Path: utzoo!utgpu!water!watmath!clyde!rutgers!ames!amdahl!dlb!plx!slvblc!dick
From: dick@slvblc.UUCP (Dick Flanagan)
Newsgroups: comp.sys.ibm.pc,att.sys.pc6300
Subject: Re: DMA BOUNDARY
Summary: The track buffer cannot cross a PHYSICAL 64K boundary
Keywords: BIOS
Message-ID: <343@slvblc.UUCP>
Date: 31 Jan 88 00:17:20 GMT
References: <1236@mtuxo.UUCP>
Sender: uupc@slvblc.UUCP
Reply-To: dick@slvblc.UUCP (Dick Flanagan)
Organization: SLV Systems Group, Ben Lomond, CA
Lines: 36
Disclaimer: none

In article <1236@mtuxo.UUCP> rayii@mtuxo.UUCP (XMRJ4-R.HAYES) writes:
> I am currently writing a program that uses interrupt 13h function 05h.
> format track. At home i use a IBM PC/XT. After coding the proper values
> for all the registers and executing the program everything works fine.
> BUT when i execute the same code on pc6300 (at work) i receive an error
> 09h DMA BOUNDARY ERROR OCCURRED DATA DOES NOT RESIDE ON 64K BYTE DMA AREA

The track buffer you are writing to/from must not cross a *PHYSICAL* 64Kb
boundary within the 20-bit address space.  Because the pc6300 has a
different resident environment (smaller/larger OS, more/fewer BUFFERS,
etc), your program's buffer happened to fall across one of those
boundaries when it was loaded on that particular machine.

One solution is to dynamically allocate (e.g., malloc()) the buffer
and then test to see if it crosses a 20-bit 64Kb boundary.  If it does,
calculate where that boundary is within the buffer, and then how much
larger the buffer would have to be if your I/O buffer were to start AT
that 64Kb boundary.  Finally, release the buffer you just allocated
and allocate another one the size you just calculated would be large
enough--taking care to adjust your I/O buffer to start AT the physical
64Kb boundary, not somewhere in front of it.

In a static, single-user system like DOS, this should be sufficient.
In a system where the memory you get from an allocation may not be the
most recent memory you released, the test would need to be repeated
and/or refined.

Good luck--and welcome to the world of the classic "Gotcha's!"

Dick

--
Dick Flanagan, W6OLD                        GEnie: FLANAGAN
UUCP: ...!sun!plx!slvblc!dick               Voice: +1 408 336 3481
USPO: PO Box 155, Ben Lomond, CA 95005      LORAN: N37 05.5 W122 05.2
--