SNES Development
Memory Mapping

This is a document intended to describe the SNES memory map. It will NOT include information on the MAD-1 or any other address decoder.

Hardware

The SNES has one 8-bit data bus and two address busses typically known as "Address Bus A" and "Address Bus B".

Data Bus

The data bus is 8 bits. If nothing sets a value on the data bus for any particular read request, the last value placed on the bus is read instead (this is known as "Open Bus").

LINE | CART | EXPAND
-----+------+--------
 D0  |  19  |   11
 D1  |  20  |   12
 D2  |  21  |   13
 D3  |  22  |   14
 D4  |  50  |   15
 D5  |  51  |   16
 D6  |  52  |   17
 D7  |  53  |   18
Address Bus A

This address bus is 24 bits, along with read and write lines (/RD and /WR) and 2 auxiliary lines (/CART and /WRAM). WRAM is connected to this bus, as well as the cart connector.

LINE | CART          LINE  | CART
-----+------        -------+------
 A0  |  17            A14  |  39
 A1  |  16            A15  |  40
 A2  |  15            A16  |  41
 A3  |  14            A17  |  42
 A4  |  13            A18  |  43
 A5  |  12            A19  |  44
 A6  |  11            A20  |  45
 A7  |  10            A21  |  46
 A8  |   9            A22  |  47
 A9  |   8            A23  |  48
 A10 |   7            /RD  |  23
 A11 |   6            /WR  |  54
 A12 |  37           /CART |  49
 A13 |  38           /WRAM |  32
Address Bus B

This address bus is 8 bits, along with read and write lines (/RD and /WR). d /WRAM). WRAM, PPU1, PPU2, and APU are connected to this bus, as well as the cart and expansion ports.

LINE | CART | EXPAND
-----+------+--------
 PA0 |  28  |    1
 PA1 |  59  |    2
 PA2 |  29  |    3
 PA3 |  60  |    4
 PA4 |  30  |    5
 PA5 |  61  |    6
 PA6 |   3  |    7
 PA7 |  34  |    8
/PARD|   4  |   10
/PAWR|  35  |    9

Memory Map

As far as the SNES is concerned, this is the memory map. "LoROM", "HiROM", and anything else is just the cart responding differently to the addresses placed on Address Bus A.

WRAM responds to Address Bus A whenever /WRAM is active, and to registers $2180–$2183 on Address Bus B. APU responds to $40–$7F on Address Bus B. PPU1 and PPU2 respond to $2100–$213F on Address Bus B. The cart is expected to respond when /CART is active, but it can respond to any address on either bus that is not otherwise mapped. Similarly, the device plugged into the expansion port may respond to any unmapped register on Address Bus B.

The ‘Speed’ column indicates the memory access speed for that area of memory. The SNES master clock runs at about 21MHz (probably as close to 1.89e9/88 Hz as possible). Internal operation CPU cycles always take 6 master cycles. Fast memory access cycles also take 6 master cycles, Slow memory access cycles take 8 master cycles, and XSlow memory access cycles take 12 master cycles.

  Banks  |  Addresses  | Speed | Mapping
---------+-------------+-------+---------
 $00-$3F | $0000-$1FFF | Slow  | Address Bus A + /WRAM (mirror $7E:0000-$1FFF)
         | $2000-$20FF | Fast  | Address Bus A
         | $2100-$21FF | Fast  | Address Bus B
         | $2200-$3FFF | Fast  | Address Bus A
         | $4000-$41FF | XSlow | Internal CPU registers (see Note 1 below)
         | $4200-$43FF | Fast  | Internal CPU registers (see Note 1 below)
         | $4400-$5FFF | Fast  | Address Bus A
         | $6000-$7FFF | Slow  | Address Bus A
         | $8000-$FFFF | Slow  | Address Bus A + /CART
---------+-------------+-------+---------
 $40-$7D | $0000-$FFFF | Slow  | Address Bus A + /CART
---------+-------------+-------+---------
 $7E-$7F | $0000-$FFFF | Slow  | Address Bus A + /WRAM
---------+-------------+-------+---------
 $80-$BF | $0000-$1FFF | Slow  | Address Bus A + /WRAM (mirror $7E:0000-$1FFF)
         | $2000-$20FF | Fast  | Address Bus A
         | $2100-$21FF | Fast  | Address Bus B
         | $2200-$3FFF | Fast  | Address Bus A
         | $4000-$41FF | XSlow | Internal CPU registers (see Note 1 below)
         | $4200-$43FF | Fast  | Internal CPU registers (see Note 1 below)
         | $4400-$5FFF | Fast  | Address Bus A
         | $6000-$7FFF | Slow  | Address Bus A
         | $8000-$FFFF | Note2 | Address Bus A + /CART
---------+-------------+-------+---------
 $C0-$FF | $0000-$FFFF | Note2 | Address Bus A + /CART

Note 1: The address for internal CPU registers may go out Address Bus A, however the CPU ignores the data bus. It is unknown whether the data bus is ignored for the whole memory region, or just for those addresses which are actually registers. It is also unknown whether CPU writes show up on the data bus or not. Current theory is that addresses and writes will show up, but reads may or may not, and the data bus is only ignored for those bits of those registers actually mapped (e.g., data bus is ignored for only bit 7 of $4211).

Note 2: If bit 1 of CPU register $420d is set, the speed is Fast, otherwise it is Slow.

History

Version 1.1: Jun 18, 2003 * Tested the memory access speed of all 256-byte memory blocks, and filled in the table with the findings.

Version 1.0: * Initial version.

by Anomie (anomie@users.sourceforge.net)