Fetch-Decode-Execute Cycle (OCR A Level Computer Science)

Exam Tip

• You’ll need to know how to write assembly language using the Little Man Computer instruction set but now we’re just covering how the registers are used in assembly language

Worked Example

A program written in the Little Man Computer instruction set is given below.

      INP    
      STA num
loop 	LDA total    
      ADD num    
      STA total    
      LDA count    
      ADD one    
      STA count    
      SUB num    
      BRZ end    
      BRA loop
end   LDA total    
      OUT    
      HLT
one 	DAT 1
num 	DAT 0
count 	DAT 0
total 	DAT 0

Explain which registers are used and their values when the line STA count is executed and the accumulator is holding the value 9. The label count refers to memory location 16.

[2]

How to answer this question:

• The instruction being executed in this example is STA count, so the registers used will be:

  • ACC - the accumulator is holding the value "9"

  • MDR - the value "9" from the ACC will be copied to the MDR

  • MAR - the value 16 will be stored here so the data is sent to memory location 16

• The value that’s in the accumulator (ACC) is going to be stored in memory (RAM)

• To work out where in memory it will be stored we need to know what count represents

• In this question, we've been told it's 16 - "the label count refers to memory location 16"

• You're not always told this in your exam, so you should also be able to count the lines of code

• It’s best to write the line numbers on the code in the question, to see which line count is on. (Don’t forget that the first line is memory location 0!)

• "Count DAT 0 "is on line 16, so the value "16" is what goes to the MAR register. (You already know this from the question above)

• It is always the value from the accumulator (ACC) that is stored, so the value "9" must go to the MDR as data can only be sent to memory from the MDR

• Then the value is sent to the memory location, in this case 16

Answer:

 Example answer that gets full marks:

The contents of the accumulator (9 in this case) will be copied to the MDR [1 mark] and then 9 is copied to location 16. [1 mark]

Acceptable answers you could have given instead:

The value 16 is copied to the MAR.   [1 mark]

Exam Tip

• When answering a question about registers be specific about the contents of the register based on the question to make sure you get the marks

• If your answer to the question above didn't include the values 16 and 9 then you wouldn't get the marks despite knowing which registers are used. This is because the question is asking you which registers are used and their values when the line STA count is executed