Int 13h buffer 64k boundaries

Fred Cisin cisin at
Thu Apr 19 14:14:33 CDT 2018

Yes, it was a "beginner" mistake to not already know that the DMA couldn't 
span a 64K boundary.
It is obvious.  Once you've already run into it.

I have no difficulty admitting that I didn't, and don't, have Chuck's 
level of experience and knowledge.
My entire venture into microcomputers was a hobby that got out of hand.

> > I'm learning a lot these days that would have been handy back then!
There are numerous people here whose posts present significant 

Grumpy Ol' Fred     		cisin at

On Wed, 18 Apr 2018, Chuck Guzis via cctalk wrote:

> Really?  64K boundary issues cropping up in MS-DOS?
> Egad, that would have been known in DOS 1.0.  Certainly, for anyone
> writing his/her own low-level disk I/O, it was obvious.
> Now, I'll add that if you wrote your own specialized device driver, DOS
> did not guarantee handing your driver a buffer that obeyed the 64K
> boundary rule.  I suspect that some DOS errors were reported to MS
> because of third-party driver bugs.
> And if you wrote a low-level driver that used 16-bit I/O, the magic
> number was 128K.
> But even in the earlies DOS 2.0 device drivers that I wrote, I included
> code to split the transfer up to get around the 64K problem if needed.
> --Chuck

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