.MD File Format – Memory map

68000 address space

The 68000 has 24 address lines, so 16 MiB. The console divides it like this:

Ranges not listed are not decoded by the console (some are claimed by the Tera Drive: $AE0000–$BFFFFF). Reading or writing a range that nothing answers is a hardware matter this page does not cover; Plutiedev notes that an access to $C00020–$C0003F hangs the console.

The viewer’s Memory map table lists these ranges for the program being disassembled, and the code view names the addresses in the register tables.

VDP port addresses

All VDP ports are mirrored every other word, so that a longword access reaches the same port twice (the VDP sees two word accesses). For example the control port is at both $C00004 and $C00006. The whole group repeats every $40 bytes up to $DFFFFF.

What the ports do is on VDP ports and registers.

Cartridge-slot decoding

The cartridge slot carries the full 68000 address, so a cartridge may answer at any address. For ordinary use only a few signals matter (Plutiedev’s cartridge-slot page):

A read always fetches a word and the 68000 picks the bytes it wants.

The /CART line

The console can boot from the cartridge or the expansion slot. /CART chooses:

A bootable cartridge pulls it low. The Mega CD’s backup-RAM cartridges are not bootable and show up at $400000.

The Z80’s view

The Z80 has its own 16-bit address space, of which the Mega Drive uses these parts (Plutiedev’s Using the Z80 and Genesis Plus GX’s memz80.c):

Facts to know when reading or writing the Z80’s memory (Plutiedev): the Z80 program must run from address $0000 and fit in 8 KiB; the 68000 must use byte accesses to Z80 RAM (word accesses do not work); Z80 RAM cannot be reached while the Z80 is in reset or running, so the 68000 requests the bus first; the YM2612 is on the Z80 bus too, so the 68000 must own the bus to use it. Z80 DMA clashes: a video DMA started while the Z80 reads the 68000 bus can load garbage on some console revisions (it never happens in emulators), and the usual workaround is to hold the Z80 bus while a DMA runs.

The Z80 sound program is a block of Z80 code in the ROM that the 68000 copies into Z80 RAM; nothing in the header marks where it is. The viewer therefore disassembles it only where you give its offset and length.

Sources

• Memory map – Plutiedev: the 68000 address-space table and the VDP port list (read in full)
• Cartridge slot – Plutiedev: the signals, standard and advanced decoding, /CART, /M3, ranges reserved by the Mega CD, 32X and Tera Drive
• Using the Z80 – Plutiedev: Z80 RAM at $A00000, byte accesses, the bus handshake, DMA clash
• Hardware issues – Plutiedev: YM2612 status read at $A04000 / $4000, the PSG on the 68000 bus
• memz80.c – Genesis Plus GX: how the Z80 map is decoded ($0000–$3FFF RAM mirrored, $4000–$5FFF YM2612, $6000 bank register, $7F00–$7FFF VDP, $8000–$FFFF banked)

Not from a source read here, so inferred: that the 68000’s $A00000–$A0FFFF window shows the Z80’s $0000–$7FFF at the same low address (Plutiedev gives only $A00000 for RAM and $A04000 for the YM2612, which agree with that).