DOS ain't dead - Booting lDOS from non-standard disk size

Booting lDOS from non-standard disk size (Announce)

posted by ecm , Düsseldorf, Germany, 29.08.2025, 16:04

> Sorry for asking something irrelevant here.
> I wonder if lDOSboot can be booted from non-standard disk size? for
> example, booting from DMF disk or disk formatted with 2M?

As for the boot sector loader, you can force some things using the instsect tool. With /Q NONE the geometry is read from the boot sector instead of querying the ROM-BIOS with int 13h function 08h, which may be needed for unusual disk sizes. /G SECTORS=x and /G HEADS=x allow to change the geometry values written to the boot sector. /L NONE forces the use of CHS access, which may be needed if the int 13h interface has bugs.

The lDOS boot iniload loader has the query patch site to allow modifying its behaviour. It has one byte for HDD boot (unit >= 80h) and another for diskette boot (unit < 80h). This allows forcing LBA, forcing CHS, and trusting the geometry passed from the prior loader (instead of querying it using int 13h function 08h, again). There is a special flag, 80h, to pass the query patch settings to the kernel. This is observed by lDebug (modifying its BOOTUNITFLxx variable that corresponds to the load unit). It isn't observed by the lDOS kernel yet.

Boot time access is purely a function of the boot sector loader and iniload. After boot time, the kernel sets up UPBs and DPBs for each recognised drive and accesses them in the usual way. That means if an unusual format is supported at all post-boot time then the ability to boot off this file system is a question about ldosboot (the boot sector loader and iniload).

If ldosboot is not able to load from a file system, it may be possible to get lDebug to boot from it, using either ldosboot (but then that may fail too) or a different boot loader. (lDOS/lDebug iniload kernels can run as io.sys, ibmbio.com, etc.) The loader in lDebug may then be able to chainload a kernel where ldosboot would have failed. The commands to run would be at least boot protocol ldos followed by q, plus whatever may be needed in addition to make the load succeed.

If you can provide me with a VM setup that reproduces a boot failure using the current ldosboot + kernel I may be able to modify things so there is a way for them to work.

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l

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Complete thread:

lDOS iniload + fdkernpl used in warez (+ FreeDOS kernel) - ecm, 04.01.2025, 21:54 (Announce)
- lDOS iniload + fdkernpl used in warez (+ FreeDOS kernel) - DosWorld, 05.01.2025, 02:25
  - lDOS iniload + fdkernpl used in warez (+ FreeDOS kernel) - mceric, 05.01.2025, 13:40
    - lDOS iniload + fdkernpl used in warez (+ FreeDOS kernel) - DosWorld, 05.01.2025, 13:52
- lDOS iniload + fdkernpl used in warez (+ FreeDOS kernel) - roytam, 29.08.2025, 05:31
  - Booting lDOS from non-standard disk size - ecm, 29.08.2025, 16:04