RCBUS 128k Ram + 512k ROM

Memory Card 128k RAM and 512k ROM

Credit to PickledDog on github for the design of the electronics behind this board.

This RCBUS-compatible board provides RAM and flash ROM. For ROM, a SST39SF040 512k x 8 flash chip is used. It can be programmed in-place using Will Sowerbutts' FLASH4 utility (included with RomWBW). For RAM, an AS6C1008 128k x 8 SRAM is used, providing 128K of addressable memory. Both are divided into 32k pages, and any page of either chip can be paged into the lower 32k of the address space. The upper 32k is permanently mapped to the last page of the RAM chip.

Paging

Upon reset, the first page of the ROM is mapped to the lower 32k. If left alone, the board will behave like any 32k ROM+32k RAM RC2014 system. A config register is used to set the currently mapped page, presented as an I/O device at 0x78 (the address used by most "large" Z80 memory systems). The low nibble sets the page number (0-15 for ROM, 0-3 for RAM), and high bit 7 selects the chip (0 for ROM, 1 for RAM).

The port address and layout are chosen to be (mostly) compatible with the RetroBrew SBC. The SBC has separate ROM and RAM registers (on 0x78 and 0x7C); this board uses a single register for both, which can be written to from either address.