Internal ROM and MODE0/MODE1 Pins
The processor exists in two versions: 7810 without internal ROM, and 7811 with 4K internal ROM. Normally, one would use the ROM-less 7810 if one doesn't use the internal ROM - however, for it seems to be quite common to use a 7811 with its internal ROM disabled. The uPD781x datasheet doesn't contain useful info on how to do this, but the uPD78C1x (CMOS version) datasheet contains at least some fragments on how the MODE0 and MODE1 pins affect internal/external ROM mapping. Judging from that information, the following four combinations seem to be supported:
|LOW||LOW||External ROM, 4K at 0000h..0FFFh|
|LOW||HIGH||External ROM, 16K at 0000h..3FFFh|
|HIGH||LOW||Internal ROM, 4K at 0000h..0FFFh|
|HIGH||HIGH||External ROM, 64K at 0000h..FFFFh|
Examples for hardware that uses a uPD7811 with its internal ROM disabled: Schneider NLQ401 printer, Roland TR-909 Drumcomputer. Reportedly it can also happen that dealers for electronic components are shipping 7811 chips when one has ordered a 7810. Unknown why that is happening (possibly they might do contain BIOSes that turned out to contain software bugs, that were produced in too high quantities).