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From: roelof@idca.tds.PHILIPS.nl (R. Vuurboom)
Newsgroups: comp.lang.c
Subject: Re: Ambiguous C?
Message-ID: <111@ssp1.idca.tds.philips.nl>
Date: 26 Apr 89 13:36:41 GMT
Organization: Philips Telecommunication and Data Systems, The Netherlands
Lines: 52



The following piece of code got me into trouble.
I needed to access a register which only allows long accesses.

The intention of the code is to do an int (on the motorola a long) 
access and then determine the 14th bit.

The compiler generated a byte access for 2 bytes further and then tested
the 6th bit. For locations in memory this will deliver the same result
however a byte access to the register caused a bus timeout error to occur.

There are basically (at least) 2 interpretations to the code:

1. dereference the location as an int then bit and with bit 13. In which
   case the generated code is probably wrong.

2. Determine the status of the 14th bit at the given location in which
   case the generated code is probably right.

Does C specify which (if any) interpretation is correct?




#define	SCUCMD		0 
#define	SCU_BDID	0x2000		/* bit 14 */

error()
{
	if ( (*(int *)(SCUCMD)) & SCU_BDID )
		;
}



	.text
	.globl	_error
_error
	jra	.L40000
.L40001
	btst.b	#5,2			<== dereferenced as byte
	jeq	.L13			
.L13
.L12
	unlk	a6
	rts
.L40000
	link.w	a6,#-4
	jra	.L40001
	.data
	.data