128K 512K RAM/ROM card for RC2014

This RC2014-compatible board provides RAM and flash ROM, software-pageable in 32k pages. 16 ROM or 4 RAM pages may be paged into the lower 32k of Z80 memory. This paging scheme is compatible with Wayne Warthen's RomWBW.

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.

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.

Many RC2014 ROM images can be used (selecting the right image for your serial board), but some will require modification to accommodate the different paging system. The 32k BASIC images work unmodified. Stephen C Cousins' excellent Small Computer Monitor can be used as well - choose the R3 configuration. CP/M loading won't work without code changes.

But most importantly, this board is compatible with RomWBW, providing several CP/M versions, ROM applications, and maximized free application memory. It will need to be built from source with configuration changes, although this is fairly straightforward (RomWBW includes its entire build toolchain in the source tree). The build process is covered in this page from the RomWBW documentation (including Linux and Mac instructions). An example image can be found in the firmware directory.

With the source tree unpacked, go into Source/HBIOS/Config and make a copy of RCZ80_std.asm (call it RCZ80_pd.asm). Edit this new file, and add the following lines:

RAMSIZE		.SET	128		; SIZE OF RAM IN KB (MUST MATCH YOUR HARDWARE!!!)
MEMMGR		.SET	MM_SBC		; MEMORY MANAGER: MM_[SBC|Z2|N8|Z180]
MPCL_RAM	.EQU	$78		; SBC MEM MGR RAM PAGE SELECT REG (WRITE ONLY)
MPCL_ROM	.EQU	$7C		; SBC MEM MGR ROM PAGE SELECT REG (WRITE ONLY)
;

As of RomWBW 3.1, it is possible to have a ROMdisk without a RAMdisk (which this board does not support). If building RomWBW 3.1 or later, add this line too:

MDRAM		.SET	FALSE		; MD: DISABLE RAM DISK
;

With our new file, we can go ahead and build our ROM image. Under Windows, this is done with the following commands in Source:

• BuildShared
  • Wait for this to complete, ignoring the harmless program origin warnings
• BuildROM
  • Enter RCZ80 at the Platform prompt
  • Enter pd at the Configuration prompt

Once the build is complete, we are done! Back at the root of the source tree, go into Binary and retrieve our new RCZ80_pd.rom. This can now be burned to the flash chip using an EPROM programmer, such as a TL866.

Bill Of Materials and part references are below. I recommend using gold-plated header for the bus connection - I use these ones from Pololu or Sparkfun. A nice detail of these, is that they sit at the same height as double-row header in a backplane.

The specified parts are just the ones I used, and can be substituted as needed - Mouser links provided for convenience and reference.