Addressing Modes

Assembler coding - Lesson 4

eZine lover (@eZine)

Published in 

Assembler coding

 · 2 years ago

Addressing Modes

Here we will discuss some addressing modes of the C64.

Below are some of the easier Addressing Techniques available on the C64.

Implicit Addressing

This is where the instruction operates exclusively inside the processor.

Example

  10 TAY - Transfer Contents of Accumulator to Y Register

Immediate Addressing

This is where the op-code is followed by a constant.

Example

  10 LDA #50

Absolute Addressing

This is where the op-code is followed by a 16 bit memory address.

Example

  10 LDA $4000

or

  10 LDA 16384 (This is the same but just in decimal)

Indexed Addressing

This method is used to access a block of data. Here a 16 bit address and an 8 bit register are added together to give the final memory location.

Examples

  10 LDA BLOCK,X - (Load value into accumulator from address BLOCK + X register) 
  10 LDA $4000,X 
  10 STA TABLE,Y

Other modes are available but not readily used in early programming techniques.

The Programmers Reference Guide lists all op-codes and addressing modes available for each instruction.