mirror of
https://github.com/ekeeke/Genesis-Plus-GX.git
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79 lines
2.5 KiB
C
79 lines
2.5 KiB
C
/****************************************************************************
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* Genesis Plus 1.2a
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*
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* Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003 Charles Mac Donald
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
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*
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* SRAM MANAGER
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***************************************************************************/
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#include "shared.h"
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T_SRAM sram;
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/****************************************************************************
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* A quick guide to SRAM on the Genesis
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*
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* This is based on observations only - it may be completely wrong!
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*
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* The SRAM definition is held at offset 0x1b0 of the ROM header.
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* From looking at several ROMS, an ID appears:
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*
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* 0x1b0 : 0x52 0x41 0xF8 0x20 0x00200001 0x0020ffff
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*
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* Assuming 64k SRAM / Battery RAM throughout
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****************************************************************************/
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void SRAM_Init ()
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{
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memset (&sram, 0, sizeof (T_SRAM));
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memset (&sram.sram[0], 0xFF, 0x10000);
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sram.crc = crc32 (0, &sram.sram[0], 0x10000);
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if ((cart_rom[0x1b0] == 0x52) && (cart_rom[0x1b1] == 0x41))
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{
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sram.on = 1;
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sram.write = 1;
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sram.detected = 1;
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sram.start = READ_WORD_LONG(cart_rom, 0x1b4);
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sram.end = READ_WORD_LONG(cart_rom, 0x1b8);
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/* some games have incorrect header informations */
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if ((sram.start > sram.end) || ((sram.end - sram.start) >= 0x10000))
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sram.end = sram.start + 0xffff;
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sram.start &= 0xfffffffe;
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sram.end |= 1;
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/* game using serial EEPROM as external RAM */
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if (sram.end - sram.start < 2) EEPROM_Init();
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}
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else
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{
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/* set SRAM memory region by default */
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sram.start = 0x200000;
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sram.end = 0x20ffff;
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/* set SRAM ON by default if game is smaller than 2M */
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if (genromsize <= 0x200000)
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{
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sram.on = 1;
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sram.write = 1;
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}
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/* some games using EEPROM don't have the correct header */
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EEPROM_Init();
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}
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}
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