The worksheetDownload the PDF
Answers

A9.5 Assembly language: the Little Man Computer

Computer architecture · A level · OCR H446 1.2.4, AQA 7517 4.7.3.5 · about 25 min

BugBotLab

What this lesson is about

OCR's LMC instruction set: tracing and writing programs with selection and iteration, run in Python.

Questions 5 marks in all

  1. [1 mark]In the LMC, what does BRP do?

    1. ABranches if the accumulator is zero or positive
    2. BBranches if the accumulator is positive only
    3. CBranches always
    4. DBranches if the accumulator is zero
    Answer: A. BRP is branch if positive, and zero counts: it only fails to branch when the result is negative.

  2. [1 mark]Which LMC mnemonic stores the accumulator in a mailbox?

    Answer: STA. STA, code 3xx, stores; LDA, code 5xx, loads.

  3. [1 mark]An LMC program is INP, STA 9, INP, SUB 9, OUT, HLT. The inputs are 20 then 35. What is output?

    Answer: 15. The first input is stored in mailbox 9; the second, 35, has 20 subtracted from it.

  4. [1 mark]Why are DAT lines put after HLT?

    1. ASo the little man never fetches data and tries to execute it as an instruction
    2. BBecause the assembler only reads DAT at the end
    3. CTo make the program run faster
    4. DBecause data must be in mailbox 99
    Answer: A. The PC starts at mailbox 0; data placed first would be fetched as instructions.

  5. [1 mark]These mailboxes hold an assembled LMC program, run by this code. What does it output?

    mailboxes = [901, 308, 508, 902, 209, 308, 802, 0, 0, 1]
acc, pc, inputs = 0, 0, [2]
while True:
    ins = mailboxes[pc]
    pc += 1
    op, xx = ins // 100, ins % 100
    if ins == 0: break
    elif op == 2: acc -= mailboxes[xx]
    elif op == 3: mailboxes[xx] = acc
    elif op == 5: acc = mailboxes[xx]
    elif op == 8 and acc >= 0: pc = xx
    elif ins == 901: acc = inputs.pop(0)
    elif ins == 902: print(acc)
    Answer:
    2
1
0

    This is the countdown program: with input 2 it outputs 2, 1 and 0, then the accumulator goes negative and BRP does not branch.

The task: the larger of each pair

Write an LMC program in the string program. It repeatedly inputs a pair of numbers, first and second, and outputs the larger of the two (either one if they are equal). If the first number of a pair is 0, it stops at once without reading a second number. The inputs are whole numbers from 0 to 999. The task's inputs are 58, 23, 7, 19, 30, 30, 0, so the program must output OUT 58, OUT 19 and OUT 30, and nothing else. The folded helpers assemble and run_lmc are given: run_lmc prints each output as OUT and the value, and reads each input with input(). The robot does not move.

Any program that meets the task's checks is marked correct in the simulator; this is one way, not the only way.