DOS ain't dead

Memory at segment 50h available for DOS? (Developers)

posted by ecm  , Düsseldorf, Germany, 15.12.2008, 14:14

> A copy of floppy disk parameters (11- or 14-byte table) is typically
> placed by DOS at 0:522h, and the INT 1Eh vector points there. The table is
> moved there either at DOS BIOS (IBMBIO.COM/IO.SYS) initialization (in PC
> DOS, MS-DOS 6-, DR-DOS), or even earlier, by the bootstrap loader in the
> Boot Record (in MS-DOS 7+). (The purpose of copying the table from ROM
> BIOS to RAM is to optimize head settle time and to set the maximum sectors
> per track for high-capacity formats.)

Thanks; I once knew that table was also somewhere there. It doesn't conflict with me using the high part for another table.

> The memory area at 0:500h is also used temporarily at boot time, e.g., PC
> DOS and MS-DOS 6- bootstrap loaders read directory sectors there. But this
> is probably non-conflicting with your idea.

Yes.

> Anyway, it is rather risky to assume that the remaining area is not used
> at all by any software. What do you exactly need this space for? Is there
> any good reason to use it, other than saving some Conventional Memory for
> applications?

Well, beside that, there isn't really a good reason. I'm relocating the second Interrupt Restoration Table (IRT) which should allow to reboot with Int19 at any time, just like Jemm's FASTBOOT option. (Of course some software like EMMs have to return to real mode in their Int19 code for this to work.) However neither the table nor the Int19 handler itsef can move into the HMA or even UMA. A20 might be disabled and the XMM may not be available to enable it for using the HMA; and DOS's UMA is probably just extended memory mapped in by an (already disabled) EMM.

So, yes, I just want to save some LMA space.

I found that the DOS-C user documentation (file config.txt) states that "a reasonably safe choice" for relocating the Int09 keyboard buffer (KEYBUF command) should be offset 0x140 to 0x1BF (of segment 40h), including. I would currently place the IRT at 0x1D0, so it won't conflict with a keyboard buffer in this area. (Other locations mentioned in config.txt include the few bytes for PrnScrn/DOS single-floppy change and the BASICA data.)

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l

Complete thread:

• Memory at segment 50h available for DOS? - ecm, 14.12.2008, 21:35 (Developers)
  • Memory at segment 50h available for DOS? - marcov, 15.12.2008, 10:13
  • Memory at segment 50h available for DOS? - mht, 15.12.2008, 11:18
    • Memory at segment 50h available for DOS? - ecm, 15.12.2008, 14:14