Slot 0 Flash Downgrade

Purpose

Replacing the internal ROM with a 512KB flash chip allows for the Z88 to be upgraded to a more modern OZ version without consuming a flash card in Slot 1.

However some useful Z88 software does not function on the newer OZ versions, since authors were known to deviate from the Cambridge developer guidelines. Vision2 VT100 terminal is one such application. Using a Z88 as a portable serial terminal, with its full size keyboard and long battery life, remains a useful proposition even today.

Historically, once a Z88 had a Slot 0 flash modification and an OZ upgrade was performed, it could no longer run legacy applications… until now!

Legacy OZ Version Selection

One of the most cumbersome constraints of OZ 3.0 and earlier was the dependence on a RAM card in Slot 1 for Expanded mode, which Vision requires. However, by using OZ 4.0 on a Z88 with an internal RAM upgrade we can achieve a fully backward compatible Expanded machine without needing any other cards.

OZ ROM Modifications

By copying the card header and the Application Front DOR to the end of the 512KB flash memory, editing the card size, and by amending the last of the linked application DORs in the OZ ROM image - we can add extra applications to the Index. We can also fix several bugs in OZ 4.0 at the same time. Both processes are explained on Dennis Gröning's website.

As was done with OZ 4.3 onwards, we can then add a file area to make use of the remaining unused flash memory. We can pre-populate this with the BBC BASIC versions of Installer, RomCombiner, and RomUpdate. These have the advantage of being usable without needing to be blown to a card and, since we will be running OZ 4.0, their requirement for an Expanded Z88 will always be met (assuming a Slot 0 RAM upgrade).

The simplest way to create the file area data is to use OZvm emulator, add a new flash card, format a file area, add the files, eject the card, and choose to save the 16KB banks.

Bundled Tools

With 512KB of flash memory available, we can add software to address many of the shortcomings of early OZ versions:

• XY-Modem - for fastest file transfers over serial.

• EazyLink2 - fallback option for reliable serial comms.

• FlashStore - for managing flash file storage.

• RomUpdate - for simple application ROM blowing to flash, and to update OZ (to return to 4.7.1 or 5.0).

• RomCombiner - for managing multiple application ROMs on flash, and erasing cards.

• Installer - to install and run application ROMs from RAM (no flash card needed, but a RAM card is required).

After using OZ 4.7.1 or OZ 5.0 it's easy to forget just how irritating it can be returning to one of the stock versions. Simply transferring files is a chore, often painfully slow since EazyLink2 standalone is limited to 9600bps. However, Dennis Gröning's XY-Modem can achieve around a 10x speed improvement, explained in this guide.

Since my own updates in 2022, all of the essential flash tools above have been updated to add compatibility for newer flash memory types - so they remain current even running on old OZ versions.

Bundled Application Software

• Vision2 - a VT100, VT102, VT220, and VT320 terminal emulator. The main reason I embarked on this project.

• Spell-Master - a companion to PipeDream which would be very useful to have available at all times, without having to dedicate an entire card to it.

Installer Constraints

Note that to use RAM apps, Installer will require a dedicated RAM card because OZ 4.0 is not capable of detecting applications installed to the RAM portion of a hybrid RAM/flash card, nor in Slot 0 RAM. Installer cannot in fact use hybrid card RAM but, even were it possible, OZ 4.0 would only evaluate the Front DOR at bank $3f (63) at the top of the card (in flash), and that would contain no reference to any RAM app. The newer OZ versions have been modified to also scan bank $1f (31) of hybrid cards (top of RAM).

Patching Applications for Slot 0

Vision's code binds its ROM banks using static addressing which fails when not running from a 32KB Eprom. Furthermore it can only run in card slots 2 or 3. It needed to have this fixed prior to inclusion in this ROM image:Vision2 Fix Patch

Likewise Spell-Master has two banks of ROM code but it somewhat unconventionally pages in six more banks of dictionary data. It uses static bank addressing (relative only to the current card slot) which also needed to be converted to fully relative addressing prior to inclusion: Spell-Master Fix Patches

XY-Modem was able to fit within some unused space in the top bank of Spell-Master, but it needed to be recompiled with a new org address to achieve this.

Flashing the ROM

ROM image for OZvm emulator:

Downgrade files:

OZ version	Downgrade Method	Notes

OZ version	Downgrade Method	Notes
5.0	Index ◇ U P D	None
4.7.1	Index ◇ U P D	The screen may occasionally stay off after the write to flash and the Z88 may not reset automatically. If that happens, leave for 5 minutes to be sure it has completed, then reset manually.
< 4.7.1	RUN"ROMUPDATE.BAS" from BASIC (the Z88 application version of RomUpdate does not work here)	Copy both romupdate.bas and romupdate.crc to RAM. The screen will corrupt as the flash process begins. One row will blink. This is normal. Wait until complete. Open

Downgrade ROM Manifest