Memory Mapping

=============================================================================
 Anomie's SNES Memory Mapping Doc
 $Revision: 1160 $
 $Date: 2008-12-21 12:40:39 -0500 (Sun, 21 Dec 2008) $
 <anomie@users.sourceforge.net>
=============================================================================

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, 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 0 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.