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512 KByte geoRAM Cartridge
Type: RAM Expansion
Producer: Berkeley Softworks
Price: $125 [1]
Released: 1990
Discontinued: 1993
Memory: 512 kB
  • 2MB upgrade available.
  • 4MB upgrade theoretical limit.
Type: RAM Expansion
Producer: Shareware Plus
Price: $50
Released: 2016
Memory: 512 Kb
Info: Modern 100% compatible remake.

GeoRAM is a 512KByte memory expansion for the C64 and C128 from the company Berkeley Softworks. It uses the registers at $dffe and $dfff to determine what part of the geoRAM memory should be mapped to $de00$deff.

The register at $dfff selects which 16k block to map, and $dffe selects a 256-byte page in that block. Since there are only 64 × 256 byte pages inside of 16k, the value in $dffe ranges from 0–63. The number of 16k blocks that is available depends on the size of the GeoRAM:

geoRAM Memory Blocks
RAM 16 kB Blocks Range
512 kB 0–31 $00–$1f
1024 kB 0–63 $00–$3f
2048 kB 0–127 $00–$7f
4096 kB 0–255 $00–$ff

The two registers are write-only. Attempting to read them will only return random values. If you need to know the current values of the registers, you need to write a copy in normal RAM when you set them.



  • 1541_Ultimate by Gideon Zweijtzer, an implementation of GeoRAM with size 128k to 16Mb. For 8 and 16Mb the 16k page is 32k and 64k. See forum link on size detection below.

References[edit | edit source]

  1. Robert Bixby: "The GEOS Column: Closeup on GEORAM", Compute!'s Gazette, Issue 83, Vol. 8, No. 5, May 1990, p 14

Links[edit | edit source]

WP-W11.png Wikipedia: GeoRAM