From:	Ben Peddell <killer DOT lightspeed AT bigpond DOT com>
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Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: New to djgpp. Questions about protected mode, etc.
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Date:	Tue, 22 Jul 2003 16:52:57 GMT
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To:	djgpp AT delorie DOT com
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Teece wrote:
> Please bear with me since I am not all that sure about how to ask the 
> questions that I have.
>  
> I have an application that I want to develop for the PC with special 
> hardware that will run on the PCI bus. I will need to send data from the 
> program directly to the hardware. It is not necessary that I use a 
> sophisticated operating system like XP or even any version of Windows. 
> Therefore I can avoid the difficulty of writing Windows device drivers 
> for the new hardware, etc.
>  
> I suppose that I could go a couple of ways. One way would be to use good 
> old DOS, which does not run in protected mode, and use DJGPP (or VC1.52, 
> I think) to obtain an executable.  Since DOS does not know about 
> protected mode I would not cause a segmentation fault when I directly 
> access the PCI hardware. Is this correct?
>  
Yes.
You can also use the PCI BIOS in real mode or protected mode.
You can read and write PCI Configuration Registers using the PCI BIOS, 
then do direct I/O to the memory and ports of the device.

> I was reading in the DJGPP documentation that DJGPP has a DOS extender. 
> This was described as a layer of software that "traps the call, switches 
> the CPU to real mode, reissues the call, waits for the service to do its 
> thing, then switches the CPU back into protected mode, and returns to 
> the application code that called the real-mode service".
>  
Actually, DJGPP uses DPMI. It only switches to V86 mode when it needs to 
issue a real-mode call (such as Disk I/O), or an interrupt which is not 
fully handled by the program.

> Does Windows have a DOS extender? Is it not possible to run old DOS 
> programs on XP in what passes for a DOS emulator?
>  
Windows has DPMI. Alas, Windows NT/2k/XP do not permit direct port I/O 
without being severly hampered ("helped").

> Does Linux have a DOS extender? Will an executable made with DJGPP run 
> on Linux?
>  
A compiled DJGPP executable will not run natively on Linux.
However, a DJGPP program can be easily ported to Linux (provided you 
don't use too much DJGPP-specific stuff).

You'd need to make a kernel module to access the ports of the device.

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