Addressing modes

CISC processors support multiple addressing modes for instructions that require transferring data between memory and registers. RISC processors have a more limited number of addressing modes. Each processor architecture defines its collection of addressing modes based on an analysis of the anticipated memory access patterns that software will use on that architecture.

To introduce the 6502 addressing modes, this section employs a simple example of 6502 code that adds together four data bytes. To avoid extraneous details, the example will ignore any carry from the 8-bit sum.

Immediate addressing mode

In immediate addressing, the operand value immediately follows the opcode in memory. For the first example, assume we are given the values of the four bytes to sum and asked to write a 6502 program to perform that task. This allows us to enter the byte values directly into our code. The bytes in this example are $01 through $04. We’ll be adding the bytes...