AS · Practice questions · Addressing modes

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Six original Cambridge-style questions on addressing modes, with mark-scheme style solutions.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

One question per mode.

Immediate (#): the operand is the value.
Direct: go to the address once.
Indirect: go to the address, read another address, go there.
Indexed: add IX to the operand first. Relative: offset from the current instruction.

Recall

[3 marks]

Describe immediate, direct and indexed addressing.

Immediate: the operand is the value to be used. ✓
Direct: the operand is the address that holds the value. ✓
Indexed: the address used is the operand plus the contents of the index register. ✓

Trace

[4 marks]

The index register holds 3. Memory holds:

Address	500	501	502	503	504
Contents	502	9	500	14	31

Give the contents of the ACC after each instruction is executed: (a) LDD 502, (b) LDI 502, (c) LDX 501, (d) LDM #502.

(a) 500 (the contents of 502). ✓
(b) 502 (502 holds 500; 500 holds 502). ✓
(c) 31 (501 + 3 = 504, which holds 31). ✓
(d) 502 (the value itself). ✓

Explain

[2 marks]

Explain the difference between indirect addressing and indexed addressing.

Indirect: the operand is the address of a location that holds the address of the data. ✓
Indexed: the address of the data is found by adding the contents of the index register to the operand. ✓

Apply

[2 marks]

Using the memory in question 2, with IX holding 3, the ACC holds 500. State whether the compare in each instruction is True or False: (a) CMI 500, (b) CMP 503.

(a) 500 holds 502; 502 holds 500; the ACC is 500, so True. ✓
(b) 503 holds 14; the ACC is 500, so False. ✓

Explain

[2 marks]

Explain why relative addressing allows a program to be relocated in memory.

Each address is given as an offset from the address of the current instruction. ✓
So if the whole program is moved, the offsets still point to the correct locations (no addresses need changing). ✓

Analysis

[2 marks]

A programmer wants to add 10 to the accumulator but writes ADD 10. Explain the error and correct it.

ADD 10 uses direct addressing: it adds the contents of address 10. ✓
It should be ADD #10 (immediate addressing) to add the number 10. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.