🚀 go-pugleaf

In <18798@brahms.udel.edu> boutell@brahms.udel.edu (Thomas B Boutell) writes:

> .COM files must fit within one 64k region, I believe. (Sorry, I can't 
> find my DOS bible; somebody borrowed it about a year ago. )-: Otherwise
> I'd give you the interrupt number.) 

INT 27H.

.COM files must be < 64K for two reasons:

	1) SS==DS==ES==CS on startup.  That means that your stack pointer is
	   pointing into your code segment at the top of the first 64K, and
	   can randomly trash stuff there due to hardware interrupts.  In
	   reality, the limit is something less than 64K, dependant on the
	   stack usage of ISR handlers, and maybe a little dependant on
	   luck. :-)

	2) DOS will simply not load a .COM file larger than 64K.  It's
	   easily possible to produce one (though most programming tools -
	   specifically linkers - tend to discourage it), but it won't
	   be too successful when it comes to executing it.
	   
	   And I'm talking about REAL .COM format binaries, not just .EXE's
	   which are renamed to .COM.  .EXE format is already known by DOS
	   and is tested for prior loading any binary file for execution,
	   despite it's extension which is why it's possible to rename a
	   .COM to a .EXE with no serious effects other than changing the
	   "execution priority" from the command line of that file.


> However, in versions of DOS from 2.0 on up, there is a call that will 
> allocate as many pages (16- byte increments) as you wish, and of course 
> nobody seriously uses versions of DOS prior to 2.0 any longer.

Just for reference, INT 21H, function 31H.

One important aspect of using this function is that you can pass an
errorlevel back to the parent, which is not possible with INT 27H.


	david
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