Next Memory Bank Select

Number	$DFFD
Decimal
Short desc.	Provides additional bank select bits for extended memory.
Bit Mask	%1101 1111 1111 1101
Readable	No
Writable	Yes
Subsystem	Memory map

bit 7 = 1 to set pentagon 512K mode (hard reset = 0) †

bits 3:0 = most significant bits of the 16K RAM bank selected in Memory Paging Control ($7FFD / 32765) (soft reset = 0)

Port 0xDFFD bits 3:0 are combined as the highest bits with the bank selected in port 0x7FFD to form a 7-bit bank number.

Bit 7 can only be modified when pentagon timing is not active.

† Pentagon 512K mode is applied only when the machine is set to pentagon timing (Machine Type Register ($03)). In this mode the selected 16K bank comes from port 0x7ffd {bits 7:6, bits 2:0}