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The SNES ROM layout describes how the ROM banks appear in a ROM image and in the SNES address space. Most games use either LoROM or HiROM layout; some emulators or ROM tools will tell you whether a ROM image uses LoROM or HiROM. Assemblers and emulators use LoROM or HiROM to map between ROM offsets and SNES addresses.

Banks Edit

The maximum size of a LoROM or HiROM image is 4 MB. All ROM banks within the image have the same size.

Each LoROM bank is $8000 bytes (32 kilobytes).
Each HiROM bank is $10000 bytes (64 kilobytes) in HiROM.
An image contains only LoROM banks or only HiROM banks, not both.

Some tools will try to handle any size ROM (not above the 4 MB limit), but a better way is to have a whole number of ROM banks, and the best way is to match the number of ROM banks with the ROM size byte at address $ffd7 in the SNES header, which would imply that the number of ROM banks is a power of 2. The next table shows the number of ROM banks in common ROM sizes.

ROM size byte	number of ROM banks
$08 (256 kilobytes)	8 LoROM banks	4 HiROM banks
$09 (512 kilobytes)	16 LoROM banks	8 HiROM banks
$0a (1 megabyte)	32 LoROM banks	16 HiROM banks
$0b (2 megabytes)	64 LoROM banks	32 HiROM banks
$0c (4 megabytes)	128 LoROM banks	64 HiROM banks

LoROM Edit

LoROM maps each ROM bank into the upper half (being addresses $8000 to $ffff) of each SNES bank, starting with SNES bank $00, and starting again with SNES bank $80.

offsets in headerless ROM image	SNES addresses First mirror	SNES addresses Second mirror
x000000 to x007fff	$008000 to $00ffff	$808000 to $80ffff
x008000 to x00ffff	$018000 to $01ffff	$818000 to $81ffff
x010000 to x017fff	$028000 to $028fff	$828000 to $82ffff
x018000 to x018fff	$038000 to $038fff	$838000 to $83ffff
...
x3e0000 to x3e7fff	$7c8000 to $7cffff	$fc8000 to $fcffff
x3e8000 to x3effff	$7d8000 to $7dffff	$fd8000 to $fdffff
x3f0000 to x3f7fff	replaced with RAM	$fe8000 to $feffff
x3f8000 to x3fffff	replaced with RAM	$ff8000 to $ffffff

The system RAM always appears in SNES banks $7e and $7f. If you want to access ROM at offsets x3f0000 to x3fffff, then you must use the second mirror at SNES banks $fe and $ff.

RAM in LoROM cartridge Edit

If the cartridge contains its own RAM or save-RAM, then LoROM will map this RAM at the start of bank $70, and again and the start of bank $f0. The maximum size of the RAM is 32 kilobytes. This does not overlap the mapped ROM.

offsets in SRM image	SNES addresses First mirror	SNES addresses Second mirror
x0000 to x7fff	$700000 to $707fff	$f00000 to $f07ffff

HiROM Edit

HiROM maps each ROM bank into the whole (being addresses $0000 to $ffff) of each SNES bank, starting with SNES bank $40, and starting again with SNES bank $80.

offsets in headerless ROM image	SNES addresses First mirror	SNES addresses Second mirror
x000000 to x00ffff	$400000 to $40ffff	$c00000 to $c0ffff
x010000 to x01ffff	$410000 to $41ffff	$c10000 to $c1ffff
x020000 to x02ffff	$420000 to $42ffff	$c20000 to $c2ffff
x030000 to x03ffff	$430000 to $43ffff	$c30000 to $c3ffff
...
x3c0000 to x3cffff	$7c0000 to $7cffff	$fc0000 to $fcffff
x3d0000 to x3dffff	$7d0000 to $7dffff	$fd0000 to $fdffff
x3e0000 to x3effff	replaced with RAM	$fe0000 to $feffff
x3f0000 to x3fffff	replaced with RAM	$ff0000 to $ffffff

The system RAM always appears in SNES banks $7e and $7f. If you want to access ROM at offsets x3e0000 to x3fffff, then you must use the second mirror at SNES banks $fe and $ff.

The upper half of each ROM bank has two more mirrors in HiROM. The upper halves ($8000 to $ffff) of SNES banks $c0 to $ff have mirrors in SNES banks $80 to $bf and again in SNES banks $00 to $3f.

SNES addresses Second mirror	SNES addresses Third mirror	SNES addresses Fourth mirror
$c08000 to $c0ffff	$008000 to $00ffff	$808000 to $80ffff
$c18000 to $c1ffff	$018000 to $01ffff	$810000 to $81ffff
...
$fe8000 to $feffff	$3e8000 to $3effff	$be8000 to $beffff
$ff8000 to $ffffff	$3f8000 to $3fffff	$bf8000 to $bfffff

Of the upper-half mirrors, the most important one is $008000 to $00ffff, because the SNES header and the interrupt handlers need to be in SNES bank $00. If you put the interrupt handlers in the upper half of SNES bank $40 or SNES bank $c0, then they will appear in SNES bank $00. The SNES header will appear at $00ffc0, $40ffc0, $80ffc0 and $c0ffc0.

RAM in HiROM cartridge Edit

If the cartridge contains its own RAM or save-RAM, then HiROM will map this RAM at the start of bank $20, and again and the start of bank $a0. The maximum size of the RAM is 32 kilobytes. This does not overlap the mapped ROM.

offsets in SRM image	SNES addresses First mirror	SNES addresses Second mirror
x0000 to x7fff	$200000 to $207fff	$a00000 to $a07ffff

ExLoROM and ExHiROM Edit

This wiki does not yet document ExLoROM and ExHiROM, which are layouts to allow ROM size being larger than 4 MB.

ROM status $ffd5 Edit

Address $ffd5 in the SNES header has bit $01 set for HiROM or clear for LoROM. Bit $20 is always set. Thus the value of $ffd5 tends to be $20 for LoROM or $21 for HiROM. Some users refer to LoROM as "mode 20" and to HiROM as "mode 21".

FastROM also sets bit $10, so the value may become $30 for LoROM or $31 for HiROM.

Formulas to convert addresses Edit

The source code for xkas, in file xkas/libxkas/arch/snes.cpu.cpp, has these C++ formulas to convert between ROM image offsets "fileaddr" and SNES addresses "archaddr". In this code, header is true if and only if the image has SMC header.

unsigned xkasSNESCPU::archaddr(unsigned addr) {
  if(header) addr -= 512;

  switch(mapper) {
    case LoROM: {
      addr = ((addr & 0x7f8000) << 1) + 0x8000 + (addr & 0x7fff);
    } break;

    case HiROM: {
      addr = 0xc00000 + (addr & 0x3fffff);
    } break;
  }

  return addr;
}

unsigned xkasSNESCPU::fileaddr(unsigned addr) {
  switch(mapper) {
    case LoROM: {
      addr = ((addr & 0x7f0000) >> 1) + (addr & 0x7fff);
    } break;

    case HiROM: {
      addr = addr & 0x3fffff;
    } break;
  }

  if(header) addr += 512;
  return addr;
}

References Edit

xkas formulas shown on this page
memmap.cpp in the snes9x-gtk source code
snesmem.zip from Zophar's Domain

Wikia

ROM Hack City

SNES ROM layout

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