/* $OpenBSD: ffs.S,v 1.2 1999/05/09 18:49:10 mickey Exp $ */ /* * Copyright (c) 1990, 1991, 1992, 1994, The University of Utah and * the Computer Systems Laboratory at the University of Utah (CSL). * All rights reserved. * * Permission to use, copy, modify and distribute this software and its * documentation is hereby granted, provided that both the copyright * notice and this permission notice appear in all copies of the * software, derivative works or modified versions, and any portions * thereof, and that both notices appear in supporting documentation. * * THE UNIVERSITY OF UTAH AND CSL ALLOW FREE USE OF THIS SOFTWARE IN ITS "AS * IS" CONDITION. THE UNIVERSITY OF UTAH AND CSL DISCLAIM ANY LIABILITY OF * ANY KIND FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE. * * CSL requests users of this software to return to csl-dist@cs.utah.edu any * improvements that they make and grant CSL redistribution rights. * */ #include /* * ffs(bitmask) * * Return the position of the "most significant" bit in `bitmask'. * Since this is similar to the VAX ffs instruction, bits in a word * are numbered as "32, 31, ... 1", 0 is returned if no bits are set. */ ENTRY(ffs) comb,= arg0,r0,ffsdone ; If arg0 is 0 or r0,r0,ret0 ; return 0 ldi 32,ret0 ; Set return to high bit extru,= arg0,31,16,r0 ; If low 16 bits are non-zero addi,tr -16,ret0,ret0 ; subtract 16 from bitpos shd r0,arg0,16,arg0 ; else shift right 16 bits extru,= arg0,31,8,r0 ; If low 8 bits are non-zero addi,tr -8,ret0,ret0 ; subtract 8 from bitpos shd r0,arg0,8,arg0 ; else shift right 8 bits extru,= arg0,31,4,r0 ; If low 4 bits are non-zero addi,tr -4,ret0,ret0 ; subtract 4 from bitpos shd r0,arg0,4,arg0 ; else shift right 4 bits extru,= arg0,31,2,r0 ; If low 2 bits are non-zero addi,tr -2,ret0,ret0 ; subtract 2 from bitpos shd r0,arg0,2,arg0 ; else shift right 2 bits extru,= arg0,31,1,r0 ; If low bit is non-zero addi -1,ret0,ret0 ; subtract 1 from bitpos ffsdone: bv 0(rp) nop EXIT(ffs)