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gameboy: update MBC1--MBC5 mapper documentation

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Signed-off-by: Tauwasser <tauwasser@tauwasser.eu>
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Tauwasser 2017-06-09 06:16:47 +02:00
parent e807364394
commit 0210c87222
1 changed files with 31 additions and 50 deletions
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src/mame/drivers/gb.cpp
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@ -25,65 +25,55 @@ MBC1 Mapper
===========
The MBC1 mapper has two modes: 2MB ROM/8KB RAM or 512KB ROM/32KB RAM.
Initially, the mapper operates in 2MB ROM/8KB RAM mode.
This mode is selected by writing into the 6000-7FFF memory area:
0bXXXXXXXB - B=0 - 2MB ROM/8KB RAM mode
B=1 - 512KB ROM/32KB RAM mode
The default behaviour is to be in 2MB ROM/8KB RAM mode.
Writing a value ( 0bXXXBBBBB ) into the 2000-3FFF memory area selects the
lower 5 bits of the ROM bank to select for the 4000-7FFF memory area. If a
value of 0bXXX00000 is written then this will autmatically be changed to
0bXXX00001 by the mbc chip.
Writing a value (0bXXXXXXBB ) into the 4000-5FFF memory area either selects
the RAM bank to use or bits 6 and 7 for the ROM bank to use for the 4000-7FFF
memory area. This behaviour depends on the memory moddel chosen.
The RAM sections are enabled by writing the value 0bXXX1010 into the 0000-1FFF
memory area. Writing any other value disables the RAM section.
Some unanswered cases:
#1 - Set mode 0
- Set lower bank bits to 1F
- Set high bank bits to 01 => bank #3F
- Set mode 1
- What ROM bank is now at 4000-7FFF, bank #1F or bank #3F?
#2 - Set mode 1
- Set ram area #1
- Set mode 0
- What ram area is now at A000-BFFF, ram bank 00 or ram bank 01?
0000-1FFF - Writing to this area enables (value 0x0A) or disables (not 0x0A) the
SRAM.
2000-3FFF - Writing a value 0bXXXBBBBB into the 2000-3FFF memory area selects the
lower 5 bits of the ROM bank to select for the 4000-7FFF memory area.
If a value of 0bXXX00000 is written then this will autmatically be
changed to 0bXXX00001 by the mbc chip. Initial value 00.
4000-5FFF - Writing a value 0bXXXXXXBB into the 4000-5FFF memory area either selects
the RAM bank to use or bits 6 and 7 for the ROM bank to use for the 4000-7FFF
memory area. This behaviour depends on the memory moddel chosen.
These address lines are fixed in mode 1 and switch depending on A14 in mode 0.
In mode 0 these will drive 0 when RB 00 is accessed (A14 low) or the value set
in 4000-5FFF when RB <> 00 is accessed (A14 high).
Switching between modes does not clear this register. Initial value 00.
6000-7FFF - Writing a value 0bXXXXXXXB into the 6000-7FFF memory area switches the mode.
B=0 - 2MB ROM/8KB RAM mode
B=1 - 512KB ROM/32KB RAM mode
Regular ROM aliasing rules apply.
MBC2 Mapper
===========
The MBC2 mapper includes 512x4bits of builtin RAM.
0000-1FFF - Writing to this area enables (value 0bXXXX1010) or disables (any
0000-3FFF - Writing to this area enables (value 0bXXXX1010) or disables (any
other value than 0bXXXX1010) the RAM. In order to perform this
function bit 12 of the address must be reset, so usable areas are
0000-00FF, 0200-02FF, 0400-04FF, 0600-06FF, ..., 1E00-1EFF.
2000-3FFF - Writing to this area selects the rom bank to appear at 4000-7FFF.
function bit 8 of the address must be reset, so usable areas are
0000-00FF, 0200-02FF, 0400-04FF, 0600-06FF, ..., 3E00-3EFF,
0000-3FFF - Writing to this area selects the rom bank to appear at 4000-7FFF.
Only bits 3-0 are used to select the bank number. If a value of
0bXXXX0000 is written then this is automatically changed into
0bXXXX0001 by the mapper.
In order to perform the rom banking bit 12 of the address must be
set, so usable areas are 2100-21FF, 2300-23FF, 2500-25FF, 2700-
27FF, ..., 3F00-3FFF.
Some unanswered cases:
#1 - Set rom bank to 8 for a 4 bank rom image.
- What rom bank appears at 4000-7FFF, bank #0 or bank #1 ?
In order to perform the rom banking bit 8 of the address must be
set, so usable areas are 0100-01FF, 0300-03FF, 0500-05FF, 0700-
07FF,..., 3F00-3FFF,
Regular ROM aliasing rules apply.
MBC3 Mapper
===========
The MBC3 mapper cartridges can include a RTC chip.
0000-1FFF - Writing to this area enables (value 0x0A) or disables (0x00) the
RAM and RTC registers.
0000-1FFF - Writing to this area enables (value 0x0A) or disables (not 0x0A) the
SRAM and RTC registers.
2000-3FFF - Writing to this area selects the rom bank to appear at 4000-7FFF.
Bits 6-0 are used to select the bank number. If a value of
0bX0000000 is written then this is autmatically changed into
@ -102,21 +92,12 @@ The MBC3 mapper cartridges can include a RTC chip.
6000-7FFF - Writing 0x00 followed by 0x01 latches the RTC data. This latching
method is used for reading the RTC registers.
Some unanswered cases:
#1 - Set rom bank to 8(/16/32/64) for a 4(/8/16/32) bank image.
- What rom bank appears at 4000-7FFF, bank #0 or bank #1 ?
MBC4 Mapper
===========
Stauts: not supported yet.
Regular ROM aliasing rules apply.
MBC5 Mapper
===========
0000-1FFF - Writing to this area enables (0x0A) or disables (0x00) the RAM area.
0000-1FFF - Writing to this area enables (0x0A) or disables (not 0x0A) the SRAM area.
2000-2FFF - Writing to this area updates bits 7-0 of the rom bank number to
appear at 4000-7FFF.
3000-3FFF - Writing to this area updates bit 8 of the rom bank number to appear