Slot 0 Flash Downgrade

Use Case

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, remains a useful proposition even today.

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

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.

Bundled Tools

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

• 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.

• Installer - to install and run application ROMs from RAM (no flash card needed).

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.

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. 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).

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 Vision2 for Slot 0

Vision's code binds its ROM banks using static addressing which fails when not running from a 32KB Eprom. It needed to have this fixed prior to inclusion in this ROM image: https://cambridgez88.jira.com/wiki/spaces/ZP/pages/2702114818

Downgrade ROM Manifest

To Do

There is over 256KB remaining on the file area - enough to accommodate SpellMaster. It needs some disassembly to locate and patch its bank switching code.