Path: utzoo!attcan!uunet!wuarchive!uwm.edu!rutgers!rochester!pt.cs.cmu.edu!b.gp.cs.cmu.edu!Ralf.Brown@B.GP.CS.CMU.EDU
From: Ralf.Brown@B.GP.CS.CMU.EDU
Newsgroups: comp.os.msdos.programmer
Subject: Re: Int 10H Function 13H - How Do I access BP from C ?
Message-ID: <27288847@ralf>
Date: 26 Oct 90 19:02:31 GMT
Sender: ralf@b.gp.cs.cmu.edu
Organization: Carnegie Mellon University School of Computer Science
Lines: 31
In-Reply-To: <685@digigw.digital.co.jp>

In article <685@digigw.digital.co.jp>, gday@digigw.digital.co.jp (Gordon Day) wrote:
}inline assembler).  The value of a specific register cannot be read or changed
}from C on the PC which is why the interrupt-type functions provided by the MSC
}libraries are implemented in assembly.  The reason is that the definition 
}of the language doesn't support the idea of "named" registers, that is, a 
}mapping of a special variable name directly to a physical machine register.  
}There is nothing stopping a compiler implementor from doing this (e.g. the 
}VAX based BSD4.3 C compiler), but no compilers (to my knowledge) on the PC do,

Don't tell Turbo C, which has let you directly manipulate the CPU registers
since v1.00.  I regularly write code like

	_AX = 0x2B00 ;
	_CX = 0x4445 ;
	_DX = 0x5351 ;
	geninterrupt(0x21) ;
	if (_AL != 0xFF)
	   DESQview_version = _BX ;
	else
	   DESQview_version = 0 ;

though it takes some care when putting things other than numeric literals
into a register (since many complex assignments clobber other registers).
TC will let you mess with BP, DS, SS, and SP, but one does so at one's own
peril, since those registers are vital to the continued running of the
compiled C code.
--
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