SAMx8: Dragon 64 512K expansion

The SAMx8 is a 512K SRAM expansion and MMU for the Dragon 64. It enables arbitrary mapping of 16K pages in two tasks, optionally making either of the top two 4K areas common to all memory maps.

The speed of modern SRAM allows the SAMx8 to include a RAM access cycle for video data even in the FAST CPU rate setting. Additionally, the video base address can be set to any multiple of 32 bytes across the entire 512K.

Some SAM features are lost, mostly those irrelevant to accessing the newly-provided RAM. But for space reasons the address-dependent CPU rate has been removed, as has undocumented video address "glitching". Given the new board can let the system run at double speed, and provides relatively fine-grained video base address configuration, these were considered to be acceptable losses.

Warning: Fitting this board requires a modification to the original motherboard. Although the Dragon 64 PCB is pretty robust, this is old hardware, so I do not recommend trying this if you're not confident in your desoldering skills. Lifted pads or tracks may ensue.

• Download SAMx8 schematic, PCB layout & VHDL source
• SAMx8 manual

Schematic and PCB layout are for KiCad 9. VHDL source buildable with Xilinx ISE.

The download linked above is for release candidate 4 of version 1.0. While I'm reasonably sure this can now be considered finished, I do need to write a program to thoroughly exercise the functionality.