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https://github.com/ekeeke/Genesis-Plus-GX.git
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49 lines
1.4 KiB
C
49 lines
1.4 KiB
C
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/*
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memvdp.c --
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Memory handlers for when the VDP reads the V-bus during DMA.
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*/
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#include "shared.h"
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unsigned int vdp_dma_r(unsigned int address)
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{
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switch((address >> 21) & 7)
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{
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case 0: /* Cartridge ROM */
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case 1:
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return *(uint16 *)(cart_rom + address);
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case 2: /* Unused */
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case 3:
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return 0xFF00;
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case 4: /* Work RAM */
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case 6:
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case 7:
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return *(uint16 *)(work_ram + (address & 0xffff));
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case 5: /* Z80 area and I/O chip */
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/* Z80 area always returns $FFFF */
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if(address <= 0xA0FFFF)
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{
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/* Return $FFFF only when the Z80 isn't hogging the Z-bus.
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(e.g. Z80 isn't reset and 68000 has the bus) */
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return (zbusack == 0) ? 0xFFFF : *(uint16 *)(work_ram + (address & 0xffff));
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}
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/* The I/O chip and work RAM try to drive the data bus which
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results in both values being combined in random ways when read.
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We return the I/O chip values which seem to have precedence, */
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else if (address <= 0xA1001F)
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{
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uint8 temp = io_read((address >> 1) & 0x0F);
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return (temp << 8 | temp);
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}
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/* All remaining locations access work RAM */
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else return *(uint16 *)(work_ram + (address & 0xffff));
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}
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return -1;
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}
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