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Z80/Z180 System Software


RomWBW provides a complete software system for all Z80/Z180 CPU-based systems produced by the RetroBrew Computers Community including the SBC 1/2, Zeta 1/2, N8, and Mark IV. Virtually all RetroBrew hardware is supported including floppy, hard disk (IDE, CF Card, SD Card), Video, and keyboard. VT-100 terminal emulation is built-in for all video adapters.

In addition to a simple system monitor, RomWBW includes robust adaptations of both CP/M-80 2.2 and Z-System (ZCPR + ZSDOS). Either operating system can be loaded directly from ROM or installed and loaded from disk storage. A selection of standard/useful applications is provided via a built-in ROM disk. A RAM disk is also provided for fast/temporary file storage.

A pre-built ROM image is included for each platform. If desired, system customization is achieved by making simple modifications to a configuration file and running a build script to generate a custom ROM image. All source and build tools are included in the distribution. The build scripts run under any modern 32 or 64 bit version of Microsoft Windows.

John Coffman's UNA hardware BIOS is fully supported by RomWBW. See the section below for more information on running RomWBW on top of UNA.


In general, you will just program your system's ROM with the appropriate ROM image from the RomWBW distribution. This can be done with a ROM programmer. Alternatively, if your system supports in-situ ROM programming, you can use Will Sowerbutts' FLASH application which has been included in recent RomWBW distributions.

The pre-built ROM images will detect and support a reasonable range of devices including serial ports, video adapters, on-board disk interfaces, and PropIO/ParPortProp boards without building a custom ROM.

RomWBW is distributed as a package (zip archive) containing the pre-built ROM images, source code, and all required build tools. To use a pre-built ROM image, download the distribution package to a modern computer (Windows, Mac, Linux, etc.). Unzip the archive using any of the common zip management applications. You will see that the distribution is broken up into a few sub-directories. The Binary directory contains the pre-built ROM images. Based on the table below, pick the appropriate ROM image:

PlatformROM Image FileBuilt-in Device Support
SBC V1/V2SBC_std.romSupports onboard PPIDE disk, VGA3/CVDU video/kbd, PropIO (video, kbd, SD Card)
Zeta V1ZETA_std.romSupports ParPortProp (video, kbd, SD Card)
Zeta V2ZETA2_std.romSupports ParPortProp (video, kbd, SD Card)
N8N8_std.romSupports onboard SD Card, video, keyboard; production board assumed (date code >= 2312)
Mark IVMK4_std.romSupports onboard SD Card, IDE Disk, VGA3/CVDU video/kbd, PropIO (video, kbd, SD Card)

Your system does not need to have the devices listed above to use the pre-built ROM. The devices will be detected and used only if present. All you need to have is the CPU board with ROM/RAM and a serial port. All pre-built ROM images are simple 512KB binary images. If your system utilizes a 1MB ROM, you can just program the image into the first 512KB of the ROM. The ReadMe.txt file in the Binary directory has more detailed information on the pre-built ROM images.

Connect a serial terminal or computer with terminal emulation software to the primary RS-232 port of your CPU board. A null-modem connection will generally be required. Set the line characteristics to 38400 baud, 8 data bits, 1 stop bit, no parity, and no flow control. Select VT-100 terminal emulation.

Upon power-up, your terminal should display a sign-on banner within 2 seconds followed by hardware inventory and discovery information. When hardware initialization is completed, a boot loader prompt allows you to choose a ROM-based operating system, system monitor, or boot from a disk device.

CPU Speed

RomWBW ROM images support virtually any CPU speed your system is running. However, there are some hardware-oriented caveats to be aware of.

The use of high density floppy disks requires a CPU speed of 8 MHz or greater.

The latest X-Modem file transfer programs (XM.COM, XM-A0.COM, and XM-A1.COM) require a CPU speed of 6 MHz or greater to support the default RomWBW serial port speed of 38400 baud. Older variants of the X-Modem programs are included (XM5.COM, XM5-A0, and XM5-A1) which will handle 38400 baud on system running down to 4 MHz.

Upgrading from Previous Versions

Program a new ROM chip from an image in the new distribution. Install the new ROM chip and boot your system. At the boot loader “Boot:” prompt, select either CP/M or Z-System to load the OS from ROM.

If you have spare ROM chips for your system, it is always safest to keep your existing, working ROM chip and program a new. If the new one fails to boot, you can easily return to the known working ROM.

If you use a customized ROM image, it is recommended that you first try a pre-built ROM image first and then move on to generating a custom image.

It is entirely possible to reprogram your system ROM using the FLASH utility from Will Sowerbutts on your ROM drive (B:). In this case, you would need to transfer the new ROM image to your system using XModem. Obviously, there is some risk to this approach since any issues with the programming or ROM image could result in a non-functional system.

Once you have successfully booted your system with the new ROM, you should update the system software from previous releases that you may have copied to disk drives. This is described below.

If your system has any bootable drives, then update the OS image on each drive using SYSCOPY. For example, if C: is a bootable drive with the Z-System OS, you would update the OS image on this drive with the command:


If you have copies of any of the system utilities on drives other than the ROM disk drive, you need to copy the latest version of the programs from the ROM drive (B:) to any drives containing these programs. For example, if you have a copy of the ASSIGN.COM program on C:, you would update it from the new ROM using the COPY command:


The following programs are maintained with the ROM images and all copies of these programs should be updated when upgrading to a new ROM version:

  • FD.COM (previously FDTST.COM)
  • XM*.COM

Usage Instructions

A typical set of CP/M-80 2.2 and Z-System applications are included on the ROM file system. The Doc directory of the distribution contains the CP/M, ZSDOS, and ZCPR user manuals which provide the primary usage information. Note that the Z-System applications will generally not run under CP/M.

The following custom applications are included on the ROM disk to enhance the operation of RomWBW:

ASSIGN.COMAdd, change, and delete drive letter assignments. Use ASSIGN /? for usage instructions.
SYSCOPY.COMCopy system image to a device to make it bootable. Use SYSCOPY with no parms for usage instructions.
FD.COMFormat and test floppy disks. Menu driven interface.
OSLDR.COMLoad a new OS on the fly. For example, you can switch to Z-System when running CP/M. Use OSLDR with no parms for usage instructions.
FORMAT.COMWill someday be a command line tool to format floppy disks. Currently does nothing!
XM.COMXModem file transfer program adapted to hardware. Note that N8 and Mark IV have XM-A0 and XM-A1 corresponding to the two serial ports on those systems.
FLASH.COMWill Sowerbutts' in-situ ROM programming utility.

UNA Hardware BIOS

John Coffman has produced a new generation of hardware BIOS called UNA. In addition to the classic ROM images described above, RomWBW comes with a UNA-based image that combines the UNA BIOS with the RomWBW OS implementations and applications.

UNA is customized dynamically using a ROM based setup routine and the setup is persisted in the system NVRAM of the RTC chip. This means that a single UNA-based ROM image can be used on most of the RetroBrew platforms and is easily customized. UNA also supports FAT file system access that can be used for in-situ ROM programming and loading system images.

While John is likely to enhance UNA over time, there are currently a few things that UNA does not support:

  • Floppy Drives
  • Video/Keyboard/Terminal Emulation
  • Zeta 1 and N8 Systems
  • Some older support boards

If you wish to try the UNA variant of RomWBW, then just program your ROM with the ROM image called “UNA_std.rom” in the Binary directory. This one image is suitable on all of the platforms and hardware UNA supports.

Please refer to the UNA BIOS Firmware Page for more information on UNA.

CP/M vs. Z-System

There are two OS variants included in this distribution and you may choose which one you prefer to use. Both variants are now included in the pre-built ROM images. You will be given the choice to boot either CP/M or Z-System at startup.

The traditional Digital Research (DRI) CP/M OS is the first choice. The Doc directory contains a manual for CP/M usage (“CPM Manual.pdf”). If you are new to the RetroBrew Computer systems, I would currently recommend using the CP/M variant to start with simply because it has gone through more testing and you are less likely to encounter problems.

The other choice is to use the most popular non-DRI CP/M “clone” which is generally referred to as Z-System. Z-System is intended to be an enhanced version of CP/M and should run all CP/M 2.2 applications. It is optimized for the Z80 CPU (as opposed to 8080 for CP/M) and has some significant improvements such as date/time stamping of files. For further information on the RomWBW implementation of Z-System, see the wiki page Z-System Notes. Additionally, the official documentation for Z-System is included in the RomWBW distribution Doc directory (“ZSDOS Manual.pdf” and “ZCPR Manual.pdf”).

ROM Customization

The pre-built ROM images are configured for the basic capabilities of each platform. If you add board(s) to your system, you will need to customize your ROM image to include support for the added board(s).

Essentially, the creation of a custom ROM is accomplished by updating a small configuration file, then running a script to compile the software and generate the custom ROM image. At this time, the build process runs on Windows 32 or 64 bit versions. All tools (compilers, assemblers, etc.) are included in the distribution, so it is not necessary to setup a build environment on your computer.

For those who are interested in more than basic system customization, note that all source code is included (including the operating systems).

Note that the ROM customization process does not apply to UNA. All UNA customization is performed within the ROM setup script.

Complete documentation of the customization process is found in the ReadMe.txt file in the Source directory.

Source Code Respository

All source code and distributions are maintained on GitHub. Code contributions are very welcome.

Distribution Directory Layout

The RomWBW distribution is a compressed zip archive file organized in a set of directories. Each of these directories has it's own ReadMe.txt file describing the contents in detail. In summary, these directories are:

BinaryThe final output files of the build process are placed here. Most importantly, are the ROM images with the file names ending in “.rom”.
DocContains various detailed documentation including the operating systems, RomWBW architecture, etc.
SourceContains the source code files used to build the software and ROM images.
ToolsContains the MS Windows programs that are used by the build process or that may be useful in setting up your system.


While I have heavily modified much of the code, I want to acknowledge that much of the work is derived or copied from the work of others in the RetroBrew Computers Community including Andrew Lynch, Dan Werner, Max Scane, David Giles, John Coffman, and probably many others I am not clearly aware of (let me know if I omitted someone!).

I especially want to credit Douglas Goodall for contributing code, time, testing, and advice. He created an entire suite of application programs to enhance the use of RomWBW. However, he is looking for someone to continue the maintenance of these applications and they have become unusable due to changes within RomWBW. As of RomWBW 2.6, these applications are no longer provided.

David Giles has contributed support for the CSIO support in the SD Card driver.

UNA BIOS is a product of John Coffman.

Getting Assistance

The best way to get assistance with RomWBW or any aspect of the RetroBrew Computers projects is via the community forum.

Also feel free to email Wayne Warthen at

playground/wwarthen/start.1506961025.txt.gz · Last modified: 2017/10/02 12:17 by wwarthen
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