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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 By using OZ 4.0 on a Z88 with 512KB RAM and 512KB of flash in Slot 0 we can achieve a fully backward compatible Expanded machine without needing any other cards.
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Tools to Bundle
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 2022/23, 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 editing the card header and size in 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 DOR 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 space. 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).
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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
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Vision for Slot 0
Vision2 Vision was sold as on a 32KB EPROM. As explained in the Developer Notes, cards smaller than 1MB are addressable at repeating intervals within that Slot's address space. This allows OZ to easily scan the top bank of a card, without needing to know its size.
By default, Vision2 cannot run from a 1MB card because its DORs presume it will occupy the last two banks, but its bank switching code presumes it will occupy the first two banks. Both conditions only hold true for a 32KB card.
Consider the Z88 memory map:
| Code Block |
|---|
Banks &00 - &3F are internal Slot 0 (lower half ROM, upper half RAM)
Banks &40 - &7F are wired to Slot 1 (usually RAM for Expanded mode in old OZ)
Banks &80 - &BF are wired to Slot 2
Banks &C0 - &FF are wired to Slot 3 (usually EPROM) |
RomCombiner includes patches to lift the restrictive card limitations for both Vision2 (full version) and for Vision/L (reduced feature version). The Vision2 patch amends the following byte values in the bank switching code:
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Original
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Patched
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Impact
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C0
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FE
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Slot 3, from 1st bank to penultimate bank
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C1
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FF
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Slot 3, from 2nd bank to last bank
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80
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BE
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Slot 2, from 1st bank to penultimate bank
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81
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BF
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Slot 2, from 2nd bank to last bank
Rather than masking out the top two bits of the bank address (which would make it Slot agnostic), Vision is limited to running in Slots 2 or 3 only, which is understandable considering early OZ needs a RAM card in Slot 1 for an Expanded machine.
Returning to the memory map above, for our custom ROM will need one of these pairs of bytes patching to the Slot 0 addresses which the ROM will occupy: &0A and &0B, as seen below.It also required an Expanded Z88, which at the time would have mandated a RAM card in Slot 1. Consequently its bank switching code assumes it will only ever run from Slots 2 or 3. The quirk of
Downgrade ROM Manifest
| Code Block | ||
|---|---|---|
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CFG.V3 ; OZ ROM for slot 0 and total of 16 banks to update. OZ.0,16 ; Bank file, CRC, destination bank in slot 0. "ozs0-400.32",$9de67a39,$00 "ozs0-400.33",$1d32d297,$01 "ozs0-400.34",$65ed5623,$02 "ozs0-400.35",$e2e022c6,$03 "ozs0-400.36",$8758704a,$04 "ozs0-400.37",$b860cf23,$05 "ozs0-400.38",$d3104e3f,$06 "ozs0-400.39",$5c5de229,$07 "flashst-197.40",$88ce8262,$08 "zip-xymodem.41",$83cf1ae5,$09 "vision2-206.42",$fd18a731,$0a "vision2-206.43",$71ee7ae3,$0b "file-area.44",$1ec9d2c1,$0c "file-area.45",$6666ce43,$0d "file-area.46",$420c789c,$0e "ozs0-400.63",$3d6adf54,$1f |
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