Machine Code and Assembly Language | OCR H446

Learning Objectives

By the end of this topic you will be able to:

Understand what machine code is and why it is the only language directly executed by the CPU

Describe the structure of an assembly language instruction (opcode + operand)

Interpret and write simple assembly programs using the OCR reference language

Explain the role of the assembler

Machine Code

Machine code is the lowest-level programming language, consisting entirely of binary digits (0s and 1s). It is the only language the CPU can execute directly — all other languages must be translated into machine code first.

Each instruction is a fixed-length binary pattern specific to the CPU's instruction set architecture (ISA). Instructions are processor-specific: code for an Intel x86 CPU will not run on an ARM processor.

Machine code is very difficult for humans to read and write directly. Errors are hard to spot, and code is completely non-portable across different processor architectures.

01001000 11000011 00000001 ← example machine code instructions

Assembly Language

Assembly Language Structure

Assembly language is a low-level language that uses mnemonics (short human-readable codes) as a one-to-one mapping to machine code instructions. Each assembly instruction translates to exactly one machine code instruction.

Instruction Format

OPCODE OPERAND

LDR R1, #5
ADD R1, R1, R2
STR R1, 200

Key Terms

Opcode: the operation to perform (e.g. LDR, ADD, STR, MOV, CMP, B)

Operand: the data or address to operate on (register, literal, memory address)

OCR Reference Language

Common Assembly Instructions (OCR)

Data Movement

LDR R1, #5    ; R1 ← 5
LDR R1, 200   ; R1 ← mem[200]
STR R1, 200   ; mem[200] ← R1
MOV R1, R2   ; R1 ← R2

Arithmetic & Logic

ADD R3, R1, R2 ; R3 = R1+R2
SUB R3, R1, R2 ; R3 = R1-R2
AND R1, R1, R2
ORR R1, R1, R2
EOR R1, R1, R2
MVN R1, R2 ; bitwise NOT

Comparison & Branch

CMP R1, #5   ; set flags
B LOOP     ; unconditional
BEQ LOOP    ; if equal
BNE LOOP    ; if not equal
BGT LOOP    ; if greater
BLT LOOP    ; if less

Shifts

LSL R1, R1, #2 ; left shift
LSR R1, R1, #1 ; right shift

Left shift = × 2
Right shift = ÷ 2
per bit shifted

The Assembler

Role of the Assembler

An assembler is a program that translates assembly language source code into machine code. It produces a one-to-one translation — each mnemonic maps to exactly one machine code instruction.

Two-pass assembler (common): first pass scans code to build a symbol table of all labels and addresses; second pass translates instructions replacing labels with their resolved addresses.

Symbol table: a table maintained by the assembler that maps label names (e.g. LOOP) to their memory addresses, enabling forward references to be resolved.

Unlike a compiler, an assembler does not optimise the code — the output is a near-direct binary equivalent of the assembly source.

Exam Practice

OCR H446 Style · 4 marks

Trace through the following assembly program and state the value in R3 at the end.
LDR R1, #8 LDR R2, #3 SUB R3, R1, R2 LSL R3, R3, #1

[4 marks]

LDR R1, #8 → R1 = 8

LDR R2, #3 → R2 = 3

SUB R3, R1, R2 → R3 = 8 − 3 = 5

LSL R3, R3, #1 → R3 = 5 × 2 = 10 (left shift by 1 = multiply by 2)

Common Mistakes

Don’t Lose Marks

Saying assembly and machine code are the same — assembly uses human-readable mnemonics; machine code is binary. The assembler translates assembly into machine code. They are not the same thing.

Getting the operand format wrong — # means an immediate (literal) value e.g. #5; without # it refers to a memory address. These are fundamentally different and examiners will penalise confusion.

LSL and LSR direction confusion — LSL (Left Shift Logical) moves bits left, equivalent to multiplying by 2 per bit. LSR moves bits right, equivalent to dividing by 2 per bit.

Machine Code &Assembly Language