Memory Map

Flash memory is split into a series of equal-sized "blocks", which is the smallest unit of data which can be read from or written to flash. A standard-sized VMU has 256 total blocks of 512 bytes each, giving a total capacity of 128KB.

The following is the generalized memory map of the flash address space for a single formatted VMU partition, along with its contents and region names:

Block	Contents	Region
0	Game File	User
...
...
...	Data Files
...
...
...
...
...
...
...	Reserved	Hidden
...
...
...
...
...	Directory	System
...
...
...	FAT
N	Root

Note: Despite the fact that there was only ever one volume memory card released commercially, and all "extended" memory cards used multiple switchable banks containing the same sized volumes, the information presented here is given for a generically-sized VMU, as these are technically supported by the Maple protocol and many commercial games recognize up to 1MB sizes.; The default values for standard-sized VMUs are also given in the field descriptions.

System Region

The system region comprises three different subregions: the Root block, the FAT blocks, and the Directory blocks. All three must be present and correctly configured for a well-behaved filesystem.

Root

The Root block (or Superblock) is essentially a single block which is reserved for describing the volume and the layout of its filesystem regions. It is typically located within the last indexable block of the filesystem.

Byte	0	2	4	6	8	A	C	E	F
0x000	Magic
0x010	Volume Label
0x020	Volume Label
0x030	Timestamp				Reserved
0x040	Volume Last	Partition	Root	FAT First	FAT Size	DIR Last	DIR Size	Icon	Sort
0x050	Hidden First	Hidden Size	Game First	Game Size	Reserved
0x060	Reserved
...
0x1F0

[0x000] Magic (16 bytes):
A 16-byte region where the byte value 0x55 must be repeated in order to signify that the volume is properly formatted and initialized to the VMU and Dreamcast BIOSes.

[0x010] Volume Label (32 bytes):
From the Maple protocol's perspective, the Volume Label is described as being an arbitrary user identifier for the volume, which can contain anything. In reality, it's not used by the VMU, but it has the following specific format for the Dreamcast's BIOS:

Offset	Size	Name	Description
0x00	1	custom_color	Set to `0x01` if using custom volume color, otherwise `0x00`.
0x01	1	blue	`0-255` value for the blue channel of the custom volume color.
0x02	1	green	`0-255` value for the green channel of the custom volume color.
0x03	1	red	`0-255` value for the red channel of the custom volume color.
0x04	1	alpha	`0-255` value for the alpha channel of the custom volume color (`0`: opaque, `255`: transparent).
0x05-0x20	27	unused	All bytes set to `0x00`.

Note: Most likely these 27 additional bytes could store anything and will be gracefully ignored by the DC's BIOS.

[0x030] Timestamp (8 bytes):
The date and time at which the VMU was last formatted.

Offset	Size	Name	Description
0x00	1	century	First two digits of the AD year (`00-99`).
0x01	1	year	Last two digits of the AD year (`00-99`).
0x02	1	month	Month (`1=Jan thru 12=Dec`).
0x03	1	day	Day of the month (`1-31`).
0x04	1	hour	Hour of the day (`0-23`).
0x05	1	minute	Minute of the hour (`0-59`).
0x06	1	second	Second of the minute (`0-59`).
0x07	1	week_day	Day of the week (`0=Mon thru 6=Sun`)

Note: All values within the timestamp are in binary-coded decimal (BCD) format!

[0x040] Volume Last (2 bytes):
The last indexable block on the volume or total # of blocks in the volume minus 1. (default: 255).

[0x042] Partition (2 bytes):
Partition # for this volume (default: 0x00).

[0x044] Root (2 bytes):
Block # of (this) root block (default: 255).

[0x046] FAT First (2 bytes):
Block # of the first FAT block (default: 254).

[0x048] FAT Size (2 bytes):
Size (in blocks) of the FAT region (default: 1).

[0x04a] DIR Last (2 bytes):
Block # of the last Directory block (default: 253).

[0x04c] DIR Size (2 bytes):
Size (in blocks) of the Directory region (default: 13).

[0x04e] Icon (1 byte):
VMU volume icon as displayed within the Dreamcast's BIOS (values: 0-123).

[0x04f] Sort (1 byte):
Unknown or unused flag (default: 0x00).

Note: Perhaps for sorting order within the DC's BIOS?

[0x050] Hidden First (2 bytes):
Block # of the first Hidden block (default: 200).

[0x052] Hidden Size (2 bytes):
Size (in blocks) of the Hidden region (default: 31).

[0x054] Game First (2 bytes):
Block # where the first block of a GAME file must be written (default: 0).

[0x056] Game Size (2 bytes):
Largest size (in blocks) of a GAME file (default: 128).

[0x058] Reserved (408+ bytes):
Claimed to be reserved, all filled with 0x00; however, it's unlikely that these bytes actually have to be zero'd out. Most likely they can be used for arbitrary storage.

FAT

The File Allocation Table (FAT) subregion within the filesystem is a contiguous array of entries, starting at the block given by FAT First and growing upwards to the block located at FAT First + FAT Size - 1.

The FAT contains a 16-bit FAT entry for every block in the filesystem, meaning its size should be 2 * Total Number of Blocks / Block size, or 1 block for the default 256 block VMU volume.

The address of a given block's FAT entry is calculated by: Fat Entry for Block N = Fat First * Block Size + 2 * N.

Each FAT Entry contains one of the following values:

Value	Name	Description
`0xfffc`	Unallocated	The block is unused or free.
`0xffff`	Damaged	The block has been damaged and its contents are to be allocated from the Hidden region.
`0xfffa`	Last In File	The block is the last one within its file and has no next block.
Other	Next In File	The block is proceeded by the another block in the file.

The FAT Entry should immediately tell you whether a block is in-use, free, or damaged. In the case of an in-use block, its FAT Entry should always contain the block # of the next block in the file sequence, creating a linked-list of blocks, whose next pointers are its FAT Entries, with the final block within the file terminating with a FAT Entry value of 0xfffa.

Note: When formatting a VMU, the FAT Entries for System blocks are such that each subregion is treated like a contiguously-stored file. The FAT Entry for the Root Block should be 0xfffa. The FAT Entries for the FAT should form a linked-list starting at FAT First, with each Entry pointing to the next block in the sequence up to the last block, which should be terminated with 0xfffa. The FAT Entries for the Directory should be linked in a similar fashion; however, they point in the opposite direction, as the Directory grows from highest-to-lowest block order.

Hidden Region

The hidden region is a special reserved region for storing information as the regular VMU blocks of other regions slowly wear out and can no longer be written to successfully. This storage redundancy is meant to be a mechanism to extend the flash's lifespan.

When a block within another region is no loner viable, it is marked with a value of 0xffff within its FAT Entry, and the storage for the block can be allocated from the Hidden Region.

When a block within the FAT becomes damaged, the entire FAT must be relocated to the Hidden Region, as it must be stored within a contiguously-allocated set of blocks.

When the entire Hidden Region becomes used, blocks at the high-end of the User Region begin being used, as the total capacity of the VMU's volume slowly becomes reduced.

Note: The Hidden Region is not supposed to be required for any particular volume, and its size can be set to 0 blocks; however, this apparently causes some issues with certain presumptuous commercial games which will not detect the extended volume properly.

User Region

The User Region designates the area where regular user-visible files are stored. Two different types of user files exist: DATA and GAME files, each with different layout requirements.

Data Files

Data files are typically save files from Dreamcast games. Their blocks are allocated from the highest to the lowest block within the User Region. Their blocks do not need to be contiguous and can jump around, as the FAT Entry for each block will point to its next block. There is also no limit on the number of data files which can be stored within a VMU (although realistically the size of the Directory and number of entries within it will become a bottleneck).

Game File

Game files are special files which can be executed from the VMU when used in standalone mode. These are typically in the form of minigames which get saved from a main Dreamcast game. Only one of these files may be present at a time on a single volume. Furthermore, each game file must be allocated starting at block 0, growing CONTIGUOUSLY up towards the Game Size limit set within the Root Block.

Warning: Since data files are stored from highest-to-lowest block and have no requirement for continuity, whereas game files are stored from lowest-to-highest block and must be contiguous, a scenario can easily be encountered where a volume technically has enough free blocks to store a game, yet these blocks are not contiguous or do not start at block 0. It is imperative that any filesystem manager for such a volume be able to perform a "defragmentation" operation for such a circumstance, in order for game files to be written properly.

Byte	0	1	2	3	4	5	6	7	8	9	A	B	C	D	E	F
0x000	Type	Copy	Start FAT		File Name
0x010	Timestamp								Size		Header		Reserved

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