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

White Flame (aka David Holz)
http://www.white-flame.com/