[CDG5] Booting 9 from driver-less disks

[Elliot Nunn]

No, the internal drive.

> On 24 Aug 2019, at 9:26 am, Daniel B-J <danielbj314 at verizon.net> wrote:
> 
> Are you doing the mini test on a CD? 
> 
> CD drives are ATAPI.
>> On Aug 23, 2019, at 9:19 PM, Elliot Nunn <elliotnunn at fastmail.com> wrote:
>> 
>> Got it! …kind of
>> 
>> This commit adds an ATALoad patch to the tbxi-patches library:
>> 
>> https://github.com/elliotnunn/tbxi-patches/commit/9351898
>> 
>> ATALoad reliably falls back on the ROM ATADisk driver when no driver partition
>> is present. In QEMU, this enables a successful boot. Not so on my Mac mini. On
>> that machine, the ROM driver does not recognise the bootable partition as being
>> bootable, and we get stuck at the question mark.
>> 
>>> On 20 Aug 2019, at 2:32 pm, Elliot Nunn <elliotnunn at fastmail.com> wrote:
>>> 
>>> Hi all,
>>> 
>>> The classic Mac OS requires non-floppy disks to have a driver partition to be
>>> bootable. The ROM ATA/SCSI/FW/USB Manager has a special code path to load a DRVR
>>> from this partition, and then the DRVR takes over. (After boot is completed,
>>> DRVRs can come from other sources.)
>>> 
>>> This was fine when Macintosh formatting utilities routinely wrote out this
>>> driver partition. But many 9-compatible Macs either shipped with, or have ended
>>> up with, driver-less HFS boot disks. Open Firmware can read the NewWorld ROM
>>> from these disks, but the ROM then leaves the machine with an OldWorld-style
>>> blinking-question-mark-on-floppy-disk. (For reasons that I don't understand yet,
>>> the ROM sometimes relents after 25 seconds of blinking, and uses its own DRVR.)
>>> 
>>> For Mac OS 8/9 to boot, the disk then needs to be reinitialised by Drive Setup
>>> or similar. This is inconvenient for tinkerers, and a show-stopper on machines
>>> with limited I/O options.
>>> 
>>> By the time this problem is encountered, the NewWorld ROM is already safely
>>> loaded into RAM. Now, with our ability to patch the NewWorld ROM, we have an
>>> opportunity to produce a workaround.
>>> 
>>> I have looked into this, and I think I am close to a solution. These are some of
>>> my findings about the ROM boot process, which I don't think are documented
>>> elsewhere. They focus on the ATA Manager, but the Monster Disk Driver Technote
>>> suggests that SCSI will work the same way.
>>> 
>>> The .ATALoad DRVR in ROM (ID -20175) uses accRun time to scan for new ATA
>>> drives, and load their ATA drivers. It can be signalled to do this by the 10.x
>>> ROM function at offset 0x5D940, and possibly by another mechanism as more disks
>>> come online. The 0x5D940 function is called in every search iteration of the ROM
>>> function FindStartupDevice. The ATA Manager has a horrible and poorly documented
>>> calling convention, but using IDA I have gotten a decent idea of the ATA Manager
>>> calls that .ATALoad makes. ATA Manager calls are used to load the driver from
>>> disk, then the driver has a special entry point at offset 8 to determine whether
>>> a given partition is bootable. At this point, I think, the driver calls the ATA
>>> Manager to "open" itself and get a slot in the unit table.
>>> 
>>> So one option is to patch ATALoad to "see" a driver partition where there is
>>> none. It would be difficult to automate such a patch to work with different
>>> versions, because the relevant function is compiled from C, so register
>>> allocations might change frequently.
>>> 
>>> At the moment I am not near a real Mac, but in Qemu, I have noticed that the 68k
>>> ROM glue to the StartLib function GetStartupDevice returns an error when the
>>> driver partition is missing. This suggests that the native StartLib parcel PEF
>>> is unable to interpret the "bootpath" property of the "chosen" node without the
>>> help of the DRVR. I think the most elegant solution would still be to fake the
>>> presence of a driver partition.
>>> 
>>> Cheers,
>>> 
>>> Elliot
>>> 
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>> 
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