summaryrefslogtreecommitdiffstats
path: root/src/VBox/Runtime/common/asm/ASMBitLastSetU16.asm
blob: 446e1fb5a53bf41fca0e50083a1e476b65335f27 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
; $Id: ASMBitLastSetU16.asm $
;; @file
; IPRT - ASMBitLastSetU16().
;

;
; Copyright (C) 2006-2023 Oracle and/or its affiliates.
;
; This file is part of VirtualBox base platform packages, as
; available from https://www.virtualbox.org.
;
; This program is free software; you can redistribute it and/or
; modify it under the terms of the GNU General Public License
; as published by the Free Software Foundation, in version 3 of the
; License.
;
; This program is distributed in the hope that it will be useful, but
; WITHOUT ANY WARRANTY; without even the implied warranty of
; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
; General Public License for more details.
;
; You should have received a copy of the GNU General Public License
; along with this program; if not, see <https://www.gnu.org/licenses>.
;
; The contents of this file may alternatively be used under the terms
; of the Common Development and Distribution License Version 1.0
; (CDDL), a copy of it is provided in the "COPYING.CDDL" file included
; in the VirtualBox distribution, in which case the provisions of the
; CDDL are applicable instead of those of the GPL.
;
; You may elect to license modified versions of this file under the
; terms and conditions of either the GPL or the CDDL or both.
;
; SPDX-License-Identifier: GPL-3.0-only OR CDDL-1.0
;


;*******************************************************************************
;* Header Files                                                                *
;*******************************************************************************
%include "iprt/asmdefs.mac"

BEGINCODE

;;
; Finds the last bit which is set in the given 16-bit integer.
;
; Bits are numbered from 1 (least significant) to 16.
;
; @returns (xAX)        index [1..16] of the last set bit.
; @returns (xAX)        0 if all bits are cleared.
; @param   msc:cx gcc:di x86:stack u16  Integer to search for set bits.
;
; @cproto DECLASM(unsigned) ASMBitLastSetU16(uint32_t u16);
;
RT_BEGINPROC ASMBitLastSetU16
%if ARCH_BITS == 16
        CPU     8086
        push    bp
        mov     bp, sp

        mov     cx, [bp + 2 + 2]
        test    cx, cx                  ; check if zero (eliminates checking dec ax result)
        jz      .return_zero

        mov     ax, 16
.next_bit:
        shl     cx, 1
        jc      .return
        dec     ax
        jmp     .next_bit

.return_zero:
        xor     ax, ax
.return:
        pop     bp
        ret

%else
        xor     eax, eax
 %if    ARCH_BITS == 64
  %ifdef ASM_CALL64_GCC
        bsr     ax, si
  %else
        bsr     ax, cx
  %endif
 %elif ARCH_BITS == 32
        bsr     ax, word [esp + 4]
 %else
  %error "Missing or invalid ARCH_BITS."
 %endif
        jz      .return_zero
        inc     eax
.return:
        ret
.return_zero:
        xor     eax, eax
        ret
%endif
ENDPROC ASMBitLastSetU16