ASM Hacking for Dummies

Introduction ### {: #introduction}

This document was written for the aspiring ROM hacker with little to no experience in 65c816 programming. The basics have been thoroughly covered and I believe this should prove a significant aid to anyone interested in 65c816 ROM hacking. Hopefully this proves true. If you benefit from this document or have suggestions for changes to make to it, please eMai ...

Writing the Header

Before writing your actual program's code, you need a header to describe WLA the SNES' memorymap. This is basically just a file you will include into your source. It contains all of the basic information about your ROM. The following is a basic header file for a 2 Mbit ROM. Look at the comments beside each line for a brief explanation of that line. I will provide a better explanation after reveali ...

Using the NMI-VBLANK

Earlier we made an interrupt table which assigned addresses for the SNES interrupts. We assigned the address to an empty handler that just returned for every interrupt except the NMI (VBLANK) interrupt. This interrupt occurs whenever the T.V.'s tracer is not tracing to the screen, making it safe to edit data in VRAM and update the screen. Not waiting for VBLANK can create awful results on the scre ...


This is a document intended to describe various aspects of SNES timing. It will probably not be useful unless you already know a good bit about the SNES.

BTW, special credit to byuu for the critical observation that the SNES returns to a known timing position on reset. Thus, a deterministic ROM (i.e. it doesn't depend on user input or any other randomness) will always give the same results on r ...

65816 Reference

Internal Registers

Mnemonic Friendly Name Description
A Accumulator The accumulator. This is the math register. It stores one of two operands or the result of most arithmetic and logical operations.
X, Y Index The index registers. These can be used to reference memory, to pass data to memory, or as counters for lo ...

SPC700 Reference


System Overview


8-bit SPC700, runs at ~1Mhz ...with the effective speed being half (each instruction takes a minimum of 2 cycles)


64KB (NOT 32KB), shared with everything.


4 8-bit I/O ports to transfer data to/from the SNES.


8x channels of ADPCM compressed sample goodness. The DSP also has special ha ...