Low-Level Programming (Cambridge (CIE) A Level Computer Science): Revision Note

Low-level programming

How do I write low-level code using addressing modes?

• Machine code and assembly language are examples of low-level languages and were introduced in section 4 - Assembly Language Basics

• Before continuing, make sure you understand the different addressing modes

Immediate addressing

• Syntax: LOAD #value

• Meaning: Load a constant value directly into the accumulator

LOAD #5       ; Load the constant value 5 into the accumulator
ADD #3        ; Add 3 to the accumulator
STORE 200     ; Store result at memory address 200

• Use when you want to work with literal values, not data in memory

Direct addressing

• Syntax: LOAD address

• Meaning: Load the value stored at the specified memory address

LOAD 100      ; Load the value stored at memory address 100
ADD 101       ; Add the value at address 101
STORE 102     ; Store result in address 102

• Use when the data is stored at known memory addresses

Indirect addressing

• Syntax: LOAD @address

• Meaning: The address given points to another address, where the data is stored

LOAD @150     ; Load value from the address stored in location 150
STORE 200     ; Store it in address 200

• Example:

  • Memory[150] = 300

  • Memory[300] = 42

  • LOAD @150 loads 42 into the accumulator.

• Use when data is stored in dynamically referenced locations

Indexed addressing

• Syntax: LOAD base[X]

• Meaning: Load from base address + index register (e.g. X)

LOAD 500[X]   ; Load value from address (500 + contents of index register X)
ADD 501[X]    ; Add value from another offset

• Assume:

  • X = 2

  • This will access address 502 and 503

• Use for working with arrays or tables, where the index changes

Relative addressing

• Syntax: BRANCH offset

• Meaning: Jump to a new instruction relative to the current instruction

BRZ +3        ; If accumulator is zero, skip 3 lines ahead
BRP -2        ; If positive, jump 2 lines back

• Used in control flow:

LOOP:  LOAD #1
       SUB 200
       BRZ END
       BR -3      ; Loop back to LOOP
END:   HALT

• Use for loops and conditional branches without needing absolute addresses