Low-Level Programming (Cambridge International A-Level Computer Science)

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

• 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

• 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

• 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)

• 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

• Used in control flow:

• Use for loops and conditional branches without needing absolute addresses