I'm a little unclear on how to handle mirroring in MAME, maybe someone can help out.
In one situation, I'm trying to mirror RAM at $F800-$FFFF across $C000-$FFFF. Here's what I've used:
AM_RANGE(0xc000,0xffff) AM_READ(MRA8_RAM) AM_MASK(0x7ff)
This allocates a 16k chunk, and correctly maps all accesses to offsets $0000-$07FF only. So it works but is a waste of space.
AM_RANGE(0xc000,0xc7ff) AM_READ(MRA8_RAM) AM_MIRROR(0xf800)
AM_RANGE(0xc000,0xffff) AM_READ(MRA8_RAM) AM_MIRROR(0xf800)
In both cases 2k is allocated, but the memory isn't mirrored above the base area ($c000-$c7ff) so the game crashes.
I had looked at the system24 driver and saw something confusing about the mirroring used:
AM_RANGE(0x000000, 0x03ffff) AM_MIRROR(0x140000) AM_ROM AM_SHARE(1)
Going by the documentation in memory.c, I would assume this maps the ROM as follows:
$000000-$03FFFF : ROM
$040000-$07FFFF : ROM (mirror)
$100000-$13FFFF : ROM (mirror)
$140000-$17FFFF : ROM (mirror)
If this means that the mirroring area can exceed the upper bound of the AM_RANGE macro ($03FFFF), then wouldn't AM_MIRROR not work for mirrored memory at a lower address with something above it? For example in my case, if the RAM was mirrored through $C000-$E7FF, but not $E800-$FFFF, AM_MIRROR would still put the mirrored RAM in the $E800-$FFFF range, right?
Now for handling mirroring of handlers, I'm stuck on this one too:
AM_RANGE(0x80, 0x80) AM_WRITE(YM2151_register_port_0_w)
AM_RANGE(0x81, 0x81) AM_WRITE(YM2151_data_port_0_w)
I'd like to mirror these from $80 to $BF each, so the first one is called for I/O ports $80, $82,..$BE, and the latter is for $81, $83,..$BF. Can the mirroring macros handle this? Or would it be better to define a handler for the full range, manage the mirroring myself, and call the above routines from there?