TITLE   RShiftU64.asm: 64-bit right shift function for IA-32

;------------------------------------------------------------------------------
;
; Copyright (c) 2004, Intel Corporation                                                         
; All rights reserved. This program and the accompanying materials                          
; are licensed and made available under the terms and conditions of the BSD License         
; which accompanies this distribution.  The full text of the license may be found at        
; http://opensource.org/licenses/bsd-license.php                                            
;                                                                                           
; THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,                     
; WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.             
; 
; Module Name:
;
;   RShiftU64.asm
; 
; Abstract:
; 
;   64-bit right shift function for IA-32
;
;------------------------------------------------------------------------------

  .686P
  .XMM
  .MODEL SMALL
  .CODE

RShiftU64 PROTO C  Operand: QWORD, Count: DWORD

RShiftU64 PROC C   Operand: QWORD, Count: DWORD

;------------------------------------------------------------------------------
; UINT64
; RShiftU64 (
;    IN UINT64   Operand,
;    IN UINTN    Count
;    )
;
; Routine Description:
;
;   This routine allows a 64 bit value to be right shifted by 32 bits and returns the 
;   shifted value.
;   Count is valid up 63. (Only Bits 0-5 is valid for Count)
;
; Arguments:
;
;   Operand - Value to be shifted
;   Count   - Number of times to shift right.
;  
; Returns:
;
;   Value shifted right identified by the Count.
;------------------------------------------------------------------------------  

  push   ecx
  
  mov    eax, dword ptr Operand[0]
  mov    edx, dword ptr Operand[4]
  
  ;
  ; CL is valid from 0 - 31. shld will move EDX:EAX by CL times but EDX is not touched
  ; For CL of 32 - 63, it will be shifted 0 - 31 so we will move edx to eax later. 
  ;
  mov    ecx, Count
  and    ecx, 63
  shrd   eax, edx, cl
  shr    edx, cl

  cmp    ecx, 32
  jc     short _RShiftU64_Done

  ;
  ; Since Count is 32 - 63, edx will have been shifted  by 0 - 31                                                     
  ; If shifted by 32 or more, set upper 32 bits to zero.
  ;
  mov    eax, edx
  xor    edx, edx

_RShiftU64_Done:
  pop    ecx
  ret

RShiftU64 ENDP

END