by White Flame
The geoRAM is a banked memory system. It uses the registers at $dffe and $dfff to determine what part of the geoRAM memory should be visible in the banked window at $de00-$deff.
$dfff - block selection, each block is 16KB $dffe - select a 256-byte page within the block (0-63)
Since there are only 64 256-byte pages that fit in 16k, the value in $dffe ranges from 0 to 63. The number of 16k blocks that is available depends on the size of the geoRAM:
512k = 0- 31, $00-$1f 1024k = 0- 63, $00-$3f 2048k = 0-127, $00-$7f
The two registers are write-only. Attempting to read them will only return bus noise. If you need to know the current values of the registers, you need to write a copy in normal RAM when you set them.
To convert a straight 24-bit offset into the geoRAM block/page format, you need to shift the two upper bits from the middle byte into the high byte:
.byte temp .byte addressLo, addressMd, addressHi lda addressMd sta temp and #%00111111 sta $dffe lda addressHi asl temp rol asl temp rol sta $dfff ldx addressLo lda $de00,x