GRAM/1D - 1MB geoRAM clone for Commodore 64/128

The geoRAM is a 512KB memory expansion for the Commodore 64 and 128 designed by Berkeley Softworks for use with GEOS. While not as fast as the Commodore REU due to its lack of DMA capabilities, it still provides a significant performance boost. GRAM/1D is a 1MB geoRAM compatible cartridge based on a two 1Mx4 DRAM devices.

The geoRAM is a banked system. The registers at $dfff and $dffe select which 256 bytes page is visible in the $de00-$deff range. The $dfff register selects a 16KB block out of the total memory, the register at $dffe selects the actual 256 bytes page within the selected block. Valid values for a standard geoRAM are 0-31 for the block register and 0-63 for the window register. It is possible to extend the block register to 8-bit to create a 4MB geoRAM.

This version is a bugfix of the previous GRAM/1D-C00. It corrects the timing issues and replaces the LS378 with LS174 for easier sourcing of the components.

The design uses two 74LS174 6-bit D-FlipFlop ICs for the block (U4) and window registers (U5). A 16V8 PLD is used to generate the control signals for the two registers and the control signals for the DRAM. The 74LS157 multiplexers (U6-U8) multiplex the address into rows and columns required by the DRAM chips. Two set of DRAMs (DIP and ZIP) are wired in parallel so the user can use either one based on availability.

Testing

Testing has been performed using the geo/neo RAM memory test and running GEOS. It is important to note that GEOS 2.0r (which was originally shipped with the geoRAM) is required to take advantage of the memory expansion.
Previous versions of GEOS only support the Commodore REU.
We were not able to locate an original GEOS 2.0r image on the internet. Our solution was to download a D81 image of GEOS 2.0 and replace GEOBOOT and CONFIGURE with the correct geoRAM version.
All the required files can be downloaded from http://cbmfiles.com/geos/index.php
A pre-made D81 image is also attached at the end of the post.