PC-9821 does not boot with HDN image - NEC PC-9801-55 emulation removed?

[handomade]

Does PiSCSI 24.4.1 support the NEC PC-9801-55 compatible HDN emulation from RaSCSI?

In old RaSCSI documentation, HDN images changed INQUIRY responses and MODE SENSE parameters for PC-98 SCSI BIOS compatibility.

I tested the same HDN image and the same SCSI hardware with RaSCSI bare metal, and it works on a PC-9821. With PiSCSI 24.4.1, the SCSI activity LED responds but the machine hangs during SCSI initialization.

Has the PC-98 HDN specific behavior been removed or changed in PiSCSI?

[handomade]

I found the root cause in the legacy RaSCSI source code.

In the old RaSCSI implementation, there was specific logic for .HDN extensions to enforce PC-98 geometry parameters (512 bytes/sector, 25 sectors, 8 heads):

if (xstrcasecmp(ext, _T(".HDN")) == 0) {
// Assume sector size 512, 25 sectors, 8 heads as default settings
imgoffset = 0;
imgsize = size;
sectorsize = 512;
sectors = 25;
heads = 8;
cylinders = (int)(size >> 9);
cylinders >>= 3;
cylinders /= 25;
}

It seems PiSCSI 24.4.1 has lost this PC-98 specific geometry translation (or handles it differently), causing it to return standard SCSI geometry instead. The legacy PC-9801-55 SCSI BIOS strictly requires this 25-sector / 8-head layout during MODE SENSE, and providing any other parameters causes the host machine to freeze during initialization.
​Could you check if this PC-98 specific geometry emulation for .hdn images was inadvertently removed or broken during recent refactoring?

[rdmark]

good findings, the PC-98 compatibility has indeed not been tested since the RaSCSI forking, however I don't think it was deliberately removed

this project doesn't have any active developers right now, but if you file a PR with a fix for this I'd be happy to review it

[handomade]

Thanks for your reply!​
That makes sense. I don't have a PiSCSI-specific board; I'm using an older direct-connect RaSCSI-compatible adapter with a Raspberry Pi Zero W.​
After some testing, I found two interesting things:​
• A STANDARD build works correctly with my hardware, while a FULLSPEC build hangs during SCSI initialization, even when no SCSI devices are attached. With the STANDARD build and no devices attached, the PC-9821 correctly continues to BASIC.​
• I also found that simply renaming an image from .HDN to .HDS allows it to boot. The same image fails as .HDN but works as .HDS, which makes me think there may be an issue specific to the HDN handling.​
I'm still testing with other PC-98 disk images to see whether this affects only certain HDN images or all of them.​
I'll continue comparing the old RaSCSI implementation with the current PiSCSI code. If I can identify the cause or prepare a fix, I'll be happy to submit a PR.