VirtualBox

source: vbox/trunk/src/VBox/Runtime/common/asm/ASMBitFirstSetU16.asm@ 76553

Last change on this file since 76553 was 76553, checked in by vboxsync, 6 years ago

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1; $Id: ASMBitFirstSetU16.asm 76553 2019-01-01 01:45:53Z vboxsync $
2;; @file
3; IPRT - ASMBitFirstSetU16().
4;
5
6;
7; Copyright (C) 2006-2019 Oracle Corporation
8;
9; This file is part of VirtualBox Open Source Edition (OSE), as
10; available from http://www.virtualbox.org. This file is free software;
11; you can redistribute it and/or modify it under the terms of the GNU
12; General Public License (GPL) as published by the Free Software
13; Foundation, in version 2 as it comes in the "COPYING" file of the
14; VirtualBox OSE distribution. VirtualBox OSE is distributed in the
15; hope that it will be useful, but WITHOUT ANY WARRANTY of any kind.
16;
17; The contents of this file may alternatively be used under the terms
18; of the Common Development and Distribution License Version 1.0
19; (CDDL) only, as it comes in the "COPYING.CDDL" file of the
20; VirtualBox OSE distribution, in which case the provisions of the
21; CDDL are applicable instead of those of the GPL.
22;
23; You may elect to license modified versions of this file under the
24; terms and conditions of either the GPL or the CDDL or both.
25;
26
27
28;*******************************************************************************
29;* Header Files *
30;*******************************************************************************
31%include "iprt/asmdefs.mac"
32
33BEGINCODE
34
35;;
36; Finds the first bit which is set in the given 16-bit integer.
37;
38; Bits are numbered from 1 (least significant) to 16.
39;
40; @returns (xAX) index [1..16] of the first set bit.
41; @returns (xAX) 0 if all bits are cleared.
42; @param msc:cx gcc:di x86:stack u16 Integer to search for set bits.
43;
44; @cproto DECLASM(unsigned) ASMBitFirstSetU16(uint16_t u16);
45;
46BEGINPROC_EXPORTED ASMBitFirstSetU16
47%if ARCH_BITS == 16
48 CPU 8086
49 push bp
50 mov bp, sp
51
52 ; 16:0
53 mov ax, 1
54 mov cx, [bp + 2 + 2 + 0]
55 test cx, cx
56 jz .return_zero
57
58 ; find the bit that was set.
59.next_bit:
60 shr cx, 1
61 jc .return
62 inc ax
63 jmp .next_bit
64
65.return_zero:
66 xor ax, ax
67.return:
68 pop bp
69 ret
70
71%else
72 xor eax, eax
73 %if ARCH_BITS == 64
74 %ifdef ASM_CALL64_GCC
75 bsf ax, si
76 %else
77 bsf ax, cx
78 %endif
79 %elif ARCH_BITS == 32
80 bsf ax, word [esp + 4]
81 %else
82 %error "Missing or invalid ARCH_BITS."
83 %endif
84 jz .return_zero
85 inc eax
86.return:
87 ret
88.return_zero:
89 xor eax, eax
90 ret
91%endif
92ENDPROC ASMBitFirstSetU16
93
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