# Sega Saturn Memory Map
Last modified 2020-01-11

Compiled from various resources, listed at end. Most of this information is
unverified, so let me know if there's issues here.

Note the following:
- Unmapped/"open bus" regions are unlisted
- "Mirrors At" is what interval the section contents are repeated at

"Mirroring & Open Bus" by Retro Game Mechanics Explained[1] is very helpful
in understanding this document.

# SH-2 (and other devices connected to the SCU)

| Start Addr | End Addr   | Size       | Mirrors At | Description      |
|------------|------------|------------|------------|------------------|
| 0x00000000 | 0x000FFFFF | 0x00100000 | 0x00080000 | Boot ROM         |
| 0x00100000 | 0x0017FFFF | 0x00100000 | 0x00000080 | SMPC registers   |
| 0x00180000 | 0x001FFFFF | 0x00080000 | 0x00010000 | Backup RAM       |
| 0x00200000 | 0x002FFFFF | 0x00100000 | ---------- | Work RAM Low     |
| 0x02000000 | 0x03FFFFFF | 0x02000000 | ---------- | A-Bus CS0        |
| 0x04000000 | 0x04FFFFFF | 0x01000000 | ---------- | A-Bus CS1        |
| 0x05800000 | 0x058FFFFF | 0x00100000 | ---------- | A-Bus CS2        |
| 0x05A00000 | 0x05AFFFFF | 0x00100000 | See notes  | Sound RAM        |
| 0x05B00000 | 0x05BFFFFF | 0x00100000 | 0x00001000 | SCSP registers   |
| 0x05C00000 | 0x05C7FFFF | 0x00080000 | ---------- | VDP1 VRAM        |
| 0x05C80000 | 0x05CFFFFF | 0x00080000 | 0x00040000 | VDP1 Framebuffer |
| 0x05D00000 | 0x05D7FFFF | 0x00080000 | ---------- | VDP1 Registers   |
| 0x05E00000 | 0x05EFFFFF | 0x00100000 | 0x00080000 | VDP2 VRAM        |
| 0x05F00000 | 0x05F7FFFF | 0x00080000 | 0x00001000 | VDP2 CRAM        |
| 0x05F80000 | 0x05FBFFFF | 0x00040000 | 0x00000200 | VDP2 registers   |
| 0x05FE0000 | 0x05FEFFFF | 0x00010000 | 0x00000100 | SCU registers    |
| 0x06000000 | 0x07FFFFFF | 0x02000000 | 0x00100000 | Work RAM High    |

- Backup RAM on odd bytes only; even bytes read $FF and write to that addr - 1
- Any attached cartridges are mapped to A-Bus CS0 and CS1 in a cart-specific
  manner.
- CD-ROM registers are mapped to A-Bus CS2
- 68k's work RAM is mirrored depending on the value of register MEM4MB (bit 9
  of $05B00400)
	- If set, all 512K of 68K-WRAM  is mapped to the first 0x800000, and
	  the remainder is open-bus
	- If cleared, the first 256K of 68K-WRAM is mirrored 4 times
- The framebuffer is the one *not* being displayed.
- Most of the VDP1 register area is unmapped
- VDP1 CRAM only works properly with word-wise writes. Byte-wise writes cause
  corruption in the un-written half of the word

## Sources
[1] https://youtu.be/PvfhANgLrm4
[2] https://wiki.yabause.org/index.php5?title=SH-2CPU
[3] https://antime.kapsi.fi/sega/files/ST-097-R5-072694.pdf