DOS can't see over 64 megs, traditionally.  This is what's in config.sys
DEVICE=C:\WINDOWS\HIMEM.SYS /EISA
DEVICE=C:\WINDOWS\SMARTDRV.EXE /DOUBLE_BUFFER
DOS=HIGH,UMB
DEVICE=C:\WINDOWS\EMM386.EXE NOEMS
FILES=30
BUFFERS=20
MEM reports that I have *over* 64 megs, which is good, but MAME acts like there's still a limit there!  Any ideas on if I've done something wrong, or perhaps on an old  memory manager out there?