Assembly language is a low-level language where each statement maps to one machine code instruction. It gives direct control of the processor and shows what high-level code really turns into.

Instruction format and addressing modes

An assembly instruction has an opcode and usually an operand. The Cambridge 9618 instruction set uses a single accumulator, ACC, and an index register, IX.

Common instructions:

Instruction Meaning
LDM #n Load the number n into ACC (immediate)
LDD addr Load the contents of addr into ACC (direct)
LDI addr Load using the address stored at addr (indirect)
LDX addr Load from addr + IX (indexed)
STO addr Store ACC in addr
ADD addr Add contents of addr to ACC
SUB addr Subtract contents of addr from ACC
INC / DEC reg Add or subtract 1
CMP addr Compare ACC with contents of addr
JMP addr Jump to addr
JPE addr / JPN addr Jump if last comparison was equal / not equal
IN / OUT Input or output a character
END Stop the program

Addressing modes: immediate (value is in the instruction), direct (instruction gives the address), indirect (address holds another address), indexed (address plus IX), and relative.

Writing a program

Labels name addresses so you do not count them by hand. This program adds 5 and 7 and stores the result:

        LDM #5
        ADD value
        STO result
        END
value:  7
result: 0

Here ADD value adds the number stored in memory at value to ACC. Stored data is declared after END.

A loop that counts from 0 to 4:

        LDM #0
        STO count
loop:   LDD count
        INC ACC
        STO count
        CMP limit
        JPN loop
        END
count:  0
limit:  5

CMP sets a flag; JPN jumps back to loop while the values are not equal. Tracing it with a table of ACC and count per step is a standard exam task.

Assemblers

An assembler translates assembly into machine code. A two-pass assembler reads the program once to build a table of label addresses, then a second time to produce the machine code with the real addresses filled in. Assembly is used for device drivers, embedded systems and speed-critical code, but it is hard to write, not portable between processor types, and slow to develop.

Check your understanding

What does LDM #5 do?

  • Loads the contents of address 5
  • Loads the value 5 into ACC
  • Stores 5 in memory
  • Jumps to address 5

Answer: Loads the value 5 into ACC. The # marks an immediate value.

Which addressing mode adds IX to the operand address?

  • Immediate
  • Direct
  • Indexed
  • Indirect

Answer: Indexed. It is useful for stepping through arrays.

What is the job of an assembler?

  • Run the program line by line
  • Translate assembly language into machine code
  • Compress files
  • Debug the CPU

Answer: Translate assembly language into machine code. It also resolves labels to addresses.

Why is assembly language not portable?

  • It only uses letters
  • Its instructions are specific to one processor family
  • It cannot use memory
  • It has no loops

Answer: Its instructions are specific to one processor family. Each CPU has its own instruction set.

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