mirror of
https://github.com/ekeeke/Genesis-Plus-GX.git
synced 2024-11-13 22:35:10 +01:00
49 lines
1.4 KiB
C
49 lines
1.4 KiB
C
|
/*
|
||
|
|
memvdp.c --
|
||
|
|
Memory handlers for when the VDP reads the V-bus during DMA.
|
||
|
|
*/
|
||
|
|
|
||
|
|
#include "shared.h"
|
||
|
|
|
||
|
|
unsigned int vdp_dma_r(unsigned int address)
|
||
|
|
{
|
||
|
|
switch((address >> 21) & 7)
|
||
|
|
{
|
||
|
|
case 0: /* Cartridge ROM */
|
||
|
|
case 1:
|
||
|
|
return *(uint16 *)(cart_rom + address);
|
||
|
|
|
||
|
|
case 2: /* Unused */
|
||
|
|
case 3:
|
||
|
|
return 0xFF00;
|
||
|
|
|
||
|
|
case 4: /* Work RAM */
|
||
|
|
case 6:
|
||
|
|
case 7:
|
||
|
|
return *(uint16 *)(work_ram + (address & 0xffff));
|
||
|
|
|
||
|
|
case 5: /* Z80 area and I/O chip */
|
||
|
|
/* Z80 area always returns $FFFF */
|
||
|
|
if(address <= 0xA0FFFF)
|
||
|
|
{
|
||
|
|
/* Return $FFFF only when the Z80 isn't hogging the Z-bus.
|
||
|
|
(e.g. Z80 isn't reset and 68000 has the bus) */
|
||
|
|
return (zbusack == 0) ? 0xFFFF : *(uint16 *)(work_ram + (address & 0xffff));
|
||
|
|
}
|
||
|
|
|
||
|
|
/* The I/O chip and work RAM try to drive the data bus which
|
||
|
|
results in both values being combined in random ways when read.
|
||
|
|
We return the I/O chip values which seem to have precedence, */
|
||
|
|
else if (address <= 0xA1001F)
|
||
|
|
{
|
||
|
|
uint8 temp = io_read((address >> 1) & 0x0F);
|
||
|
|
return (temp << 8 | temp);
|
||
|
|
}
|
||
|
|
/* All remaining locations access work RAM */
|
||
|
|
else return *(uint16 *)(work_ram + (address & 0xffff));
|
||
|
|
}
|
||
|
|
|
||
|
|
return -1;
|
||
|
|
}
|
||
|
|
|