Occasionally questions come up about the special “hidden” control register in the 7800, and I usually end up searching back through the forum archive for the answer, so I thought I’d finally document it somewhere I can find it!

The purpose of this control register is to switch the 7800 from 7800 mode to 2600 mode. The register can be written to using any address between $0000 and $001F. This address range overlaps the TIA so once the register is set you need to set the lock bit (see below) before you can use the TIA.

There are 4 bits in the register and they work as follows:

Bit 0 (lock mode) – When set to 1 this bit locks the control register so that no more writes will affect it. The only way to clear the lock is to power cycle the console.

Bit 1 (MARIA Enable) - 1 = enable MARIA (also enables system RAM), 0 = disable MARIA (only RIOT RAM is available). 0 also disables the EXTRAM signal from the expansion connector.

Bit 2 (EXT) - 0 = enable BIOS at $8000-$FFFF, 1 = disable BIOS / enable cartridge

Bit 3 (TIA-EN) - 1 = enable TIA video pull-ups (video output is TIA instead of MARIA) and also disables 2 button joystick mode, 0 = disable TIA video pull-ups (video output is MARIA instead of TIA).

Besides the functions controlled by the register bits, it also controls the HALT input to the 6502. When the 7800 is first powered up HALT is blocked from getting to the 6502, but after 2 writes to the control register (the data written doesn’t make a difference), the HALT input will be enabled. I am not sure why this function exists. I can only speculate that it’s to prevent the MARIA from spuriously halting the 6502 until it’s properly initialized.