Date:	Sun, 20 Aug 1995 13:08:58 +0300 (IDT)
From:	Eli Zaretskii <eliz AT is DOT elta DOT co DOT il>
To:	Matt Emmett <matt DOT emmett AT bbs DOT logicnet DOT com>
Cc:	djgpp AT sun DOT soe DOT clarkson DOT edu
Subject:	Re: BIOS data area

On 15 Aug 1995, Matt Emmett wrote:

>   i'm wondering to access the Bios data area, specifically the address
> at 40h:42h. anyhelp would be appreciated. thanx.

This is explained in the DJGPP FAQ list (available as faq102.zip from the 
same place you get DJGPP):


18.4  Q: How can I move data between my program and the transfer buffer?
      Q: How do I access my peripheral card which is memory-mapped to an
         address between 640K and 1M?
      Q: How can I read or change a value of one of the variables in the
         BIOS data area?
      Q: How can I peek at an address whose far pointer I get from an INT
         21h call?
      A: Depending on your specific needs, you can use one of three
         methods:

           * If you want to access a byte, a 16-bit word, or a 32-bit
             double word, use the ``far pointer'' functions documented on
             the sys/farptr.h header file.  You should convert any
             real-mode far pointer segment:offset pair into a linear
             address (segment*16 + offset), and use
             _go32_conventional_mem_selector() to get the selector which
             allows access to conventional memory, like this:

              u_char value = _farpeekb(_go32_conventional_mem_selector(),
                                       segment*16 + offset);

             Use _farpeekw() to peek at 16-bit shorts and _farpeekl() to
             peek at 32-bit longs.  If you need to access several
             (non-contiguous) values in a loop, use corresponding
             _farnspeekX() functions which allow you to set the selector
             only once, as opposed to passing it with every call.

             There is a corresponding set of _farpokeX() and _farnspokeX()
             functions to poke (change the values of) such memory
             locations.

             These functions have an advantage of emitting inline assembly
             code when you compile with optimizations, so they are very
             fast.

           * If you need to access more than 4 contiguous bytes, use
             dosmemget() and dosmemput() library functions (documented in
             libcref.i file).  They also require that you convert the
             segment:offset pair into a linear address, but they don't
             need the conventional memory selector.

             Note that some memory-mapped peripheral devices might require
             16-bit word accesses to work properly, so if dosmemXXX()
             yields garbled results, use farptr functions or movedata()
             (see below).
           * For moving buffers larger than a few tens of bytes, it's best
             to use movedata() library function (also documented in
             libcref.i file).  It requires that you pass selector and
             offset for both the conventional memory address and for the
             buffer in your program's address space.  Use the function
             _go32_my_ds() to get the selector of any variable in your
             program, and its address as its ``offset'' or linear address.
             Movedata() is faster because it moves by 32-bit longs, but be
             careful with its use when moving data to and from peripheral
             cards: many of them only support 8- or 16-bit wide data path,
             so moving data 4 bytes at a time won't gain you much, and
             might even get you in trouble with some buggy BIOSes.

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