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AY-3-8910 unused bits in registers read back as 0 [hap]

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This commit is contained in:
Michaël Banaan Ananas 2010-12-23 17:55:31 +00:00
parent fc27445a95
commit 0120446c89
1 changed files with 16 additions and 1 deletions
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17
src/emu/sound/ay8910.c
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@ -868,6 +868,7 @@ void ay8910_write_ym(void *chip, int addr, int data)
int ay8910_read_ym(void *chip)
{
ay8910_context *psg = (ay8910_context *)chip;
device_type chip_type = psg->device->type();
int r = psg->register_latch;
if (r > 15) return 0;
@ -898,7 +899,21 @@ int ay8910_read_ym(void *chip)
logerror("%s: warning - read 8910 '%s' Port B\n",cpuexec_describe_context(psg->device->machine),psg->device->tag());
break;
}
return psg->regs[r];
/* Depending on chip type, unused bits in registers may or may not be accessible.
Untested chips are assumed to regard them as 'ram'
Tested and confirmed on hardware:
- AY-3-8910: inaccessible bits (see masks below) read back as 0
- YM2149: no anomaly
*/
if (chip_type == AY8910) {
const UINT8 mask[0x10]={
0xff,0x0f,0xff,0x0f,0xff,0x0f,0x1f,0xff,0x1f,0x1f,0x1f,0xff,0xff,0x0f,0xff,0xff
};
return psg->regs[r] & mask[r];
}
else return psg->regs[r];
}
/*************************************