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 or both of the top two 4K regions 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.

There are some trade-offs: The register that selected address-dependent CPU rate now simply duplicates selection of fast mode. Support for undocumented video address "glitching" is removed in favour of the finer-grained video address control. Also, any feature that was only there to support DRAM has been removed.

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
• FUZIX for Dragon + SAMx8 + CoCoSDC [source]; samx8 branch
• memtstx8 memory tester, adapted from code by Stewart Orchard [source]

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