BDOS register clobbering

[Robb Bates]

Is it safe to say that nowhere does any BDOS calls ever affect IX and IY?  I'm assuming so because IX and IY are Z80 only and any CP/M variant is designed to run on 8080 as well.

[Volker Pohlers]

There are various BDOS replacements such as ZSDOS, CP/A, etc., which are optimized for the Z80 and utilize all registers, including IX.

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Robb Bates <robb...@gmail.com> schrieb am So. 9. Nov. 2025 um 18:00:

Is it safe to say that nowhere does any BDOS calls ever affect IX and IY?  I'm assuming so because IX and IY are Z80 only and any CP/M variant is designed to run on 8080 as well.

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[Robb Bates]

Ah.  So preserve IX and IY.  Got it.

[Robb Bates]

Oh wait... is that what the Z in Z-systems is for?  ZPM3 and such?

Derp!

[Alan Cox]

On Sun, 9 Nov 2025 at 17:00, Robb Bates <robb...@gmail.com> wrote:

Is it safe to say that nowhere does any BDOS calls ever affect IX and IY?  I'm assuming so because IX and IY are Z80 only and any CP/M variant is designed to run on 8080 as well.

 Theory yes, practice no. A BIOS or BDOS using IX or IY will or should save those registers. There were one or two that didn't always so everyone ended up at least saving IX to deal with them. Most software saves IX and IY because it's better not to assume the code you depend upon is perfect.

Alan