Date   : Tue, 26 Jul 1994 08:51:12 EST
From   : Stephen Quan <quan@...>
Subject: Re: Erk! I've got stuck again!

Chris Rae writes:

> No sooner than I've started, I've come across an STA &FE30.

Doesn't some hardware attach itself to these locations?
I remember there were a few bytes up there that you set which gave
you direct access to the user port and perhaps a few other things.

In my highschool days and matriculation, we worked with programming
the user port, and all we had was a basic program that read and
wrote a few bytes.  Does anyone have a memory map of this region?
I seem to remember the names FRED, JIM and SHIELA could anyone post
info on this too?

Anyhow, I have been doing some more programming on MSDOS.  The way how
I see it, is that you implement the STA instruction under assembly
similar to the following

       ....  /* Precalculate the STA address and have it sitting in SI. */
             test si,si
             js   @rom_no_sta
             mov  [cs:si],al
@...

If you are doing it in C, you could do.

      if ((signed short) address >= 0)
        *address = newbyte;
-- 
Stephen Quan (quan@...                 ), SysAdmin, Analyst/Programmer.
Centre for Spatial Information Studies, University of Tasmania, Hobart.
GPO BOX 252C, Australia, 7001.  Local Tel: (002) 202898 Fax: (002) 240282
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