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- added VFAT support and old files saved - corrected some docs - added SRAM crc calculation after SRAM saving on SDCARD - modified reload_rom actions order - fixed some vdp status bits
190 lines
8.9 KiB
Plaintext
190 lines
8.9 KiB
Plaintext
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SSFII GENESIS TECHNICAL INFORMATION
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Second Edition (July 26, 2000)
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Bart Trzynadlowski
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The purpose of this document is to discuss the workings of the Genesis port of
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Capcom's "Super Street Fighter II The New Challengers". Virtually all of this
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information was originally found by others' efforts.
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Please credit "those who contributed information" rather than me if
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you use any of this information. You may pass this document around freely, but
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please keep it unaltered. If you see any errors or have more info, contact me.
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I can be reached at trzy@powernet.net.
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Second Edition: July 26, 2000
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- Major update with correct information thanks to Tim Meekins
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First Edition: July 21, 2000
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- Initial release
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0. THE SSFII CHALLENGE
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The challenge behind getting "Super Street Fighter II The New Challengers"
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(SSFII) to work on a Genesis emulator lies primarily in the fact that the game
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is stored on a 40 megabit (5 megabyte) cartridge.
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The Sega Genesis maps ROM from 0x000000 to 0x3FFFFF which means that
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the CPU can only see 4 megabytes of cartridge ROM. SSFII needed more space for
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the backgrounds and thus Capcom opted for a special 5 megabyte cartridge with
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bankswitching in order to access the area of ROM outside of normal range.
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1. THE BANKSWITCHING MECHANISM
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The bankswitching mechanism probably resides on the cartridge. I'm 99% sure of
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this. Sega documentation does offer a description of the mechanism, though,
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which led me to suspect for a moment that perhaps the mechanism was on the
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Genesis itself.
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The idea is unlikely though, as that range of registers is used by the
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32X for an entirely different purpose. I have been informed that that range
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(which is marked as "SEGA RESERVED" in the Genesis developer's manual) can be
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used for extra devices which may be present on the cartridge.
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The bankswitching mechanism is very simple. It views the addressable 4 mega-
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bytes of ROM as 8 512KB regions. The first area, 0x000000-0x07FFFF is fixed
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and cannot be remapped because that is where the vector table resides.
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The banking registers on the cartridge work by allocating the 512KB
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chunk to a given part of the addressable 4MB ROM space. Below are the
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registers and what range they correspond to. The value written to a register
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will cause the specified 512KB page to be mapped to that region. A page is
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specified with 6 bits (bits 7 and 6 are always 0) thus allowing a possible 64
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pages (SSFII only has 10, though.)
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0xA130F3: 0x080000 - 0x0FFFFF
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0xA130F5: 0x100000 - 0x17FFFF
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0xA130F7: 0x180000 - 0x1FFFFF
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0xA130F9: 0x200000 - 0x27FFFF
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0xA130FB: 0x280000 - 0x2FFFFF
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0xA130FD: 0x300000 - 0x37FFFF
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0xA130FF: 0x380000 - 0x3FFFFF
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The registers are accessed through byte writes. I haven't seen SSFII try to
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read any of the registers, so I don't know if it's possible or not. I don't
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emulate anything besides byte writes in my emulator.
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There is also a register 0xA130F1. Apparently the regions specified by
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0xA130F9-0xA130FF (0x200000-0x3FFFFF) can be either ROM or RAM and can be
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write-protected. Here is the layout of the register as far as I know:
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7 6 5 4 3 2 1 0
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+-----------------------+
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|??|??|??|??|??|??|WP|MD|
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+-----------------------+
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MD: Mode -- 0 = ROM, 1 = RAM
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WP: Write protect -- 0 = writable, 1 = not writable
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Emulation of the 0xA130F1 register is not necessary, SSFII initializes it at
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start-up by writing 0 I believe, which signifies ROM and no write protection
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to the regions at 0x200000-0x3FFFFF (ROM isn't writable anyway, there isn't a
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need for protection.)
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Examples:
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If 0x01 is written to register 0xA130FF, 0x080000-0x0FFFFF is visible
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at 0x380000-0x3FFFFF.
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If 0x08 is written to register 0xA130F9, the first 512KB of the
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normally invisible upper 1MB of ROM is now visible at 0x200000-
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0x27FFFF.
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The registers simply represent address ranges in the 4MB ROM area and you can
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page in data to these ranges by specifying the bank # -- it's that simple!
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2. CHECKSUM
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SSFII contains a checksum routine which calculates the checksum of the first
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4MB of the ROM; it then pages in the last 1MB of ROM to 0x300000-0x3FFFFF.
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It does this by writing to the bank registers. The following code is taken
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directly from the US ROM dumped by Genesis Power:
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... Checksum first 4MB of ROM (0x000000-0x3FFFFF) ...
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0x003BEC: move.b #$08, ($A130FD)
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0x003BF4: move.b #$09, ($A130FF)
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0x003BFC: lea ($300000), a0
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... Checksum uppper 1MB of ROM now mapped at 0x300000-0x3FFFFF ...
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The bankswitching hardware must be properly emulated before the game starts
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working. If it is not, the checksum will fail and the game will hang with a
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red screen.
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In the Genesis Power US dump, you can jump to 0x003C3C to avoid the
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checksum routine, this is where program flow transfers to if the checksum was
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found to be valid. The game doesn't do anything important before the checksum
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code as far as emulation is concerned. It writes the SEGA message to the
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Trademark Security System register and does some joypad init stuff.
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3. EMULATING SSFII
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SSFII is fairly straightforward to emulate. All you need to do is detect the
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game, load up all 5MB, and emulate the banking registers. The easiest way to
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do it, in my opinion, is to keep a second copy of the ROM and do memcpy()s
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from that second copy to the ROM being emulated.
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For example, here is how Genital does it... I've got genesis.rom which is the
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buffer where ROMs are loaded and executed. There is also genesis.rom_ssf2
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which is another 5MB buffer where I load the SSFII ROM to as well.
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The bankswitching register emulation is done in my IOWriteByte()
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function (which handles byte writes to the 0xA00000-0xBFFFFF range). The
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following is the code I use:
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if (addr >= 0xa130f3 && config.ssf2_emu)
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{
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switch (addr & 0xf)
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{
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case 0x3: /* 080000-0FFFFF */
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memcpy((genesis.rom + 0x080000), (genesis.rom_ssf2 + (data * 0x80000)), 0x80000);
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break;
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case 0x5: /* 100000-17FFFF */
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memcpy((genesis.rom + 0x100000), (genesis.rom_ssf2 + (data * 0x80000)), 0x80000);
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break;
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case 0x7: /* 180000-1FFFFF */
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memcpy((genesis.rom + 0x180000), (genesis.rom_ssf2 + (data * 0x80000)), 0x80000);
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break;
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case 0x9: /* 200000-27FFFF */
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memcpy((genesis.rom + 0x200000), (genesis.rom_ssf2 + (data * 0x80000)), 0x80000);
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break;
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case 0xb: /* 280000-2FFFFF */
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memcpy((genesis.rom + 0x280000), (genesis.rom_ssf2 + (data * 0x80000)), 0x80000);
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break;
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case 0xd: /* 300000-37FFFF */
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memcpy((genesis.rom + 0x300000), (genesis.rom_ssf2 + (data * 0x80000)), 0x80000);
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break;
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case 0xf: /* 380000-3FFFFF */
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memcpy((genesis.rom + 0x380000), (genesis.rom_ssf2 + (data * 0x80000)), 0x80000);
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break;
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default:
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break;
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}
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return;
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}
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Pardon the bad formatting, it just wouldn't fit correctly in the 80 columns
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that the rest of the document sticks to.
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The first line checks whether the address is 0xA130F3 or above (notice
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how 0xA130F1 is ignored, SSFII doesn't really use it) and if SSFII emulation
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is enabled (in Genital, I have a flag -- config.ssf2_emu -- which is set to 1
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to indicate SSFII emulation should occur.) The remainder of the code is a
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switch () statement which handles the register emulation.
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Since 0x80000 = 512KB, and the data variable holds the byte written to
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the register, data * 0x80000 yields the offset into the ROM that has been
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requested to be mapped.
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At first glance, memcpy()s may seem inefficient and slow. Although it
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is true, this doesn't apply to emulating SSFII since the game doesn't do much
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(if any) bankswitching during gameplay. No slowdown due to the memcpy()s is
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noticable.
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If you are using an emulator like Starscream which requires the ROMs to be
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byteswapped, make certain both ROM images are byteswapped. If you don't use
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2 copies of the ROM, you will find the game will still have several glitches
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because the memcpy()s will be destroying the data they write over.
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4. CONCLUSION
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That's all there is to it. This should be sufficient information for anyone
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wishing to implement SSFII support in their Sega Genesis emulator. If you have
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any questions at all (I'm not great at making things clear) feel free to email
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me at trzy@powernet.net.
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