Memory Map

Table of Contents

• RAM Memory Space
• ROM Memory Space
• Work RAM

Map of the VMU's various memory spaces.

The VMU is a strange beast with multiple disjoint address spaces for data, instructions, and more. Understanding the various segments of the memory map and their layout is required to master the VMU's architecture from a software perspective.

RAM Memory Space

The following table shows the various segments and banks of the main RAM or the internal data address space. This address space is the target of regular data transfer instructions, unless otherwise specified. Spanning the memory space is XRAM, SFRs, and RAM. XRAM has 3 banks of 64, 64, and 16 bytes respectively. The SFR range spans 143 bytes. Main system memory is broken into two banks of 256 bytes each. Bank 0 contains the stack and is reserved for use by the BIOS and OS routines exclusively. Bank 1 is where the user's generic application memory resides.

Address	Bank 0	Bank 1	Bank 2
1B0-1BF	XRAM - Upper Half of Framebuffer	XRAM - Lower Half of Framebuffer	UNUSED
1A0-1AF
190-19F
180-18F			XRAM - Icons
170-17F	Special Function Registers	UNUSED	UNUSED
160-16F
150-15F
140-14F
130-13F
120-12F
110-11F
100-10F
0F0-0FF	Stack Area	Application/User RAM
0E0-0EF
0D0-0DF
0C0-0CF
0B0-0BF
0A0-0AF
090-09F
080-08F	System/BIOS RAM
070-07F
060-06F
050-05F
040-04F
030-03F
020-02F
010-01F
000-00F

ROM Memory Space

Due to the fact that the VMU can execute either directly from the BIOS, residing on a built-in ROM chip, or from a GAME user application residing in flash, the device treats the instruction space as a generic memory space, which can swap between sourcing instructions between ROM and flash depending on the CPU state. ROM is a single chip spanning 64KB of space, while flash is 2 banks of 64KB each.

Address	ROM	Flash (Bank 0)	Flash (Bank 1)
F0FF-FFFF	UNUSED	For GAME/DATA Use	For DATA Use Only
E000-EFFF	OS Program Area
4000-DFFF	Reserved Area
0000-3FFF	System Program Area

Work RAM

Work RAM is an additional, separate memory space from main RAM. It offers the VMU an additional 512 bytes which can be utilized as more general-purpose storage when in standalone mode. While connected to a Dreamcast, this storage functions as a Maple TX/RX packet buffer for the Maple communications stack. WRAM is accessed one byte at a time, with a target address given by the two VRMAD SFRs (VRMAD1 and VRMAD2) and read/writes being issued via accessing the VTRBF SFR.

Address	Description
1FF	512 Bytes Additional Work RAM
...
000

Note

Interestingly, the contents of WRAM will persist even when the user changes MODEs to the BIOS and back to GAME. Apparently the BIOS does not use WRAM. Unfortunately, the minute the VMU is plugged into a Dreamcast controller, WRAM will be overwritten by incoming and outgoing Maple packets, though.