86 lines
2.5 KiB
Plaintext
86 lines
2.5 KiB
Plaintext
/*
|
|
* File......: BITCLR.PRG
|
|
* Author....: Forest Belt, Computer Diagnostic Services, Inc.
|
|
* CIS ID....: ?
|
|
*
|
|
* This is an original work by Forest Belt and is placed in the
|
|
* public domain.
|
|
*
|
|
* Modification history:
|
|
* ---------------------
|
|
*
|
|
* Rev 1.2 15 Aug 1991 23:02:50 GLENN
|
|
* Forest Belt proofread/edited/cleaned up doc
|
|
*
|
|
* Rev 1.1 14 Jun 1991 19:50:58 GLENN
|
|
* Minor edit to file header
|
|
*
|
|
* Rev 1.0 01 Apr 1991 01:00:40 GLENN
|
|
* Nanforum Toolkit
|
|
*
|
|
*/
|
|
|
|
|
|
/* $DOC$
|
|
* $FUNCNAME$
|
|
* FT_BITCLR()
|
|
* $CATEGORY$
|
|
* String
|
|
* $ONELINER$
|
|
* Clear (reset) selected bit in a byte
|
|
* $SYNTAX$
|
|
* FT_BITCLR( <cByte>, <nBitPos> ) -> cByte
|
|
* $ARGUMENTS$
|
|
* <cByte> is a character from CHR(0) to CHR(255).
|
|
*
|
|
* <nBitPos> is a number from 0 to 7 conforming to standard
|
|
* right-to-left bit numbering convention and representing the
|
|
* position of the bit within the byte.
|
|
* $RETURNS$
|
|
* Returns new byte, with designated bit cleared (reset).
|
|
* If parameters are faulty, returns NIL.
|
|
* $DESCRIPTION$
|
|
* In effect, ANDs argument byte with a byte that has all bits set except
|
|
* the target bit. If bit is already clear (0), it remains clear.
|
|
* Note: Calls FT_ISBIT() which is also in this Library.
|
|
*
|
|
* This function is presented to illustrate that bit-wise operations
|
|
* are possible with Clipper code. For greater speed, write .C or
|
|
* .ASM versions and use the Clipper Extend system.
|
|
* $EXAMPLES$
|
|
* This code would clear bit 4 in a byte represented by CHR(115):
|
|
*
|
|
* cNewByte := FT_BITCLR( CHR(115), 4 )
|
|
* ? ASC( cNewbyte ) // result: 99
|
|
* ? cNewByte // result: 'c'
|
|
*
|
|
* This code would clear bit 5 in the byte represented by letter 'A':
|
|
*
|
|
* FT_BITCLR( 'A', 5 ) // result: 'A', since
|
|
* // bit 5 already clear
|
|
*
|
|
* For a demonstration of Clipper bit manipulations, compile and
|
|
* link the program BITTEST.PRG in the Nanforum Toolkit source code.
|
|
* $SEEALSO$
|
|
* FT_BITSET() FT_ISBIT()
|
|
* $END$
|
|
*/
|
|
|
|
FUNCTION FT_BITCLR(cInbyte, nBitpos)
|
|
|
|
LOCAL cByte
|
|
|
|
IF valtype(cInbyte) != "C" .or. valtype(nBitpos) != "N" // parameter check
|
|
cByte := NIL
|
|
ELSE
|
|
IF (nBitPos > 7) .or. (nBitPos < 0) .or. (nBitPos != int(nBitPos))
|
|
cByte := NIL
|
|
ELSE
|
|
cByte := iif( .not. FT_ISBIT(cInByte, nBitpos), cInByte, ;
|
|
chr(asc(cInByte) - (2 ^ nBitpos)))
|
|
ENDIF
|
|
ENDIF
|
|
|
|
RETURN cByte
|
|
|