Assembly - shr instruction turn on carry flag?
I saw the next code:
shr AL, 1
jnc bit_0
I don't understand when the carry flag is turn on due to shr instrucrion?
Thanks
assembly x86
asked Mar 22 '12 at 14:13
Adam ShAdam Sh
2,573164372
add a comment |
I saw the next code:
shr AL, 1
jnc bit_0
I don't understand when the carry flag is turn on due to shr instrucrion?
Thanks
assembly x86
asked Mar 22 '12 at 14:13
Adam ShAdam Sh
2,573164372
2
It's testing whether AL is even or odd.
– harold
Mar 22 '12 at 16:04
add a comment |
I saw the next code:
shr AL, 1
jnc bit_0
I don't understand when the carry flag is turn on due to shr instrucrion?
Thanks
assembly x86
asked Mar 22 '12 at 14:13
Adam ShAdam Sh
2,573164372
I saw the next code:
shr AL, 1
jnc bit_0
I don't understand when the carry flag is turn on due to shr instrucrion?
Thanks
assembly x86
assembly x86
asked Mar 22 '12 at 14:13
Adam ShAdam Sh
2,573164372
asked Mar 22 '12 at 14:13
Adam ShAdam Sh
2,573164372
asked Mar 22 '12 at 14:13
Adam ShAdam Sh
2,573164372
asked Mar 22 '12 at 14:13
Adam ShAdam Sh
2,573164372
asked Mar 22 '12 at 14:13
Adam ShAdam Sh
2,573164372
2,573164372
2
It's testing whether AL is even or odd.
– harold
Mar 22 '12 at 16:04
add a comment |
2
It's testing whether AL is even or odd.
– harold
Mar 22 '12 at 16:04
2
2
It's testing whether AL is even or odd.
– harold
Mar 22 '12 at 16:04
It's testing whether AL is even or odd.
– harold
Mar 22 '12 at 16:04
add a comment |
2 Answers
2
active
oldest
votes
shr AL, 1 moves all of the bits in AL right one place.
The original rightmost bit is shifted out of the AL register into the carry flag (and the new leftmost bit is set to 0). For example:
+------------------------+ +------------------------+
| 1 0 0 1 0 1 0 1 | | 0 1 1 0 0 1 0 0 |
+------------------------+ +------------------------+
or
+------------------------+ CF +------------------------+ CF
|(0) 1 0 0 1 0 1 0 | 1 |(0) 0 1 1 0 0 1 0 | 0
+------------------------+ +------------------------+
In your comment on another answer:
My book gives the next example: Before shift: AL = 10101110 ; shr AL, 1 ; After shift: 01011100, CF = 1 ; Is it mistake?
If that's what it says, then yes. That's a left shift (shl AL, 1), not a right shift: the bits in AL have all been moved left by one place, and the carry flag has been set to the bit that was shifted out of the left-hand end.
answered Mar 24 '12 at 17:07
Matthew SlatteryMatthew Slattery
35.5k382105
add a comment |
to add some clarifications :
shr AL, 1 ; it will (SH)ift (R)ight (1)x the value contained in AL register, which is 8 lower bits of AX register. Hence it will divide the value contained in AL by 2.
If AL was previously even like 0b100 (4) it will become 0b10 (2) and put 0 in the carry flag.
Carry Flag is bit 0 in the flag register https://en.wikipedia.org/wiki/FLAGS_register
If AL was previously an odd value like 0b101 (5) then it will become 0b10 (2) and put 1 in the flag register. Hence the carry flag will act like a remainder if you divide by 2.
jnc bit_0 ; It will (J)ump to label 'bit_0' if (N)o (C)arry flag was set, i.e if the value was even (like 0b100 in the above example) before the shift.
edited Nov 13 '18 at 14:24
Mustafa Enes Batur
54
answered Jan 23 '18 at 11:43
Antonin GAVRELAntonin GAVREL
567621
add a comment |
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2 Answers
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active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
shr AL, 1 moves all of the bits in AL right one place.
The original rightmost bit is shifted out of the AL register into the carry flag (and the new leftmost bit is set to 0). For example:
+------------------------+ +------------------------+
| 1 0 0 1 0 1 0 1 | | 0 1 1 0 0 1 0 0 |
+------------------------+ +------------------------+
or
+------------------------+ CF +------------------------+ CF
|(0) 1 0 0 1 0 1 0 | 1 |(0) 0 1 1 0 0 1 0 | 0
+------------------------+ +------------------------+
In your comment on another answer:
My book gives the next example: Before shift: AL = 10101110 ; shr AL, 1 ; After shift: 01011100, CF = 1 ; Is it mistake?
If that's what it says, then yes. That's a left shift (shl AL, 1), not a right shift: the bits in AL have all been moved left by one place, and the carry flag has been set to the bit that was shifted out of the left-hand end.
answered Mar 24 '12 at 17:07
Matthew SlatteryMatthew Slattery
35.5k382105
add a comment |
shr AL, 1 moves all of the bits in AL right one place.
The original rightmost bit is shifted out of the AL register into the carry flag (and the new leftmost bit is set to 0). For example:
+------------------------+ +------------------------+
| 1 0 0 1 0 1 0 1 | | 0 1 1 0 0 1 0 0 |
+------------------------+ +------------------------+
or
+------------------------+ CF +------------------------+ CF
|(0) 1 0 0 1 0 1 0 | 1 |(0) 0 1 1 0 0 1 0 | 0
+------------------------+ +------------------------+
In your comment on another answer:
My book gives the next example: Before shift: AL = 10101110 ; shr AL, 1 ; After shift: 01011100, CF = 1 ; Is it mistake?
If that's what it says, then yes. That's a left shift (shl AL, 1), not a right shift: the bits in AL have all been moved left by one place, and the carry flag has been set to the bit that was shifted out of the left-hand end.
answered Mar 24 '12 at 17:07
Matthew SlatteryMatthew Slattery
35.5k382105
add a comment |
shr AL, 1 moves all of the bits in AL right one place.
The original rightmost bit is shifted out of the AL register into the carry flag (and the new leftmost bit is set to 0). For example:
+------------------------+ +------------------------+
| 1 0 0 1 0 1 0 1 | | 0 1 1 0 0 1 0 0 |
+------------------------+ +------------------------+
or
+------------------------+ CF +------------------------+ CF
|(0) 1 0 0 1 0 1 0 | 1 |(0) 0 1 1 0 0 1 0 | 0
+------------------------+ +------------------------+
In your comment on another answer:
My book gives the next example: Before shift: AL = 10101110 ; shr AL, 1 ; After shift: 01011100, CF = 1 ; Is it mistake?
If that's what it says, then yes. That's a left shift (shl AL, 1), not a right shift: the bits in AL have all been moved left by one place, and the carry flag has been set to the bit that was shifted out of the left-hand end.
answered Mar 24 '12 at 17:07
Matthew SlatteryMatthew Slattery
35.5k382105
shr AL, 1 moves all of the bits in AL right one place.
The original rightmost bit is shifted out of the AL register into the carry flag (and the new leftmost bit is set to 0). For example:
+------------------------+ +------------------------+
| 1 0 0 1 0 1 0 1 | | 0 1 1 0 0 1 0 0 |
+------------------------+ +------------------------+
or
+------------------------+ CF +------------------------+ CF
|(0) 1 0 0 1 0 1 0 | 1 |(0) 0 1 1 0 0 1 0 | 0
+------------------------+ +------------------------+
In your comment on another answer:
My book gives the next example: Before shift: AL = 10101110 ; shr AL, 1 ; After shift: 01011100, CF = 1 ; Is it mistake?
If that's what it says, then yes. That's a left shift (shl AL, 1), not a right shift: the bits in AL have all been moved left by one place, and the carry flag has been set to the bit that was shifted out of the left-hand end.
answered Mar 24 '12 at 17:07
Matthew SlatteryMatthew Slattery
35.5k382105
answered Mar 24 '12 at 17:07
Matthew SlatteryMatthew Slattery
35.5k382105
answered Mar 24 '12 at 17:07
Matthew SlatteryMatthew Slattery
35.5k382105
answered Mar 24 '12 at 17:07
Matthew SlatteryMatthew Slattery
35.5k382105
35.5k382105
add a comment |
add a comment |
to add some clarifications :
shr AL, 1 ; it will (SH)ift (R)ight (1)x the value contained in AL register, which is 8 lower bits of AX register. Hence it will divide the value contained in AL by 2.
If AL was previously even like 0b100 (4) it will become 0b10 (2) and put 0 in the carry flag.
Carry Flag is bit 0 in the flag register https://en.wikipedia.org/wiki/FLAGS_register
If AL was previously an odd value like 0b101 (5) then it will become 0b10 (2) and put 1 in the flag register. Hence the carry flag will act like a remainder if you divide by 2.
jnc bit_0 ; It will (J)ump to label 'bit_0' if (N)o (C)arry flag was set, i.e if the value was even (like 0b100 in the above example) before the shift.
edited Nov 13 '18 at 14:24
Mustafa Enes Batur
54
answered Jan 23 '18 at 11:43
Antonin GAVRELAntonin GAVREL
567621
add a comment |
to add some clarifications :
shr AL, 1 ; it will (SH)ift (R)ight (1)x the value contained in AL register, which is 8 lower bits of AX register. Hence it will divide the value contained in AL by 2.
If AL was previously even like 0b100 (4) it will become 0b10 (2) and put 0 in the carry flag.
Carry Flag is bit 0 in the flag register https://en.wikipedia.org/wiki/FLAGS_register
If AL was previously an odd value like 0b101 (5) then it will become 0b10 (2) and put 1 in the flag register. Hence the carry flag will act like a remainder if you divide by 2.
jnc bit_0 ; It will (J)ump to label 'bit_0' if (N)o (C)arry flag was set, i.e if the value was even (like 0b100 in the above example) before the shift.
edited Nov 13 '18 at 14:24
Mustafa Enes Batur
54
answered Jan 23 '18 at 11:43
Antonin GAVRELAntonin GAVREL
567621
add a comment |
to add some clarifications :
shr AL, 1 ; it will (SH)ift (R)ight (1)x the value contained in AL register, which is 8 lower bits of AX register. Hence it will divide the value contained in AL by 2.
If AL was previously even like 0b100 (4) it will become 0b10 (2) and put 0 in the carry flag.
Carry Flag is bit 0 in the flag register https://en.wikipedia.org/wiki/FLAGS_register
If AL was previously an odd value like 0b101 (5) then it will become 0b10 (2) and put 1 in the flag register. Hence the carry flag will act like a remainder if you divide by 2.
jnc bit_0 ; It will (J)ump to label 'bit_0' if (N)o (C)arry flag was set, i.e if the value was even (like 0b100 in the above example) before the shift.
edited Nov 13 '18 at 14:24
Mustafa Enes Batur
54
answered Jan 23 '18 at 11:43
Antonin GAVRELAntonin GAVREL
567621
to add some clarifications :
shr AL, 1 ; it will (SH)ift (R)ight (1)x the value contained in AL register, which is 8 lower bits of AX register. Hence it will divide the value contained in AL by 2.
If AL was previously even like 0b100 (4) it will become 0b10 (2) and put 0 in the carry flag.
Carry Flag is bit 0 in the flag register https://en.wikipedia.org/wiki/FLAGS_register
If AL was previously an odd value like 0b101 (5) then it will become 0b10 (2) and put 1 in the flag register. Hence the carry flag will act like a remainder if you divide by 2.
jnc bit_0 ; It will (J)ump to label 'bit_0' if (N)o (C)arry flag was set, i.e if the value was even (like 0b100 in the above example) before the shift.
edited Nov 13 '18 at 14:24
Mustafa Enes Batur
54
answered Jan 23 '18 at 11:43
Antonin GAVRELAntonin GAVREL
567621
edited Nov 13 '18 at 14:24
Mustafa Enes Batur
54
edited Nov 13 '18 at 14:24
Mustafa Enes Batur
54
edited Nov 13 '18 at 14:24
Mustafa Enes Batur
54
54
answered Jan 23 '18 at 11:43
Antonin GAVRELAntonin GAVREL
567621
answered Jan 23 '18 at 11:43
Antonin GAVRELAntonin GAVREL
567621
answered Jan 23 '18 at 11:43
Antonin GAVRELAntonin GAVREL
567621
567621
add a comment |
add a comment |
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It's testing whether AL is even or odd.
– harold
Mar 22 '12 at 16:04