kopia lustrzana https://github.com/Jean-MarcHarvengt/MCUME
55 wiersze
1.5 KiB
C
55 wiersze
1.5 KiB
C
/*
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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 readw_swap_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 READ_WORD(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)
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? 0xFFFF
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: READ_WORD(work_ram, address & 0xFFFF);
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}
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else
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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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if(address <= 0xA1001F)
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{
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uint8 temp = gen_io_r((address >> 1) & 0x0F);
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return (temp << 8 | temp);
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}
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else
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/* All remaining locations access work RAM */
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return READ_WORD(work_ram, address & 0xFFFF);
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}
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return -1;
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}
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