`js` and `jb` instructions in assembly
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I am having a hard time understanding what exactly js and jb instruction do. I understand that jb is jump if below. But, what would be the difference between jb and jle. And similarly, js seems to me that it is equivalent to jb, as it means jump if signed. Any help would be appreciated.
assembly x86 att
asked Nov 23 '18 at 18:55
konstantkonstant
391214
add a comment |
0
I am having a hard time understanding what exactly js and jb instruction do. I understand that jb is jump if below. But, what would be the difference between jb and jle. And similarly, js seems to me that it is equivalent to jb, as it means jump if signed. Any help would be appreciated.
assembly x86 att
asked Nov 23 '18 at 18:55
konstantkonstant
391214
jstests the Sign flag, andjbtests the Carry flag.jleis more complex and your assembler / processor text books are the place to begin. One uses a different set of flag tests for signed and unsigned arithmetic, as the processor does not (usually) distinguish the two.
– Weather Vane
Nov 23 '18 at 19:01
2
The instruction set reference clearly shows which flags each test.jsexamines the sign flag so you can check for negative numbers.jbis for unsigned comparison as it uses the carry flag.jlis for signed comparison which handles overflow as well. In the given codejsjust checks whether the MSB ofeaxis set or not (meaning it's negative if viewed as signed). Thejbis just doing an unsigned less-than.
– Jester
Nov 23 '18 at 19:02
1
felixcloutier.com/x86/Jcc.html
– Peter Cordes
Nov 23 '18 at 19:04
The conditional jumps check the flags (eflagregister), they are not aware of which instruction did produce the flag. I.e.stcjb somewherewill take the branch tosomewherealways, becausestcis "set carry flag" - yet it doesn't have anything to do with human concept of "below" term (and reasonable programmer would rather write thejcalias there to not confuse reader by "below", but make it clear that he is interested into "carry" status).
– Ped7g
Nov 23 '18 at 22:43
add a comment |
0
0
0
I am having a hard time understanding what exactly js and jb instruction do. I understand that jb is jump if below. But, what would be the difference between jb and jle. And similarly, js seems to me that it is equivalent to jb, as it means jump if signed. Any help would be appreciated.
assembly x86 att
asked Nov 23 '18 at 18:55
konstantkonstant
391214
I am having a hard time understanding what exactly js and jb instruction do. I understand that jb is jump if below. But, what would be the difference between jb and jle. And similarly, js seems to me that it is equivalent to jb, as it means jump if signed. Any help would be appreciated.
assembly x86 att
assembly x86 att
asked Nov 23 '18 at 18:55
konstantkonstant
391214
asked Nov 23 '18 at 18:55
konstantkonstant
391214
edited Nov 24 '18 at 1:11
konstant
asked Nov 23 '18 at 18:55
konstantkonstant
391214
asked Nov 23 '18 at 18:55
konstantkonstant
391214
asked Nov 23 '18 at 18:55
konstantkonstant
391214
391214
jstests the Sign flag, andjbtests the Carry flag.jleis more complex and your assembler / processor text books are the place to begin. One uses a different set of flag tests for signed and unsigned arithmetic, as the processor does not (usually) distinguish the two.
– Weather Vane
Nov 23 '18 at 19:01
2
The instruction set reference clearly shows which flags each test.jsexamines the sign flag so you can check for negative numbers.jbis for unsigned comparison as it uses the carry flag.jlis for signed comparison which handles overflow as well. In the given codejsjust checks whether the MSB ofeaxis set or not (meaning it's negative if viewed as signed). Thejbis just doing an unsigned less-than.
– Jester
Nov 23 '18 at 19:02
1
felixcloutier.com/x86/Jcc.html
– Peter Cordes
Nov 23 '18 at 19:04
The conditional jumps check the flags (eflagregister), they are not aware of which instruction did produce the flag. I.e.stcjb somewherewill take the branch tosomewherealways, becausestcis "set carry flag" - yet it doesn't have anything to do with human concept of "below" term (and reasonable programmer would rather write thejcalias there to not confuse reader by "below", but make it clear that he is interested into "carry" status).
– Ped7g
Nov 23 '18 at 22:43
add a comment |
jstests the Sign flag, andjbtests the Carry flag.jleis more complex and your assembler / processor text books are the place to begin. One uses a different set of flag tests for signed and unsigned arithmetic, as the processor does not (usually) distinguish the two.
– Weather Vane
Nov 23 '18 at 19:01
2
The instruction set reference clearly shows which flags each test.jsexamines the sign flag so you can check for negative numbers.jbis for unsigned comparison as it uses the carry flag.jlis for signed comparison which handles overflow as well. In the given codejsjust checks whether the MSB ofeaxis set or not (meaning it's negative if viewed as signed). Thejbis just doing an unsigned less-than.
– Jester
Nov 23 '18 at 19:02
1
felixcloutier.com/x86/Jcc.html
– Peter Cordes
Nov 23 '18 at 19:04
The conditional jumps check the flags (eflagregister), they are not aware of which instruction did produce the flag. I.e.stcjb somewherewill take the branch tosomewherealways, becausestcis "set carry flag" - yet it doesn't have anything to do with human concept of "below" term (and reasonable programmer would rather write thejcalias there to not confuse reader by "below", but make it clear that he is interested into "carry" status).
– Ped7g
Nov 23 '18 at 22:43
js tests the Sign flag, and jb tests the Carry flag. jle is more complex and your assembler / processor text books are the place to begin. One uses a different set of flag tests for signed and unsigned arithmetic, as the processor does not (usually) distinguish the two.– Weather Vane
Nov 23 '18 at 19:01
js tests the Sign flag, and jb tests the Carry flag. jle is more complex and your assembler / processor text books are the place to begin. One uses a different set of flag tests for signed and unsigned arithmetic, as the processor does not (usually) distinguish the two.– Weather Vane
Nov 23 '18 at 19:01
2
2
The instruction set reference clearly shows which flags each test.
js examines the sign flag so you can check for negative numbers. jb is for unsigned comparison as it uses the carry flag. jl is for signed comparison which handles overflow as well. In the given code js just checks whether the MSB of eax is set or not (meaning it's negative if viewed as signed). The jb is just doing an unsigned less-than.– Jester
Nov 23 '18 at 19:02
The instruction set reference clearly shows which flags each test.
js examines the sign flag so you can check for negative numbers. jb is for unsigned comparison as it uses the carry flag. jl is for signed comparison which handles overflow as well. In the given code js just checks whether the MSB of eax is set or not (meaning it's negative if viewed as signed). The jb is just doing an unsigned less-than.– Jester
Nov 23 '18 at 19:02
1
1
felixcloutier.com/x86/Jcc.html
– Peter Cordes
Nov 23 '18 at 19:04
felixcloutier.com/x86/Jcc.html
– Peter Cordes
Nov 23 '18 at 19:04
The conditional jumps check the flags (
eflag register), they are not aware of which instruction did produce the flag. I.e. stc jb somewhere will take the branch to somewhere always, because stc is "set carry flag" - yet it doesn't have anything to do with human concept of "below" term (and reasonable programmer would rather write the jc alias there to not confuse reader by "below", but make it clear that he is interested into "carry" status).– Ped7g
Nov 23 '18 at 22:43
The conditional jumps check the flags (
eflag register), they are not aware of which instruction did produce the flag. I.e. stc jb somewhere will take the branch to somewhere always, because stc is "set carry flag" - yet it doesn't have anything to do with human concept of "below" term (and reasonable programmer would rather write the jc alias there to not confuse reader by "below", but make it clear that he is interested into "carry" status).– Ped7g
Nov 23 '18 at 22:43
add a comment |
2 Answers
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There exists a handy table that does explain very well which Jcc instruction to use:
Jump conditions and flags:
Mnemonic Condition tested Description
jo OF = 1 overflow
jno OF = 0 not overflow
jc, jb, jnae CF = 1 carry / below / not above nor equal
jnc, jae, jnb CF = 0 not carry / above or equal / not below
je, jz ZF = 1 equal / zero
jne, jnz ZF = 0 not equal / not zero
jbe, jna CF or ZF = 1 below or equal / not above
ja, jnbe CF or ZF = 0 above / not below or equal
js SF = 1 sign
jns SF = 0 not sign
jp, jpe PF = 1 parity / parity even
jnp, jpo PF = 0 not parity / parity odd
jl, jnge SF xor OF = 1 less / not greater nor equal
jge, jnl SF xor OF = 0 greater or equal / not less
jle, jng (SF xor OF) or ZF = 1 less or equal / not greater
jg, jnle (SF xor OF) or ZF = 0 greater / not less nor equal
answered Nov 23 '18 at 19:57
zx485zx485
15.4k133248
add a comment |
0
jb (and ja) branch based on the unsigned result of the flags, as opposed to the signed branch condition for jg, jge, jl, and jle.
In an unsigned comparison, the MSB is included as part of the number itself and not an indication of its sign. For example:
; Intel ; ; AT&T
mov eax, 08000000h ; mov $0x8000000, %eax
mov ecx, 00000001h ; mov $0x0000001, %ecx
cmp eax, ecx ; cmp %ecx, %eax
jl mybranch ; branch taken ; jl mybranch ; branch taken
Whereas:
mov eax, 08000000h ; mov $0x8000000, %eax
mov ecx, 00000001h ; mov $0x0000001, %ecx
cmp eax, ecx ; cmp %ecx, %eax
jb mybranch ; branch not taken ; jb mybranch ; branch not taken
js will branch based solely on the state of the sign flag in the (R|E)FLAGS register
edited Nov 23 '18 at 20:02
Matteo Italia
103k15150253
answered Nov 23 '18 at 19:09
Govind ParmarGovind Parmar
13.1k53764
1
The OP is using GAS, which only supports0x...syntax for hex, not DOS-style0...h. (And they have it in AT&T syntax mode, not.intel_syntax noprefix).
– Peter Cordes
Nov 23 '18 at 19:11
@PeterCordes I was going to edit my post but it looks like some one beat me to it :)
– Govind Parmar
Nov 23 '18 at 20:48
add a comment |
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2 Answers
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There exists a handy table that does explain very well which Jcc instruction to use:
Jump conditions and flags:
Mnemonic Condition tested Description
jo OF = 1 overflow
jno OF = 0 not overflow
jc, jb, jnae CF = 1 carry / below / not above nor equal
jnc, jae, jnb CF = 0 not carry / above or equal / not below
je, jz ZF = 1 equal / zero
jne, jnz ZF = 0 not equal / not zero
jbe, jna CF or ZF = 1 below or equal / not above
ja, jnbe CF or ZF = 0 above / not below or equal
js SF = 1 sign
jns SF = 0 not sign
jp, jpe PF = 1 parity / parity even
jnp, jpo PF = 0 not parity / parity odd
jl, jnge SF xor OF = 1 less / not greater nor equal
jge, jnl SF xor OF = 0 greater or equal / not less
jle, jng (SF xor OF) or ZF = 1 less or equal / not greater
jg, jnle (SF xor OF) or ZF = 0 greater / not less nor equal
answered Nov 23 '18 at 19:57
zx485zx485
15.4k133248
add a comment |
0
There exists a handy table that does explain very well which Jcc instruction to use:
Jump conditions and flags:
Mnemonic Condition tested Description
jo OF = 1 overflow
jno OF = 0 not overflow
jc, jb, jnae CF = 1 carry / below / not above nor equal
jnc, jae, jnb CF = 0 not carry / above or equal / not below
je, jz ZF = 1 equal / zero
jne, jnz ZF = 0 not equal / not zero
jbe, jna CF or ZF = 1 below or equal / not above
ja, jnbe CF or ZF = 0 above / not below or equal
js SF = 1 sign
jns SF = 0 not sign
jp, jpe PF = 1 parity / parity even
jnp, jpo PF = 0 not parity / parity odd
jl, jnge SF xor OF = 1 less / not greater nor equal
jge, jnl SF xor OF = 0 greater or equal / not less
jle, jng (SF xor OF) or ZF = 1 less or equal / not greater
jg, jnle (SF xor OF) or ZF = 0 greater / not less nor equal
answered Nov 23 '18 at 19:57
zx485zx485
15.4k133248
add a comment |
0
0
0
There exists a handy table that does explain very well which Jcc instruction to use:
Jump conditions and flags:
Mnemonic Condition tested Description
jo OF = 1 overflow
jno OF = 0 not overflow
jc, jb, jnae CF = 1 carry / below / not above nor equal
jnc, jae, jnb CF = 0 not carry / above or equal / not below
je, jz ZF = 1 equal / zero
jne, jnz ZF = 0 not equal / not zero
jbe, jna CF or ZF = 1 below or equal / not above
ja, jnbe CF or ZF = 0 above / not below or equal
js SF = 1 sign
jns SF = 0 not sign
jp, jpe PF = 1 parity / parity even
jnp, jpo PF = 0 not parity / parity odd
jl, jnge SF xor OF = 1 less / not greater nor equal
jge, jnl SF xor OF = 0 greater or equal / not less
jle, jng (SF xor OF) or ZF = 1 less or equal / not greater
jg, jnle (SF xor OF) or ZF = 0 greater / not less nor equal
answered Nov 23 '18 at 19:57
zx485zx485
15.4k133248
There exists a handy table that does explain very well which Jcc instruction to use:
Jump conditions and flags:
Mnemonic Condition tested Description
jo OF = 1 overflow
jno OF = 0 not overflow
jc, jb, jnae CF = 1 carry / below / not above nor equal
jnc, jae, jnb CF = 0 not carry / above or equal / not below
je, jz ZF = 1 equal / zero
jne, jnz ZF = 0 not equal / not zero
jbe, jna CF or ZF = 1 below or equal / not above
ja, jnbe CF or ZF = 0 above / not below or equal
js SF = 1 sign
jns SF = 0 not sign
jp, jpe PF = 1 parity / parity even
jnp, jpo PF = 0 not parity / parity odd
jl, jnge SF xor OF = 1 less / not greater nor equal
jge, jnl SF xor OF = 0 greater or equal / not less
jle, jng (SF xor OF) or ZF = 1 less or equal / not greater
jg, jnle (SF xor OF) or ZF = 0 greater / not less nor equal
answered Nov 23 '18 at 19:57
zx485zx485
15.4k133248
answered Nov 23 '18 at 19:57
zx485zx485
15.4k133248
answered Nov 23 '18 at 19:57
zx485zx485
15.4k133248
answered Nov 23 '18 at 19:57
zx485zx485
15.4k133248
15.4k133248
add a comment |
add a comment |
0
jb (and ja) branch based on the unsigned result of the flags, as opposed to the signed branch condition for jg, jge, jl, and jle.
In an unsigned comparison, the MSB is included as part of the number itself and not an indication of its sign. For example:
; Intel ; ; AT&T
mov eax, 08000000h ; mov $0x8000000, %eax
mov ecx, 00000001h ; mov $0x0000001, %ecx
cmp eax, ecx ; cmp %ecx, %eax
jl mybranch ; branch taken ; jl mybranch ; branch taken
Whereas:
mov eax, 08000000h ; mov $0x8000000, %eax
mov ecx, 00000001h ; mov $0x0000001, %ecx
cmp eax, ecx ; cmp %ecx, %eax
jb mybranch ; branch not taken ; jb mybranch ; branch not taken
js will branch based solely on the state of the sign flag in the (R|E)FLAGS register
edited Nov 23 '18 at 20:02
Matteo Italia
103k15150253
answered Nov 23 '18 at 19:09
Govind ParmarGovind Parmar
13.1k53764
1
The OP is using GAS, which only supports0x...syntax for hex, not DOS-style0...h. (And they have it in AT&T syntax mode, not.intel_syntax noprefix).
– Peter Cordes
Nov 23 '18 at 19:11
@PeterCordes I was going to edit my post but it looks like some one beat me to it :)
– Govind Parmar
Nov 23 '18 at 20:48
add a comment |
0
jb (and ja) branch based on the unsigned result of the flags, as opposed to the signed branch condition for jg, jge, jl, and jle.
In an unsigned comparison, the MSB is included as part of the number itself and not an indication of its sign. For example:
; Intel ; ; AT&T
mov eax, 08000000h ; mov $0x8000000, %eax
mov ecx, 00000001h ; mov $0x0000001, %ecx
cmp eax, ecx ; cmp %ecx, %eax
jl mybranch ; branch taken ; jl mybranch ; branch taken
Whereas:
mov eax, 08000000h ; mov $0x8000000, %eax
mov ecx, 00000001h ; mov $0x0000001, %ecx
cmp eax, ecx ; cmp %ecx, %eax
jb mybranch ; branch not taken ; jb mybranch ; branch not taken
js will branch based solely on the state of the sign flag in the (R|E)FLAGS register
edited Nov 23 '18 at 20:02
Matteo Italia
103k15150253
answered Nov 23 '18 at 19:09
Govind ParmarGovind Parmar
13.1k53764
1
The OP is using GAS, which only supports0x...syntax for hex, not DOS-style0...h. (And they have it in AT&T syntax mode, not.intel_syntax noprefix).
– Peter Cordes
Nov 23 '18 at 19:11
@PeterCordes I was going to edit my post but it looks like some one beat me to it :)
– Govind Parmar
Nov 23 '18 at 20:48
add a comment |
0
0
0
jb (and ja) branch based on the unsigned result of the flags, as opposed to the signed branch condition for jg, jge, jl, and jle.
In an unsigned comparison, the MSB is included as part of the number itself and not an indication of its sign. For example:
; Intel ; ; AT&T
mov eax, 08000000h ; mov $0x8000000, %eax
mov ecx, 00000001h ; mov $0x0000001, %ecx
cmp eax, ecx ; cmp %ecx, %eax
jl mybranch ; branch taken ; jl mybranch ; branch taken
Whereas:
mov eax, 08000000h ; mov $0x8000000, %eax
mov ecx, 00000001h ; mov $0x0000001, %ecx
cmp eax, ecx ; cmp %ecx, %eax
jb mybranch ; branch not taken ; jb mybranch ; branch not taken
js will branch based solely on the state of the sign flag in the (R|E)FLAGS register
edited Nov 23 '18 at 20:02
Matteo Italia
103k15150253
answered Nov 23 '18 at 19:09
Govind ParmarGovind Parmar
13.1k53764
jb (and ja) branch based on the unsigned result of the flags, as opposed to the signed branch condition for jg, jge, jl, and jle.
In an unsigned comparison, the MSB is included as part of the number itself and not an indication of its sign. For example:
; Intel ; ; AT&T
mov eax, 08000000h ; mov $0x8000000, %eax
mov ecx, 00000001h ; mov $0x0000001, %ecx
cmp eax, ecx ; cmp %ecx, %eax
jl mybranch ; branch taken ; jl mybranch ; branch taken
Whereas:
mov eax, 08000000h ; mov $0x8000000, %eax
mov ecx, 00000001h ; mov $0x0000001, %ecx
cmp eax, ecx ; cmp %ecx, %eax
jb mybranch ; branch not taken ; jb mybranch ; branch not taken
js will branch based solely on the state of the sign flag in the (R|E)FLAGS register
edited Nov 23 '18 at 20:02
Matteo Italia
103k15150253
answered Nov 23 '18 at 19:09
Govind ParmarGovind Parmar
13.1k53764
edited Nov 23 '18 at 20:02
Matteo Italia
103k15150253
edited Nov 23 '18 at 20:02
Matteo Italia
103k15150253
edited Nov 23 '18 at 20:02
Matteo Italia
103k15150253
103k15150253
answered Nov 23 '18 at 19:09
Govind ParmarGovind Parmar
13.1k53764
answered Nov 23 '18 at 19:09
Govind ParmarGovind Parmar
13.1k53764
answered Nov 23 '18 at 19:09
Govind ParmarGovind Parmar
13.1k53764
13.1k53764
1
The OP is using GAS, which only supports0x...syntax for hex, not DOS-style0...h. (And they have it in AT&T syntax mode, not.intel_syntax noprefix).
– Peter Cordes
Nov 23 '18 at 19:11
@PeterCordes I was going to edit my post but it looks like some one beat me to it :)
– Govind Parmar
Nov 23 '18 at 20:48
add a comment |
1
The OP is using GAS, which only supports0x...syntax for hex, not DOS-style0...h. (And they have it in AT&T syntax mode, not.intel_syntax noprefix).
– Peter Cordes
Nov 23 '18 at 19:11
@PeterCordes I was going to edit my post but it looks like some one beat me to it :)
– Govind Parmar
Nov 23 '18 at 20:48
1
1
The OP is using GAS, which only supports
0x... syntax for hex, not DOS-style 0...h. (And they have it in AT&T syntax mode, not .intel_syntax noprefix).– Peter Cordes
Nov 23 '18 at 19:11
The OP is using GAS, which only supports
0x... syntax for hex, not DOS-style 0...h. (And they have it in AT&T syntax mode, not .intel_syntax noprefix).– Peter Cordes
Nov 23 '18 at 19:11
@PeterCordes I was going to edit my post but it looks like some one beat me to it :)
– Govind Parmar
Nov 23 '18 at 20:48
@PeterCordes I was going to edit my post but it looks like some one beat me to it :)
– Govind Parmar
Nov 23 '18 at 20:48
add a comment |
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jstests the Sign flag, andjbtests the Carry flag.jleis more complex and your assembler / processor text books are the place to begin. One uses a different set of flag tests for signed and unsigned arithmetic, as the processor does not (usually) distinguish the two.– Weather Vane
Nov 23 '18 at 19:01
2
The instruction set reference clearly shows which flags each test.
jsexamines the sign flag so you can check for negative numbers.jbis for unsigned comparison as it uses the carry flag.jlis for signed comparison which handles overflow as well. In the given codejsjust checks whether the MSB ofeaxis set or not (meaning it's negative if viewed as signed). Thejbis just doing an unsigned less-than.– Jester
Nov 23 '18 at 19:02
1
felixcloutier.com/x86/Jcc.html
– Peter Cordes
Nov 23 '18 at 19:04
The conditional jumps check the flags (
eflagregister), they are not aware of which instruction did produce the flag. I.e.stcjb somewherewill take the branch tosomewherealways, becausestcis "set carry flag" - yet it doesn't have anything to do with human concept of "below" term (and reasonable programmer would rather write thejcalias there to not confuse reader by "below", but make it clear that he is interested into "carry" status).– Ped7g
Nov 23 '18 at 22:43