SameBoy/BESS.md

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BESS Best Effort Save State

Motivation

BESS is a save state format specification designed to allow different emulators, as well as majorly different versions of the same emulator, to import save states from other BESS-compliant save states. BESS works by appending additional, implementation-agnostic information about the emulation state. This allows a single save state file to be read as both a fully-featured, implementation specific save state which includes detailed timing information; and as a portable "best effort" save state that represents a state accurately enough to be restored in casual use-cases.

Specification

Note: This specification is currently under development and will potentially change in the near future

Every integer used in the BESS specification is stored in Little Endian encoding.

BESS works by appending a detectable footer at the end of an existing save state format. The footer uses the following format:

Offset from end of file Content
-8 Offset to the first BESS Block, from the file's start
-4 The ASCII string 'BESS'

BESS blocks

BESS uses a block format where each block contains the following header:

Offset Content
0 A four-letter ASCII identifier
4 Length of the block, excluding header

Every block is followed by another blocked, until the END block is reached.

CORE block

The CORE block uses the 'CORE' identifier, and is a required block that contains both core state information, as well as basic information about the BESS version used. This block must be the first block, except for the NAME block.

The length of the CORE block is 0xD0 bytes, but implementations are expected to ignore any excess bytes. Following the BESS block header, the structure is as follows:

Offset Content
0x00 Major BESS version as a 16-bit integer
0x02 Minor BESS version as a 16-bit integer

Both major and minor versions should be 1. Implementations are expected to reject incompatible majors, but still attempt to read newer minor versions.

Offset Content
0x04 A four-character ASCII model identifier

BESS uses a four-character string to identify Game Boy models:

  • The first letter represents mutually-incompatible families of models and is required. The allowed values are 'G' for the original Game Boy family, 'S' for the Super Game Boy family, and 'C' for the Game Boy Color and Advance family.
  • The second letter represents a specific model within the family, and is optional (If an implementation does not distinguish between specific models in a family, a space character may be used). The allowed values for family G are 'D' for DMG and 'M' for MGB; the allowed values for family S are 'N' for NTSC, 'P' for PAL, and '2' for SGB2; and the allowed values for family C are 'C' for CGB, and 'A' for the various GBA line models.
  • The third letter represents a specific CPU revision within a model, and is optional (If an implementation does not distinguish between revisions, a space character may be used). The allowed values for model GD (DMG) are '0' and 'A', through 'C'; the allowed values for model CC (CGB) are '0' and 'A', through 'E'; the allowed values for model CA (AGB, AGS, GBP) are '0', 'A' and 'B'; and for every other model this value must be a space character.
  • The last character is used for padding and must be a space character.

For example; 'GD ' represents a DMG of an unspecified revision, 'S ' represents some model of the SGB family, and 'CCE ' represent a CGB using CPU revision E.

Offset Content
0x08 The value of the PC register
0x0A The value of the AF register
0x0C The value of the BC register
0x0E The value of the DE register
0x10 The value of the HL register
0x12 The value of the SP register
0x14 The value of IME (0 or 1)
0x15 The value of the IE register
0x16 Execution state (0 = running; 1 = halted; 2 = stopped)
0x17 Reserved, must be 0
0x18 The values of every memory-mapped register (128 bytes)

The values of memory-mapped registers should be written 'as-is' to memory as if the actual ROM wrote them, with the following exceptions and note:

  • Unused registers have Don't-Care values which should be ignored
  • Unused register bits have Don't-Care values which should be ignored
  • If the model is CGB or newer, the value of KEY0 (FF4C) must be valid as it determines DMG mode
  • Sprite priority is derived from KEY0 (FF4C) instead of OPRI (FF6C) because OPRI can be modified after booting, but only the value of OPRI during boot ROM execution takes effect
  • If a register doesn't exist on the emulated model (For example, KEY0 (FF4C) on a DMG), its value should be ignored.
  • BANK (FF50) should be 0 if the boot ROM is still mapped, and 1 otherwise, and must be valid.
  • Implementations should not start a serial transfer when writing the value of SB
  • Similarly, no value of NRx4 should trigger a sound pulse on save state load
  • And similarly again, implementations should not trigger DMA transfers when writing the values of DMA or HDMA5
  • The value store for DIV will be used to set the internal divisor to DIV << 8
  • Implementation should apply care when ordering the write operations (For example, writes to NR52 must come before writes to the other APU registers)
Offset Content
0x98 The size of RAM (32-bit integer)
0x9C The offset of RAM from file start (32-bit integer)
0xA0 The size of VRAM (32-bit integer)
0xA4 The offset of VRAM from file start (32-bit integer)
0xA9 The size of MBC RAM (32-bit integer)
0xAC The offset of MBC RAM from file start (32-bit integer)
0xB0 The size of OAM (=0xA0, 32-bit integer)
0xB4 The offset of OAM from file start (32-bit integer)
0xB9 The size of HRAM (=0x7F, 32-bit integer)
0xBC The offset of HRAM from file start (32-bit integer)
0xC0 The size of background palettes (=0x40 or 0, 32-bit integer)
0xC4 The offset of background palettes from file start (32-bit integer)
0xC8 The size of object palettes (=0x40 or 0, 32-bit integer)
0xCC The offset of object palettes from file start (32-bit integer)

The contents of large buffers are stored outside of BESS structure so data from an implementation's native save state format can be reused. The offsets are absolute offsets from the save state file's start. Background and object palette sizes must be 0 for models prior to Game Boy Color.

An implementation needs handle size mismatches gracefully. For example, if too large MBC RAM size is specified, the superfluous data should be ignored. On the other hand, if a too small VRAM size is specified (For example, if it's a save state from an emulator emulating a CGB in DMG mode, and it didn't save the second CGB VRAM bank), the implementation is expected to set that extra bank to all zeros.

NAME block

The NAME block uses the 'NAME' identifier, and is an optional block that contains the name of the emulator that created this save state. While optional, it is highly recommended to be included in every implementation it allows the user to know which emulator and version is compatible with the native save state format contained in this file. When used, this block should come first.

The length of the NAME block is variable, and it only contains the name and version of the originating emulator in ASCII.

XOAM block

The XOAM block uses the 'XOAM' identifier, and is an optional block that contains the data of extra OAM (addresses 0xFEA0-0xFEFF). This block length must be 0x60. Implementations that do not emulate this extra range are free to ignore the excess bytes, and to not create this block.

MBC block

The MBC block uses the 'MBC ' identifier, and is an optional block that is only used when saving states of ROMs that use an MBC. The length of this block is variable and must be divisible by 3.

This block contains an MBC-specific number of 3-byte-long pairs that represent the values of each MBC register. For example, for MBC5 the contents would look like:

Offset Content
0x0 The value 0x0000 as a 16-bit integer
0x2 0x0A if RAM is enabled, 0 otherwise
0x3 The value 0x2000 as a 16-bit integer
0x5 The lower 8 bits of the ROM bank
0x6 The value 0x3000 as a 16-bit integer
0x8 The bit 9 of the ROM bank
0x9 The value 0x4000 as a 16-bit integer
0xB The current RAM bank

An implementation should parse this block as a series of writes to be made. Values outside the 0x0000-0x7FFF and 0xA000-0xBFFF ranges are not allowed.

RTC block

The RTC block uses the 'RTC ' identifier, and is an optional block that is used while emulating an MBC3 with an RTC. The contents of this block are identical to 64-bit RTC saves from VBA, which are also used by SameBoy and different emulators such as BGB.

The length of this block is 0x30 bytes long and it follows the following structure:

Offset Content
0x00 Current seconds (1 byte), followed by 3 bytes of padding
0x04 Current minutes (1 byte), followed by 3 bytes of padding
0x08 Current hours (1 byte), followed by 3 bytes of padding
0x0C Current days (1 byte), followed by 3 bytes of padding
0x10 Current high/overflow/running (1 byte), followed by 3 bytes of padding
0x14 Latched seconds (1 byte), followed by 3 bytes of padding
0x18 Latched minutes (1 byte), followed by 3 bytes of padding
0x1C Latched hours (1 byte), followed by 3 bytes of padding
0x20 Latched days (1 byte), followed by 3 bytes of padding
0x24 Latched high/overflow/running (1 byte), followed by 3 bytes of padding
0x28 UNIX timestamp at the time of the save state (64-bit)

HUC3 block

The HUC3 block uses the 'HUC3' identifier, and is an optional block that is used while emulating an HuC3 cartridge to store RTC and alarm information. The contents of this block are identical to HuC3 RTC saves from SameBoy.

The length of this block is 0x11 bytes long and it follows the following structure:

Offset Content
0x00 UNIX timestamp at the time of the save state (64-bit)
0x08 RTC minutes (16-bit)
0x0A RTC days (16-bit)
0x0C Scheduled alarm time minutes (16-bit)
0x0E Scheduled alarm time days (16-bit)
0x10 Alarm enabled flag (8-bits, either 0 or 1)

SGB block

The SGB block uses the 'SGB ' identifier, and is an optional block that is only used while emulating an SGB or SGB2 and SGB commands enabled. Implementations must not save this block on other models or when SGB commands are disabled, and should assume SGB commands are disabled if this block is missing.

The length of this block is 0x39 bytes and it follows the following structure:

Offset Content
0x00 The size of the border tile data (=0x2000, 32-bit integer)
0x04 The offset of the border tile data (SNES tile format, 32-bit integer)
0x08 The size of the border tilemap (=0x800, 32-bit integer)
0x0C The offset of the border tilemap (LE 16-bit sequences, 32-bit integer)
0x10 The size of the border palettes (=0x80, 32-bit integer)
0x14 The offset of the border palettes (LE 16-bit sequences, 32-bit integer)
0x18 The size of active colorization palettes (=0x20, 32-bit integer)
0x1C The offset of the active colorization palettes (LE 16-bit sequences, 32-bit integer)
0x20 The size of RAM colorization palettes (=0x1000, 32-bit integer)
0x24 The offset of the RAM colorization palettes (LE 16-bit sequences, 32-bit integer)
0x28 The size of the attribute map (=0x168, 32-bit integer)
0x2C The offset of the attribute map (32-bit integer)
0x30 The size of the attribute files (=0xfd2, 32-bit integer)
0x34 The offset of the attribute files (32-bit integer)
0x38 Multiplayer status (1 byte); high nibble is player count, low nibble is current player (0-based)

If only some of the size-offset pairs are available (for example, partial HLE SGB implementation), missing fields are allowed to have 0 as their size, and implementations are expected to fall back to a sane default.

END block

The END block uses the 'END ' identifier, and is a required block that marks the end of BESS data. Naturally, it must be the last block.