Date:	Tue, 6 Jan 1998 16:06:44 +0200 (IST)
From:	Eli Zaretskii <eliz AT is DOT elta DOT co DOT il>
To:	Robert Smith <Robert DOT Smith AT rd DOT bbc DOT co DOT uk>
cc:	djgpp AT delorie DOT com
Subject:	Re: Accessing Phyical Memory Above 1 MB - Clarification
In-Reply-To:	<34B20E67.2F1CF0FB@rd.bbc.co.uk>
Message-ID:	<Pine.SUN.3.91.980106155325.2989A-100000@is>
MIME-Version:	1.0

On Tue, 6 Jan 1998, Robert Smith wrote:

> From section 18.7 of the DJGPP FAQ, I understand that I have to do 
> something like the following to initialise a pointer (base address of
> PCI board memory assumed to be 0x1000000, size of memory assumed to 
> be 16 Kbytes, error handling not included here for clarity) :

I didn't look at every detail of your code, but it seems like it should
work.  It turns out that lock_linear_region call is unnecessary, though
(the FAQ is wrong). 

> Then to access the boards memory I must do something like the following:
> 
> /* Reading a byte from the start of the boards memory */
> Data8 = _farpeekb(Sel,0l);
>  
> /* Inverting the second double word in the boards memory */
> Data32 = _farpeekl(Sel,4l);
> Data32 = ~Data32;
> farpokel(Sel,4l,Data32);

Correct, except that you can express the last 3 lines as a single line:

  _farpokel(Sel, 41, ~_farpeekl(Sel, 41);

or even faster:

  _farsetsel(Sel);
  _farnspokel(41, ~_farnspeekl(41));

> The above does not provide what I want since it is necessary to
> use the routines in <sys/farptr.h> to access the memory, whereas 
> I wish to access the memory directly.

Why?  What's wrong with using the farptr functions?

> From the DJ Delorie's response to comp.os.msdos.djgpp newsgroup posting
> Re:dos_ds, (Sun 4 Jan 1998), I believe that it is also possible for 
> me to access the boards memory as follows:

_dos_ds has nothing to do with accessing memory-mapped devices above 
1MB, and cannot be used to access memory above the address 1MB+64K.  You 
might succeed using nearptr facility to access the memory-mapped device 
directly, but I won't recommend that, since it defeats memory protection, 
but usually gains you nothing in terms of speed.

> /* Initialise board memory pointer */
> BoardMem = (struct BoardMem_s *) BOARD_BASE_ADDRESS -
> __djgpp_base_address;

This is not enough.  You still have to map the device into your address 
space, and you need to call `__djgpp_nearptr_enable' before using the 
base_address variable.  The reason for this is that BOARD_BASE_ADDRESS is 
a physical address, whereas the pointers your program uses are logical 
addresses.  Mapping the device into your address space supplies a logical 
address of the device which is the only address you can use from your 
program.

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