Using X as a pointer
Because the 65c816's index registers can be 16-bit, they can hold a 16-bit address. For programs that already use 16-bit index registers for other reasons (such as accessing level data), this can sometimes create an optimization opportunity, allowing for the use of faster addressing modes and smaller code.
Additional things to keep in mind:
absolute,xbut not any indirect addressing modes.
STY directpage,xexists, but not
- This optimization is mutually exclusive with the optimization where
DEX \ BPLis used at the end of a loop to avoid a compare.
- ROM can be accessed at 3.58 MHz, whereas RAM in the SNES is always 2.68 MHz; this means that cycles that access RAM are slower (8 master clocks) than ones that access ROM (6 master clocks).
Data in the first 8KB of RAM
This technique works best for data that's located within the first 8KB of RAM. Direct page instructions are limited to the first 64KB of address space which contains a mirror of the first 8KB of RAM.
If the direct page is at $0000 (or another multiple of $100) and index registers are 16-bit, every instruction that can be switched from
directpage,x is a byte and a cycle saved.
Data somewhere within a bank
For memory outside of the first 8KB of RAM,
directpage,x is not available, so this technique is much less likely to help if the code already uses
If the code is using an indirect addressing mode, however, the additional instructions allowed by
absolute,x may justify switching to it in some situations.
Array of structures example (ca65)
This example demonstrates how X can be used as a pointer within an array of structures. When X points to the first byte in a structure,
directpage,x can access a field in that struct.
.struct Actor Type .word PositionX .word PositionY .word VelocityX .word VelocityY .word .endstruct .segment "BSS" ActorCount = 10 ActorStart: .res ActorCount * .sizeof(Actor) ActorEnd: .segment "CODE" .a16 .i16 ; For each actor slot that's in use (type isn't zero), apply the velociy .proc ApplyVelocityToActiveActors ldx #ActorStart Loop: ; Do not process actors with a zero type lda Actor::Type,x beq Skip lda Actor::PositionX,x clc adc Actor::VelocityX,x sta Actor::PositionX,x lda Actor::PositionY,x clc adc Actor::VelocityY,x sta Actor::PositionY,x Skip: ; Move onto the next entry txa clc adc #.sizeof(Actor) tax ; Is it past the end yet? cpx #ActorEnd bcc Loop rts .endproc