移位和循环移位指令 shl, shr sal, sar, sol, sor, scl, scr, shld, shrd

移位和循环移位指令

shl, shr sal, sar, sol, sor, scl, scr, shld, shrd

逻辑移位和算术移位

• shl

  shl reg/mem, imm8/cl

  mov bl, 8fh
  shl bl, 1 ; bl = 00011110b cf = 1
  
  mov al, 10000000b
  shl al, 2 ; cf = 0, al = 00000000b
  

  当一个数多次进行左移时,进位标志保存的是最后移出最高有效位的数值

  位元乘法

• shr

  shr reg/mem, imm8/cl

  ```assembly
  

mov al, 0D0h
shr al, 1 ; al = 01101000 cf = 0

mov al, 00000010b
shr al, 2 ; al = 00000000b cf = 1
  ```

位元除法

当一个数多次进行右移时,进位标志保存的是最后移出最低有效位的数值

• sal和sar

  sal 与shl一样

  mov al, 0f0h
  sar al, 1 ; cf = 0 al = 11111000b
  
  
  ; 有符号数除法
  mov dl, -128
  sar dl, 3 ; dl = 11110000b cf = 0
  
  ; ax符号扩展到eax
  mov ax, -128
  shl eax, 16
  sar eax, 16
  

• rol

  把所有位都向左移, 最高位复制到进位标志位和最低位

  mov al, 40h
  rol al, 1 ; al = 10000000b cf = 0
  rol al, 1 ; al = 00000001b cf = 1
  rol al, 1 ; al = 00000010b cf = 9
  
  ; 循环多次
  mov al, 26h
  rol al, 4 ; al = 62h
  
  mov ax, 6a4bh
  rol ax, 4 ; ax = a4b6h
  rol ax, 4 ; ax = 4b6ah
  rol ax, 4 ; ax = b6a4h
  rol ax, r ; ax = 6a4bh
  

• ror

  把所有位都向右移, 最低位复制到进位标志位和最高位

mov al, 01h
ror al, 1 ; al = 10000000b cf = 1
ror al, 1 ; al = 01000000b cf = 0

; 循环多次
mov al, 00000100b
ror al, 3 ; al = 80h cf = 1

• rcl 和 rcr

  • rcl: 每一位左移,进位标志复制到最低位,最高位复制到进位标志
  • rcr: 每一位右移,进位标志复制到最高位,最低位复制到进位标志

  clc ; cf = 0
  mov bl, 88h
  rcl bl, 1 ; bl = 00010000b cf = 1
  rcl b1, 1 ; bl = 00100001b cf = 0
  
  ; 从进位标志位恢复位
  .data
  testVal byte 01101010b
  .code
  shr testVal, 1 ; testVal = 00110101b cf = 0
  jc exit
  rcl testVal, 1 ; testVal = 01101010b cf = 0
  
  stc ; cf = 1
  mov ah, 10h
  rcr ah, 1 ; ah = 10001000b cf = 0
  

  有符号数溢出

  mov al, +127
  rol al, 1 ; al = 11111110b of = 1
  
  mov al, -128
  shr al, 1 ; al = 01000000b of = 1
  

• shld和shrd指令

shld/shrd dest, source, count

shld/shrd reg16, reg16, cl/imm8

shld/shrd mem16, reg16, cl/imm8

shld/shrd reg32, reg32, cl/imm8

shld/shrd mem32, reg32, cl/imm8

.data
wVal word, 9ba6h
.code
mov ax, 0ac36h
shld wVal, ax, 4 ; wVal = ba6ah

mov ax, 234bh
mov dx, 7654h
shrd ax, dx, 4 ; ax = 4243

.data
array dword 648b2165h, 8c943a29h, 6dfa4b86h, 91f76c04h, 8baf9857h
.code
mov bl, 4
mov esi, offset array
mov ecx, (lengthof array) - 1
L1:
	push ecx
	mov eax, [esi + type dword]
	mov cl, bl
	shrd [esi], eax, cl
	
	add esi, type dword
	pop ecx
	loop L1
	shr dword ptr [esi], 4