Quirks for PC-9801-55 and derivatives

[s-rakowski]

Hi, and thank you for working on this great project.

Recently I've been trying to get BlueSCSI v2 to work with a NEC PC-9801FA, with its dedicated PC-9801FA-02 SCSI-1 controller. The original HDD is missing. Apparently, the dedicated controller is internally the same as the PC-9801-55 board, but I don't have that one, so I can't confirm if, for example, its ROM is different. Nevertheless, both controllers require putting Vendor=NEC in the INI file, as well as specific geometry settings.

However, even after configuring the documented settings, I couldn't get the emulated HDD to be detected.

When I made the BlueSCSI debug all the commands, I have noticed that the initiator sends a single Mode Sense 6 command, allocating 64 bytes for the answer: 0x1A 0x00 0xFF 0x00 0x40 0x00. With the current firmware, even with Toolbox disabled, the "Rigid disk geometry page" was never actually sent, and the controller would never try again.

I've noticed that this project has a separate Mode Sense implementation for PC-98: https://github.com/ztto/ArdSCSino-stm32/blob/master/src/ArdSCSIno.cpp#L1434

So I tried hacking on BlueSCSI, using the source above as a reference, and doing all of the following made it work:

• Commenting out Disconnect-reconnect page
• Commenting out Vendor page (0x00)
• Setting Interleave value in Format Device page to 0
• Explicitly setting block count in Block Descriptor to scsiDev.target->cfg->scsiSectors
• Setting bytes 2 and 3 (Tracks per zone) of Format Device page to scsiDev.target->cfg->headsPerCylinder (I don't really get this one, but that's what the ArdSCSino code above does, and leaving this field set to 0 prevents the HDD from being properly detected)

With all of the above, all returned Mode Sense pages now fit in 64 bytes, and everything seems to be working fine.

Of course, none of this is probably supposed to be the default behavior, but I think about making a PR that would add a dedicated Quirks flag to enable those hacks. Do you think it would be a good idea?

[erichelgeson]

That is really strange! The host should request the specific page if it wants disk geometry but I suppose they just assumed they were using only this specific NEC drive and not have to worry about the spec here.

You can set SectorsPerTrack and HeadsPerCylinder in the ini so no code change needed there.
scsiSectors is determined by the size of the file - you'll likely need to create one exactly the right size for this use case.

If you'd like to take a stab at this please do, happy to answer any questions.

[s-rakowski]

I did figure out the INI settings quite early:

Vendor=NEC
SectorsPerTrack=25
HeadsPerCylinder=8
EnableSCSI2=0
EnableToolbox=0

The host should request the specific page if it wants disk geometry but I suppose they just assumed they were using only this specific NEC drive and not have to worry about the spec here.

What I'm thinking is that the ROM expects a predefined set of pages in the Mode Sense answer, and just reads geometry-related fields from hard-coded offsets! However, I can't confirm it for sure, because I'm not experienced at x86 reverse engineering.

To clarify my original question, just in case - what I would like to do is to add a Quirks flag which would do roughly what the "ugly" modification does here: https://github.com/s-rakowski/BlueSCSI-v2/tree/pc98-hack
Quirks seem to be easily accessible as scsiDev.target->cfg->quirks.
Would that be OK?

[erichelgeson]

Yes looks like you're on the right track, open a PR when you're ready to review!

One thing to note is we probably want a corresponding preset so you dont have to set all the ini settings as well, so it'll set the quirks and default ini settings as you described.