Round Register up to multiple of 8
I am trying to round a 64bit register up so it's a multiple of 8. So it ends with either an 8 or 0.
Is there an efficient way to do this, I am currently doing it as follows:
xor r9, r9
_R:
add r9, 08h
cmp r8, r9
ja _R
mov r8, r9
x86-64 masm
asked Nov 14 '18 at 1:59
WillWill
1256
add a comment |
I am trying to round a 64bit register up so it's a multiple of 8. So it ends with either an 8 or 0.
Is there an efficient way to do this, I am currently doing it as follows:
xor r9, r9
_R:
add r9, 08h
cmp r8, r9
ja _R
mov r8, r9
x86-64 masm
asked Nov 14 '18 at 1:59
WillWill
1256
add a comment |
I am trying to round a 64bit register up so it's a multiple of 8. So it ends with either an 8 or 0.
Is there an efficient way to do this, I am currently doing it as follows:
xor r9, r9
_R:
add r9, 08h
cmp r8, r9
ja _R
mov r8, r9
x86-64 masm
asked Nov 14 '18 at 1:59
WillWill
1256
I am trying to round a 64bit register up so it's a multiple of 8. So it ends with either an 8 or 0.
Is there an efficient way to do this, I am currently doing it as follows:
xor r9, r9
_R:
add r9, 08h
cmp r8, r9
ja _R
mov r8, r9
x86-64 masm
x86-64 masm
asked Nov 14 '18 at 1:59
WillWill
1256
asked Nov 14 '18 at 1:59
WillWill
1256
asked Nov 14 '18 at 1:59
WillWill
1256
asked Nov 14 '18 at 1:59
WillWill
1256
asked Nov 14 '18 at 1:59
WillWill
1256
1256
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
To round a value up to a multiple of eight requires two statements and no loops:
add r9, 7
and r9, 0fffffffffffffff8H
The first moves any value that's not already a multiple of eight into the "next highest section". The second rounds that down to a multiple of eight. So you'll get results like:
orig add and
0 7 0
1 8 8
: : :
7 14 8
8 15 8
9 16 16
Note that, if you want to round up even those numbers which are already multiples of eight (e.g., 8 -> 16), just add eight instead of seven.
edited Nov 14 '18 at 2:54
Peter Cordes
123k17186314
answered Nov 14 '18 at 2:06
paxdiablopaxdiablo
632k17012451669
I need it so that it always round up. With r9 = 11h, the and instruction results in r9 = 10h.
– Will
Nov 14 '18 at 2:10
@Will, suggest you try again, that's handled by the initialadd: adding seven to11hwill give you18hwhich will thenandto18h.
– paxdiablo
Nov 14 '18 at 2:12
1
Ahah yeah you're right, I was looking at the register window after the "add r9, 07h" and I saw 11h which was correctly round to 10h. The original value before the add was 0Ah. This works perfectly. I know there would be a smart way to do it! Thanks.
– Will
Nov 14 '18 at 2:14
Fun fact: you can write0xfffffffffffffff8as-8, which is portable to all the x86 assemblers. For MASM vs. GAS you have to choose between0xfffffffffffffff8and0fffffffffffffff8Hif you want to write it out that way.
– Peter Cordes
Nov 14 '18 at 2:55
add a comment |
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1 Answer
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active
oldest
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1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
To round a value up to a multiple of eight requires two statements and no loops:
add r9, 7
and r9, 0fffffffffffffff8H
The first moves any value that's not already a multiple of eight into the "next highest section". The second rounds that down to a multiple of eight. So you'll get results like:
orig add and
0 7 0
1 8 8
: : :
7 14 8
8 15 8
9 16 16
Note that, if you want to round up even those numbers which are already multiples of eight (e.g., 8 -> 16), just add eight instead of seven.
edited Nov 14 '18 at 2:54
Peter Cordes
123k17186314
answered Nov 14 '18 at 2:06
paxdiablopaxdiablo
632k17012451669
I need it so that it always round up. With r9 = 11h, the and instruction results in r9 = 10h.
– Will
Nov 14 '18 at 2:10
@Will, suggest you try again, that's handled by the initialadd: adding seven to11hwill give you18hwhich will thenandto18h.
– paxdiablo
Nov 14 '18 at 2:12
1
Ahah yeah you're right, I was looking at the register window after the "add r9, 07h" and I saw 11h which was correctly round to 10h. The original value before the add was 0Ah. This works perfectly. I know there would be a smart way to do it! Thanks.
– Will
Nov 14 '18 at 2:14
Fun fact: you can write0xfffffffffffffff8as-8, which is portable to all the x86 assemblers. For MASM vs. GAS you have to choose between0xfffffffffffffff8and0fffffffffffffff8Hif you want to write it out that way.
– Peter Cordes
Nov 14 '18 at 2:55
add a comment |
To round a value up to a multiple of eight requires two statements and no loops:
add r9, 7
and r9, 0fffffffffffffff8H
The first moves any value that's not already a multiple of eight into the "next highest section". The second rounds that down to a multiple of eight. So you'll get results like:
orig add and
0 7 0
1 8 8
: : :
7 14 8
8 15 8
9 16 16
Note that, if you want to round up even those numbers which are already multiples of eight (e.g., 8 -> 16), just add eight instead of seven.
edited Nov 14 '18 at 2:54
Peter Cordes
123k17186314
answered Nov 14 '18 at 2:06
paxdiablopaxdiablo
632k17012451669
I need it so that it always round up. With r9 = 11h, the and instruction results in r9 = 10h.
– Will
Nov 14 '18 at 2:10
@Will, suggest you try again, that's handled by the initialadd: adding seven to11hwill give you18hwhich will thenandto18h.
– paxdiablo
Nov 14 '18 at 2:12
1
Ahah yeah you're right, I was looking at the register window after the "add r9, 07h" and I saw 11h which was correctly round to 10h. The original value before the add was 0Ah. This works perfectly. I know there would be a smart way to do it! Thanks.
– Will
Nov 14 '18 at 2:14
Fun fact: you can write0xfffffffffffffff8as-8, which is portable to all the x86 assemblers. For MASM vs. GAS you have to choose between0xfffffffffffffff8and0fffffffffffffff8Hif you want to write it out that way.
– Peter Cordes
Nov 14 '18 at 2:55
add a comment |
To round a value up to a multiple of eight requires two statements and no loops:
add r9, 7
and r9, 0fffffffffffffff8H
The first moves any value that's not already a multiple of eight into the "next highest section". The second rounds that down to a multiple of eight. So you'll get results like:
orig add and
0 7 0
1 8 8
: : :
7 14 8
8 15 8
9 16 16
Note that, if you want to round up even those numbers which are already multiples of eight (e.g., 8 -> 16), just add eight instead of seven.
edited Nov 14 '18 at 2:54
Peter Cordes
123k17186314
answered Nov 14 '18 at 2:06
paxdiablopaxdiablo
632k17012451669
To round a value up to a multiple of eight requires two statements and no loops:
add r9, 7
and r9, 0fffffffffffffff8H
The first moves any value that's not already a multiple of eight into the "next highest section". The second rounds that down to a multiple of eight. So you'll get results like:
orig add and
0 7 0
1 8 8
: : :
7 14 8
8 15 8
9 16 16
Note that, if you want to round up even those numbers which are already multiples of eight (e.g., 8 -> 16), just add eight instead of seven.
edited Nov 14 '18 at 2:54
Peter Cordes
123k17186314
answered Nov 14 '18 at 2:06
paxdiablopaxdiablo
632k17012451669
edited Nov 14 '18 at 2:54
Peter Cordes
123k17186314
edited Nov 14 '18 at 2:54
Peter Cordes
123k17186314
edited Nov 14 '18 at 2:54
Peter Cordes
123k17186314
123k17186314
answered Nov 14 '18 at 2:06
paxdiablopaxdiablo
632k17012451669
answered Nov 14 '18 at 2:06
paxdiablopaxdiablo
632k17012451669
answered Nov 14 '18 at 2:06
paxdiablopaxdiablo
632k17012451669
632k17012451669
I need it so that it always round up. With r9 = 11h, the and instruction results in r9 = 10h.
– Will
Nov 14 '18 at 2:10
@Will, suggest you try again, that's handled by the initialadd: adding seven to11hwill give you18hwhich will thenandto18h.
– paxdiablo
Nov 14 '18 at 2:12
1
Ahah yeah you're right, I was looking at the register window after the "add r9, 07h" and I saw 11h which was correctly round to 10h. The original value before the add was 0Ah. This works perfectly. I know there would be a smart way to do it! Thanks.
– Will
Nov 14 '18 at 2:14
Fun fact: you can write0xfffffffffffffff8as-8, which is portable to all the x86 assemblers. For MASM vs. GAS you have to choose between0xfffffffffffffff8and0fffffffffffffff8Hif you want to write it out that way.
– Peter Cordes
Nov 14 '18 at 2:55
add a comment |
I need it so that it always round up. With r9 = 11h, the and instruction results in r9 = 10h.
– Will
Nov 14 '18 at 2:10
@Will, suggest you try again, that's handled by the initialadd: adding seven to11hwill give you18hwhich will thenandto18h.
– paxdiablo
Nov 14 '18 at 2:12
1
Ahah yeah you're right, I was looking at the register window after the "add r9, 07h" and I saw 11h which was correctly round to 10h. The original value before the add was 0Ah. This works perfectly. I know there would be a smart way to do it! Thanks.
– Will
Nov 14 '18 at 2:14
Fun fact: you can write0xfffffffffffffff8as-8, which is portable to all the x86 assemblers. For MASM vs. GAS you have to choose between0xfffffffffffffff8and0fffffffffffffff8Hif you want to write it out that way.
– Peter Cordes
Nov 14 '18 at 2:55
I need it so that it always round up. With r9 = 11h, the and instruction results in r9 = 10h.
– Will
Nov 14 '18 at 2:10
I need it so that it always round up. With r9 = 11h, the and instruction results in r9 = 10h.
– Will
Nov 14 '18 at 2:10
@Will, suggest you try again, that's handled by the initial
add: adding seven to 11h will give you 18h which will then and to 18h.– paxdiablo
Nov 14 '18 at 2:12
@Will, suggest you try again, that's handled by the initial
add: adding seven to 11h will give you 18h which will then and to 18h.– paxdiablo
Nov 14 '18 at 2:12
1
1
Ahah yeah you're right, I was looking at the register window after the "add r9, 07h" and I saw 11h which was correctly round to 10h. The original value before the add was 0Ah. This works perfectly. I know there would be a smart way to do it! Thanks.
– Will
Nov 14 '18 at 2:14
Ahah yeah you're right, I was looking at the register window after the "add r9, 07h" and I saw 11h which was correctly round to 10h. The original value before the add was 0Ah. This works perfectly. I know there would be a smart way to do it! Thanks.
– Will
Nov 14 '18 at 2:14
Fun fact: you can write
0xfffffffffffffff8 as -8, which is portable to all the x86 assemblers. For MASM vs. GAS you have to choose between 0xfffffffffffffff8 and 0fffffffffffffff8H if you want to write it out that way.– Peter Cordes
Nov 14 '18 at 2:55
Fun fact: you can write
0xfffffffffffffff8 as -8, which is portable to all the x86 assemblers. For MASM vs. GAS you have to choose between 0xfffffffffffffff8 and 0fffffffffffffff8H if you want to write it out that way.– Peter Cordes
Nov 14 '18 at 2:55
add a comment |
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