DOS ain't dead

recursing subdirectories - redirector (Developers)

posted by ecm  , Düsseldorf, Germany, 20.10.2025, 20:30

> I looked that up:
>
> https://www.ctyme.com/intr/rb-2589.htm
>
> So it is set to point to my PSP initially, which means it will wipe out my
> command line.
>
> That's fine - and it also means that there is a known size limit to the
> size of the DTA - 0x80.
>
> Although closer inspection:
> https://www.ctyme.com/intr/rb-2977.htm#Table1626
>
> suggests that this is a fixed size that won't be expanded with new DOS
> versions.
>
> And that size is decimal 43 bytes.

That tracks, yes.

> > Alternatively you could copy the find data from a
> > single DTA and stash it elsewhere, then restore it later. In any case,
> the
> > populated find data if preserved is supposed to be able to resume
> > concurrent searches regardless of what order you call the DOS.
>
> Ok, great.
>
> > Redirectors should try to emulate this, although the find data may not
> be
> > large enough to reference their file system internals. So the find data
> can
>
> What's a redirector?

As of circa MS-DOS v3.10 and higher, there are two different kinds of file systems: local (FAT12, FAT16, or in v7.10 FAT32) and redirector. Every "DOS drive" is one or the other.

Local drives have a CDS entry, DPB, usually an msbio UPB (unless they're backed by another block device driver), and an on-disk BPB in the boot sector.

Redirector drives only have a CDS entry, which has the "net" bit set (also documented in the interrupt list). The redirector may have internal tables relating to what it provides as a DOS drive, but these are specific to the redirector and not in a generally known format.

The original redirector was the IBM/MS network client, which could allow to add DOS drives for networked file systems. Later, mscdex/shsucdx type resident CDFS extensions re-used the redirector interface to provide CD-ROM file systems as DOS drives. Other extensions such as iHPFS are also redirectors. The same is true of the "mfs" (Mach FS) part of dosemu2, which provides (optionally r/w) access to Linux host directories as DOS drives.

(shsucdx and dosemu2 mfs have some support for LFNs, which requires either cooperation by the LFN server (doslfn) in the case of shsucdx, or for the redirector extension to also hook the int 21h function 71h subfunctions in the case of mfs.)

> > contain a handle, pointing to a redirector-internal find data block,
> which
> > is cleaned up either when the search call returns "no more files", LFN
> find
> > close is called, or eg the process terminates.
>
> Ok, I looked those up.
>
> LFN FindFirst: AX = 714Eh
> https://www.ctyme.com/intr/rb-3203.htm
> LFN FindNext: AX=714Fh
> https://www.ctyme.com/intr/rb-3204.htm
> LFN FindClose: AX=714Eh
> https://www.ctyme.com/intr/rb-3211.htm

Find close is 71A1h, not 714Eh.

> And the structure that includes the LFN:
> https://www.ctyme.com/intr/rb-3203.htm#Table1779
>
> And that looks like a fixed decimal 318 byte table too.
>
> And I see that the FindFirst explicitly mentions the carry flag issue:
>
> For compatibility with DOS versions prior to v7.00, the carry flag should
> be set on call to ensure that it is set on exit

Yes, indeed, as discussed previously. But the mentions like this aren't always present.

---
l

Complete thread:

• recursing subdirectories - kerravon, 20.10.2025, 08:15 (Developers)
  • recursing subdirectories - ecm, 20.10.2025, 08:54
    • recursing subdirectories - kerravon, 20.10.2025, 18:02
      • recursing subdirectories - redirector - ecm, 20.10.2025, 20:30
        • recursing subdirectories - redirector - ecm, 20.10.2025, 20:33
        • recursing subdirectories - redirector - kerravon, 25.10.2025, 16:56
          • recursing subdirectories - redirector - usotsuki, 25.10.2025, 18:15
            • recursing subdirectories - redirector - kerravon, 26.10.2025, 03:22
              • recursing subdirectories - redirector - Rugxulo, 26.10.2025, 03:34