← IGCSE Computer Science Paper 1

Mnemonics in assembly language

A short, memorable code in assembly language, such as LDA, ADD or STO, that stands for one machine code instruction and is easier to remember than binary.

Part of: IGCSE Computer Science 0478 — 4.2 Programming Languages, Translators and IDEs — Module 11

Understanding Mnemonics in assembly language in IGCSE Computer Science Paper 1

What it does: ADD is a short, memorable word, whereas the matching binary pattern would be hard to recall and easy to mistype. Each processor has its own instruction set, so a program using its mnemonics will not run on a different type of processor without being rewritten. A mnemonic is a short code like LDA, ADD or STO. Each one stands for a single machine code instruction. Mnemonics are easier to remember than binary. The CPU only understands machine code, so an assembler must translate them first. ADD is the mnemonic, which gives the operation. The 5 is the operand, which gives the data or address to use.

Common mistakes: STO is the mnemonic for the store instruction. It copies the accumulator into memory, so the comment should say 'store result', not 'load result'. A mnemonic like ADD needs an operand to say which address to use. The line should be ADD 11.

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This module's key terms are also in a printable crossword (Modules 419–429) and word search (Modules 419–439).

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Learning

Duo

What is a mnemonic in assembly language?

A long English sentence that describes a whole programA short, memorable code such as ADD that stands for one machine code instruction

Matches

Each mnemonic such as ADD stands for a whole group of 10 machine code instructions

↓

A mnemonic stands for exactly one machine code instruction, not a group of them.

Applying

Meanings

Term

mnemonic

Meaning

A short, memorable code in assembly language, such as LDA, ADD or STO, that stands for one machine code instruction and is easier to remember than binary.

Practice

LDA 10

Testing

Gaps

Check Answer

Spots

2ADD 21 ; add value

3STO 22 ; load result

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Quick Questions

1. What is a mnemonic in assembly language?

A. A long English sentence that describes a whole program

B. A short, memorable code such as ADD that stands for one machine code instruction ✓

A mnemonic is a short code like LDA, ADD or STO. Each one stands for a single machine code instruction.

2. Which of these is an assembly language mnemonic?

A. 10110011

B. STO ✓

STO is a mnemonic (store). 10110011 is binary machine code, not a mnemonic.

3. Why do programmers use mnemonics instead of writing binary?

A. Mnemonics are easier for humans to remember and read ✓

B. Mnemonics run faster because the CPU understands them directly

Mnemonics are easier to remember than binary. The CPU only understands machine code, so an assembler must translate them first.

4. In the instruction ADD 5, what is ADD?

A. The mnemonic ✓

B. The operand

ADD is the mnemonic, which gives the operation. The 5 is the operand, which gives the data or address to use.

5. In the instruction LDA 20, what is 20?

A. The operand ✓

B. The mnemonic

20 is the operand. The mnemonic is LDA, which tells the computer what to do.

Match the Pairs

LDA 20 → A mnemonic with an operand: it loads the value stored at address 20 into the accumulator

STO 35 → A mnemonic with an operand: it stores the accumulator value into memory address 35

A mnemonic is easier for a programmer to remember than the binary instruction it stands for → ADD 12

Assembly language is machine dependent, so mnemonics only work on the type of processor they were written for → LDA 7 (written for one processor type)

Each mnemonic such as ADD stands for a whole group of 10 machine code instructions → A mnemonic stands for exactly one machine code instruction, not a group of them.

Mnemonics like LDA and STO will run on any computer because they are not binary → Mnemonics are machine dependent. They only work on processors that use the same instruction set.

Key Term

mnemonic

A short, memorable code in assembly language, such as LDA, ADD or STO, that stands for one machine code instruction and is easier to remember than binary.

Fill in the Blanks

1. LDA 20 ADD 21 STO 22

Hint: Type the mnemonic that loads the value at address 20 into the accumulator.

2. LDA 20 ADD 21 STO 22

Hint: Type the mnemonic that adds the value at address 21 to the accumulator.

3. LDA 20 ADD 21 STO 22

Hint: Type the mnemonic that stores the accumulator result into memory address 22.

4. LDA 20 ; load from address 20 ADD 21 ; add value at address 21 STO 22 ; store in address 22

Hint: Type the operand, the memory address the comment says ADD should use.

5. LDA 20 ; LDA is a mnemonic for one instruction STO 22

Hint: Type the term for a short, memorable code used in assembly language instead of binary.

Spot the Error

Question 1

A LDA 20 ; load value

B ADD 21 ; add value

C STO 22 ; load result

D END

Line C — STO is the mnemonic for the store instruction. It copies the accumulator into memory, so the comment should say 'store result', not 'load result'.

Question 2

A LDA 10

B ADD

C STO 12

D END

Line B — A mnemonic like ADD needs an operand to say which address to use. The line should be ADD 11.

Question 3

A LDA 40

B ADD 41

C STO 42 43

D END

Line C — Each mnemonic stands for one machine code instruction and takes one operand here. STO 42 43 has two operands. It should be STO 42.

Question 4

A LDA 50

B 51 ADD

C STO 52

D END

Line B — An assembly instruction is written mnemonic first, then the operand. It should be ADD 51, not 51 ADD.

Question 5

A LDA 70 ; get number

B SUB 70 ; double it

C STO 71

Line B — To double a number you add it to itself using ADD. SUB 70 would take the number away from itself and give 0. It should be ADD 70.

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