Fix interrupt handling in DOS 1 mode #107
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- In DOS 1 mode, save old interrupt and jump to it the same way it's done in normal mode, instead of trying to use the DOS 1 style SETINT and PRVINT routines. - Remove the useless SETINT and PRVINT routines (these are intended to be called by DOS 1 and DOS 2 drivers, but in Nextor the driver's interrupt routine is called explicitly by the kernel)
Konamiman
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Prerrequisite for understanding this issue: Read how interrupts and HOKSAV work in MSX-DOS.
In MSX-DOS 1 mode Nextor was incorrectly invoking
SETINTandPRVINT, which is something that don't make any sense since these routines are intended to be invoked by the drivers themselves but in the Nextor world disk drivers provide their interrupt routines at a fixed address that is invoked by the kernel.Combined with the fact that a Nextor driver not having any available device at boot time (e.g. removable device with no media inserted) will end up not having an entry in
DRVTBL, this caused the interrupt service routine of drivers in MSX-DOS ROMs to never be executed, with nasty effects (e.g. FDD motor never going off).The fix consists of just storing the previous value of
H.TIMIin an internal buffer and jumping to it at the end of the interrupt processing when booting in DOS 1 mode as well (the same thing was being done already by MSX-DOS 2 and by Nextor when booting in normal mode), this ensures that all the interrupts routines in all drivers are properly executed. And while we're at it, the code ofSETINTandPRVINThas been removed since it's completely useless in Nextor (as Nextor drivers will never invoke them).How to test
You can test this fix in an emulator using any Nextor kernel (the Sunrise IDE one will do) and a Philips NMS8280 with a duplicate disk controller, see for example this configuration in blueMSX:
The order of the slots (Nextor before the FDDs or the other way around) shouldn't matter.
With this setup and making sure that you don't have any volume actually attached to the Nextor device (e.g. in blueMSX go to "Hard disk" - "Sunrise primary", then "Sunrise secondary", and "eject" if there's something set) and that you force booting in MSX-DOS 1 mode (by e.g. pressing the "1" key while booting) you can set breakpoints in the following locations and verify that all are hit:
H.TIMI(FD9Fh)FDA1h)7900h, this corresponds to the interrupt routine of the FDD and should be hit twice, one for each FDD controller slot.Without the fix, only the first two will be hit.
Closes #64.