08916: jupace: Incorrect Z80 data bus value during interrupt

ID	Category [?]	Severity [?]	Reproducibility	Date Submitted	Last Update
08916	Core	Minor	Always	Sep 11, 2024, 23:53	Sep 13, 2024, 04:57

Tester	pgimeno	View Status	Public	Platform
Assigned To		Resolution	Open	OS
Status [?]	Confirmed			Driver	cantab/jupace.cpp
Version	0.251	Fixed in Version		Build
		Fixed in Git Commit		Github Pull Request #

Summary	08916: jupace: Incorrect Z80 data bus value during interrupt
Description	It seems that the bus value during an interrupt servicing in the jupace driver is fixed to 255. This is incorrect; the real machine's hardware actually derives the value from both text RAM and character set RAM in a nontrivial way, associated with the video hardware. I've posted the details of the correct behaviour here: https://retrocomputing.stackexchange.com/questions/20350/what-would-be-the-value-on-the-data-bus-for-a-jupiter-ace-with-the-z80-interrupt#30578 This affects the game Valkyr in particular (marked as Not Working as of this writing); possibly more. This game assumes that the data bus value during an interrupt is 20h, which is not necessarily true in general but it is true if the machine was just booted and just a LOAD command was entered.
Steps To Reproduce	- Configure the machine with at least 16K extra RAM. - Load and try to play Valkyr (m_valkyr.zip). - Observe that the machine resets or otherwise fails to work. - Optionally, modify the driver to always return 32 instead of 255 as the bus value during interrupt servicing and try again; check that the game loads an runs.
Additional Information	The value for an unused I/O port (e.g. port 255) is also derived from text RAM / character set RAM in the same way; the same fix could also provide accurate values for unused ports. I'm not aware of any program that expects a correct value though, but it would be nice to have it in the interest of accuracy. I've written a test ROM that allows verifying whether the IM2 behaviour is correct in normal situations (it's not very thorough though). https://codeberg.org/pgimeno/jupace-testrom
Github Commit

Flags	Verified with Original, Verified with Code
Regression Version
Affected Sets / Systems	jupace

Attached Files