkMax marks the section of code that checks whether the temperature setting is less than or equal to the maximum allowed value (25). It is used to continue validation only after confirming that the input is greater than or equal to the minimum temperature (15). If the temperature is within range, the program continues from checkMax to determine if it is also below the maximum; otherwise, it skips this section.

b) The output will be 0. This is because 14 is less than the minimum allowed value of 15. The program compares the input to the minimum temperature first, and since 14 is below 15, it does not continue to checkMax and instead loads and outputs the invalid value (0).

c) The value stored at the maxTemp label should be changed from 25 to 30. This allows the program to correctly validate temperatures up to and including 30 degrees when comparing the input against the maximum allowed temperature.

Acceptable answers you could have given instead:

a) Any response mentioning that checkMax it is for checking if the user’s input is below or equal to the maximum allowable temperature should be awarded marks.

b) Any answer stating that the output will be 0 because 14 is less than 15, or similar logic, should be awarded marks.

c) Any answer suggesting a change to the maxTemp DAT value to 30 should be awarded marks.