DOS ain't dead

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

posted by mht , Wroclaw, Poland, 15.12.2008, 11:18

> I'm currently moving some data around that has to stay in the low memory
> area. Is there any data between address 0h:520h and 0h:600h? (That's
> between segment 50h and 60h except the few items described in RBIL 61's
> MEMORY.LST.) Does any program use this space?

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.)

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.

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?

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