X86-assembly/Instructions/shr

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shr/sar

Description

  • The shr or sar instruction is used to shift the bits of the operand destination to the right, by the number of bits specified in the count operand.
  • Bits shifted beyond the destination are first shifted into the CF flag.
  • Equivalent to dividing by 23
  • This could be written as follows in python:
>>> def shr(dest, count):
...   return hex(dest >> count)
... 
>>> shr(0xA, 2)
'0x2'
  • shr fills with zeroes, and sar fills with the sign bit.
# Emulate the sar machine instruction
def sar_8(a, width):
    sign = a & 0x80
    a &= 0x7F
    a >>= width
    a |= sign
    return a

Syntax

shr destination, count

Example

mov eax, 0xA  ; set EAX to 0xA (1010 in binary)
shr eax, 2    ; shifts 2 bits to the right in EAX, now equal to 0x2 (0010 in binary)
              +---+---+---+---+---┬───┬───┬───┬───┐
mov eax, 0xA  | 0 | 0 | 0 | 0 | 0 │ 1 │ 0 │ 1 │ 0 │
              +---+---+---+---+---┴───┴───┴───┴───┘
                                    │   │   └───────────┐
                                    │   └───────┐       │
                                    └───────┐   │       │
              +---+---+---+---+---+---+---┬───┬───┐   ┌───┐
shr eax, 2    | 0 | 0 | 0 | 0 | 0 | 0 | 0 │ 1 │ 0 │   │ 1 │
              +---+---+---+---+---+---+---┴───┴───┘   └───┘
                                                        CF
shr eax, 1   ;Signed division by 2
shr eax, 2   ;Signed division by 4
shr eax, 3   ;Signed division by 8
shr eax, 4   ;Signed division by 16
shr eax, 5   ;Signed division by 32
shr eax, 6   ;Signed division by 64
shr eax, 7   ;Signed division by 128
shr eax, 8   ;Signed division by 256

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