Genesis Memory Domains

by Brad Corrupts

Bizhawk-Vanguard exposes the following domains in RTC:

Genplus-gx

  • 68K RAM (Rewindable): Genesis main RAM region run by the 68K. 64KB large on a 16-bit bus. Main memory containing variables, data structures, etc. Rarely, if ever, contains actual code.

  • Z80 RAM (Rewindable): SPU work ram run by the Z80. 8KB large on an 8-bit bus. Sound engine and data are stored here. Note that some games use the 68K for their sound engine (Sonic 1 being a notable example).

  • MD CART: The ROM file.

  • BOOT ROM: The BIOS for the Sega/Mega CD. Don't touch this, unless you're doing BIOS corruptions.

  • CRAM (Rewindable): Palette ram. 128 bytes. (Not recommended. Some games refresh CRAM every frame. Also looks to be broken in this build.)

  • VSRAM (Rewindable): Vertical Scroll RAM. 128 bytes. (Not recommended. Only affects vertical scroll position in the game.)

  • VRAM (Rewindable): Video ram. 96KB. Holds the actual data drawn to the screen.

  • System Bus: Theoretically allows access to everything that's mapped as it facilitates the communication between pieces of hardware. In execution, not everything mapped will be visible through this domain. It's a matter of whether it was properly exposed or not. Corrupt something here and it'll be reflected in the domains derived from it.

    • 0x000000 - 0x3FFFFF- 4MB Cartridge ROM (MD Cart)

    • 0x400000 - 0x7FFFFF- Reserved (used by the Sega CD and 32x)

    • 0x800000 - 0x9FFFFF- Reserved (used by the 32x?)

    • 0xA00000 - 0xA0FFFF- Z80 addressing space (8K RAM mapped to x0000-0x1FFF)

    • 0xA10000 - 0xA10001- Version register (read-only word-long)

    • 0xA10002 - 0xA1001F- I/O Registers

    • 0xA11000- Memory mode register

    • 0xA11100 - 0xA11101- Z80 bus request

    • 0xA11200 - 0xA11201- Z80 reset

    • 0xA14000 - 0xA14003- TMSS register

    • 0xC00000 - 0xC00009- VDP registers

    • 0xFF0000 - 0xFFFFFF- 64KB 68K RAM Sega CD Changes

    • 0x000000 0x01FFFF- BIOS ROM

    • 0x020000 0x03FFFF- "Program RAM" Bank Access

    • 0x200000 0x23FFFF- "WORD RAM"

    • 0xA12000 0xA120XX- "Gate Array"

    • 0xFFFD00 0xFFFDFF- Interrupt/Exception vectors

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