From: "A.Appleyard" <A DOT APPLEYARD AT fs2 DOT mt DOT umist DOT ac DOT uk>
To: DJGPP AT SUN DOT SOE DOT CLARKSON DOT EDU
Date: Mon, 30 Jan 1995 16:22:16 GMT
Subject: Writing to screens etc

In accordance with suggestions re how to write to the screens etc in Gnu C, I
have looked at the file c:\djgpp\include\sys\farptr.h . The functions have
these argument patterms:-

#define uns insigned
extern inline void      _farpokeb(
   uns short selector, uns long offset, uns char value);
extern inline void      _farpokew(
   uns short selector, uns long offset, uns short value);
extern inline void      _farpokel(
   uns short selector, uns long offset, uns long value);
extern inline uns char  _farpeekb(uns short selector, uns long offset);
extern inline uns short _farpeekw(uns short selector, uns long offset);
extern inline uns long  _farpeekl(uns short selector, uns long offset);
extern inline void      _farsetsel(uns short selector);
extern inline void      _farnspokeb(uns long offset, uns char value);
extern inline void      _farnspokew(uns long offset, uns short value);
extern inline void      _farnspokel(uns long offset, uns long value);
extern inline uns char  _farnspeekb(uns long offset);
extern inline uns short _farnspeekw(uns long offset);
extern inline uns long  _farnspeekl(uns long offset);

(1) Since the graphics screen is (in real mode) at 0xa0000000 etseq, can I
write an 0x14 byte to graphics screen address 0xa0000017 in Gnu C thus:- ?
    _farsetsel(0xa000); _farnspokeb(0x0017,0x14); ?   Or how?
Can I get away with always setting the selector = 0 and putting the full
address in the offset?

(2) It seems that the text screen is sometimes at 0xb0000000, sometimes at
0xb8000000. How for each PC can a program tell which is which? Is it true that
sometimes the address is different when writing to screen from when reading
from screen? If so, what happens in e.g. `*x+=2;', where x points to a screen
address?