Understanding conditional jumps
0
[org 100h]
jmp start
start:
MOV AL, 5
MOV BL, 0xFF
CMP AL, BL
JG L1
MOV AX, 0
JMP Exit
L1:
MOV AX, 1
Exit:
mov ax,0x4c00
int 21h
Why does this jump to L1 after JG? As here al < bl and it should only jump to L1 if al > bl.
Am I right though?
The end result in ax should be 0 but it is giving 1. I'm using dosbox 0.74 and I'm a beginner in assembly.
assembly x86
edited Nov 20 '18 at 18:03
Peter Cordes
129k18194329
asked Nov 20 '18 at 16:47
5D_cOOKIE5D_cOOKIE
12
|
show 6 more comments
0
[org 100h]
jmp start
start:
MOV AL, 5
MOV BL, 0xFF
CMP AL, BL
JG L1
MOV AX, 0
JMP Exit
L1:
MOV AX, 1
Exit:
mov ax,0x4c00
int 21h
Why does this jump to L1 after JG? As here al < bl and it should only jump to L1 if al > bl.
Am I right though?
The end result in ax should be 0 but it is giving 1. I'm using dosbox 0.74 and I'm a beginner in assembly.
assembly x86
edited Nov 20 '18 at 18:03
Peter Cordes
129k18194329
asked Nov 20 '18 at 16:47
5D_cOOKIE5D_cOOKIE
12
1
5 > -1, soalis greater. Why do you think it's less/equal? ( felixcloutier.com/x86/Jcc.html .. thecmpinstruction will set flags in a way that mnemonics likejg = jump if greaterandja = jump if abovework as expected ... (when you understand the basic data types and why 8 bit-1is encoded as 1111_1111 in binary, i.e. 0xFF) ... for SIGNED integers in 8 bits you have working range -128..+127 .. for unsigned integers the range is 0..255 (you are loadingblwith bit pattern "1111_1111" which can be interpreted as 255 (unint8) or -1 (int8) or anything (anything_t).
– Ped7g
Nov 20 '18 at 16:48
1
is it not 5 > 255 :@ what is wrong with me... how can i make bl 255? changing jg to ja does that?
– 5D_cOOKIE
Nov 20 '18 at 16:50
0xff is -1 in 8-bit 2's complement.
– Peter Cordes
Nov 20 '18 at 16:50
1
2) in that shortened Intel manual (link), the unsigned-related jumps areja/jb/...("below/above"), signed-related jumps arejg/jl/...("greater/less"). But keep in mind they actually don't care about their name, they just check current flag status, set up by whatever last instruction(s)... Also don't assume much about flags being set by instructions, most of the time the arithmetic instructions set them as expected, but there are exceptions, likedec/incdoes NOT modify CF (for good reasons, but it may surprise you) - so check inst. guide.
– Ped7g
Nov 20 '18 at 17:03
2
Please read Under what circumstances may I add “urgent” or other similar phrases to my question, in order to obtain faster answers? - the summary is that this is not an ideal way to address volunteers, and is probably counterproductive to obtaining answers. Please refrain from adding this to your questions.
– halfer
Nov 20 '18 at 17:55
|
show 6 more comments
0
0
0
[org 100h]
jmp start
start:
MOV AL, 5
MOV BL, 0xFF
CMP AL, BL
JG L1
MOV AX, 0
JMP Exit
L1:
MOV AX, 1
Exit:
mov ax,0x4c00
int 21h
Why does this jump to L1 after JG? As here al < bl and it should only jump to L1 if al > bl.
Am I right though?
The end result in ax should be 0 but it is giving 1. I'm using dosbox 0.74 and I'm a beginner in assembly.
assembly x86
edited Nov 20 '18 at 18:03
Peter Cordes
129k18194329
asked Nov 20 '18 at 16:47
5D_cOOKIE5D_cOOKIE
12
[org 100h]
jmp start
start:
MOV AL, 5
MOV BL, 0xFF
CMP AL, BL
JG L1
MOV AX, 0
JMP Exit
L1:
MOV AX, 1
Exit:
mov ax,0x4c00
int 21h
Why does this jump to L1 after JG? As here al < bl and it should only jump to L1 if al > bl.
Am I right though?
The end result in ax should be 0 but it is giving 1. I'm using dosbox 0.74 and I'm a beginner in assembly.
assembly x86
assembly x86
edited Nov 20 '18 at 18:03
Peter Cordes
129k18194329
asked Nov 20 '18 at 16:47
5D_cOOKIE5D_cOOKIE
12
edited Nov 20 '18 at 18:03
Peter Cordes
129k18194329
asked Nov 20 '18 at 16:47
5D_cOOKIE5D_cOOKIE
12
edited Nov 20 '18 at 18:03
Peter Cordes
129k18194329
edited Nov 20 '18 at 18:03
Peter Cordes
129k18194329
edited Nov 20 '18 at 18:03
Peter Cordes
129k18194329
129k18194329
asked Nov 20 '18 at 16:47
5D_cOOKIE5D_cOOKIE
12
asked Nov 20 '18 at 16:47
5D_cOOKIE5D_cOOKIE
12
asked Nov 20 '18 at 16:47
5D_cOOKIE5D_cOOKIE
12
12
1
5 > -1, soalis greater. Why do you think it's less/equal? ( felixcloutier.com/x86/Jcc.html .. thecmpinstruction will set flags in a way that mnemonics likejg = jump if greaterandja = jump if abovework as expected ... (when you understand the basic data types and why 8 bit-1is encoded as 1111_1111 in binary, i.e. 0xFF) ... for SIGNED integers in 8 bits you have working range -128..+127 .. for unsigned integers the range is 0..255 (you are loadingblwith bit pattern "1111_1111" which can be interpreted as 255 (unint8) or -1 (int8) or anything (anything_t).
– Ped7g
Nov 20 '18 at 16:48
1
is it not 5 > 255 :@ what is wrong with me... how can i make bl 255? changing jg to ja does that?
– 5D_cOOKIE
Nov 20 '18 at 16:50
0xff is -1 in 8-bit 2's complement.
– Peter Cordes
Nov 20 '18 at 16:50
1
2) in that shortened Intel manual (link), the unsigned-related jumps areja/jb/...("below/above"), signed-related jumps arejg/jl/...("greater/less"). But keep in mind they actually don't care about their name, they just check current flag status, set up by whatever last instruction(s)... Also don't assume much about flags being set by instructions, most of the time the arithmetic instructions set them as expected, but there are exceptions, likedec/incdoes NOT modify CF (for good reasons, but it may surprise you) - so check inst. guide.
– Ped7g
Nov 20 '18 at 17:03
2
Please read Under what circumstances may I add “urgent” or other similar phrases to my question, in order to obtain faster answers? - the summary is that this is not an ideal way to address volunteers, and is probably counterproductive to obtaining answers. Please refrain from adding this to your questions.
– halfer
Nov 20 '18 at 17:55
|
show 6 more comments
1
5 > -1, soalis greater. Why do you think it's less/equal? ( felixcloutier.com/x86/Jcc.html .. thecmpinstruction will set flags in a way that mnemonics likejg = jump if greaterandja = jump if abovework as expected ... (when you understand the basic data types and why 8 bit-1is encoded as 1111_1111 in binary, i.e. 0xFF) ... for SIGNED integers in 8 bits you have working range -128..+127 .. for unsigned integers the range is 0..255 (you are loadingblwith bit pattern "1111_1111" which can be interpreted as 255 (unint8) or -1 (int8) or anything (anything_t).
– Ped7g
Nov 20 '18 at 16:48
1
is it not 5 > 255 :@ what is wrong with me... how can i make bl 255? changing jg to ja does that?
– 5D_cOOKIE
Nov 20 '18 at 16:50
0xff is -1 in 8-bit 2's complement.
– Peter Cordes
Nov 20 '18 at 16:50
1
2) in that shortened Intel manual (link), the unsigned-related jumps areja/jb/...("below/above"), signed-related jumps arejg/jl/...("greater/less"). But keep in mind they actually don't care about their name, they just check current flag status, set up by whatever last instruction(s)... Also don't assume much about flags being set by instructions, most of the time the arithmetic instructions set them as expected, but there are exceptions, likedec/incdoes NOT modify CF (for good reasons, but it may surprise you) - so check inst. guide.
– Ped7g
Nov 20 '18 at 17:03
2
Please read Under what circumstances may I add “urgent” or other similar phrases to my question, in order to obtain faster answers? - the summary is that this is not an ideal way to address volunteers, and is probably counterproductive to obtaining answers. Please refrain from adding this to your questions.
– halfer
Nov 20 '18 at 17:55
1
1
5 > -1, so al is greater. Why do you think it's less/equal? ( felixcloutier.com/x86/Jcc.html .. the cmp instruction will set flags in a way that mnemonics like jg = jump if greater and ja = jump if above work as expected ... (when you understand the basic data types and why 8 bit -1 is encoded as 1111_1111 in binary, i.e. 0xFF) ... for SIGNED integers in 8 bits you have working range -128..+127 .. for unsigned integers the range is 0..255 (you are loading bl with bit pattern "1111_1111" which can be interpreted as 255 (unint8) or -1 (int8) or anything (anything_t).– Ped7g
Nov 20 '18 at 16:48
5 > -1, so al is greater. Why do you think it's less/equal? ( felixcloutier.com/x86/Jcc.html .. the cmp instruction will set flags in a way that mnemonics like jg = jump if greater and ja = jump if above work as expected ... (when you understand the basic data types and why 8 bit -1 is encoded as 1111_1111 in binary, i.e. 0xFF) ... for SIGNED integers in 8 bits you have working range -128..+127 .. for unsigned integers the range is 0..255 (you are loading bl with bit pattern "1111_1111" which can be interpreted as 255 (unint8) or -1 (int8) or anything (anything_t).– Ped7g
Nov 20 '18 at 16:48
1
1
is it not 5 > 255 :@ what is wrong with me... how can i make bl 255? changing jg to ja does that?
– 5D_cOOKIE
Nov 20 '18 at 16:50
is it not 5 > 255 :@ what is wrong with me... how can i make bl 255? changing jg to ja does that?
– 5D_cOOKIE
Nov 20 '18 at 16:50
0xff is -1 in 8-bit 2's complement.
– Peter Cordes
Nov 20 '18 at 16:50
0xff is -1 in 8-bit 2's complement.
– Peter Cordes
Nov 20 '18 at 16:50
1
1
2) in that shortened Intel manual (link), the unsigned-related jumps are
ja/jb/... ("below/above"), signed-related jumps are jg/jl/... ("greater/less"). But keep in mind they actually don't care about their name, they just check current flag status, set up by whatever last instruction(s)... Also don't assume much about flags being set by instructions, most of the time the arithmetic instructions set them as expected, but there are exceptions, like dec/inc does NOT modify CF (for good reasons, but it may surprise you) - so check inst. guide.– Ped7g
Nov 20 '18 at 17:03
2) in that shortened Intel manual (link), the unsigned-related jumps are
ja/jb/... ("below/above"), signed-related jumps are jg/jl/... ("greater/less"). But keep in mind they actually don't care about their name, they just check current flag status, set up by whatever last instruction(s)... Also don't assume much about flags being set by instructions, most of the time the arithmetic instructions set them as expected, but there are exceptions, like dec/inc does NOT modify CF (for good reasons, but it may surprise you) - so check inst. guide.– Ped7g
Nov 20 '18 at 17:03
2
2
Please read Under what circumstances may I add “urgent” or other similar phrases to my question, in order to obtain faster answers? - the summary is that this is not an ideal way to address volunteers, and is probably counterproductive to obtaining answers. Please refrain from adding this to your questions.
– halfer
Nov 20 '18 at 17:55
Please read Under what circumstances may I add “urgent” or other similar phrases to my question, in order to obtain faster answers? - the summary is that this is not an ideal way to address volunteers, and is probably counterproductive to obtaining answers. Please refrain from adding this to your questions.
– halfer
Nov 20 '18 at 17:55
|
show 6 more comments
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1
5 > -1, soalis greater. Why do you think it's less/equal? ( felixcloutier.com/x86/Jcc.html .. thecmpinstruction will set flags in a way that mnemonics likejg = jump if greaterandja = jump if abovework as expected ... (when you understand the basic data types and why 8 bit-1is encoded as 1111_1111 in binary, i.e. 0xFF) ... for SIGNED integers in 8 bits you have working range -128..+127 .. for unsigned integers the range is 0..255 (you are loadingblwith bit pattern "1111_1111" which can be interpreted as 255 (unint8) or -1 (int8) or anything (anything_t).– Ped7g
Nov 20 '18 at 16:48
1
is it not 5 > 255 :@ what is wrong with me... how can i make bl 255? changing jg to ja does that?
– 5D_cOOKIE
Nov 20 '18 at 16:50
0xff is -1 in 8-bit 2's complement.
– Peter Cordes
Nov 20 '18 at 16:50
1
2) in that shortened Intel manual (link), the unsigned-related jumps are
ja/jb/...("below/above"), signed-related jumps arejg/jl/...("greater/less"). But keep in mind they actually don't care about their name, they just check current flag status, set up by whatever last instruction(s)... Also don't assume much about flags being set by instructions, most of the time the arithmetic instructions set them as expected, but there are exceptions, likedec/incdoes NOT modify CF (for good reasons, but it may surprise you) - so check inst. guide.– Ped7g
Nov 20 '18 at 17:03
2
Please read Under what circumstances may I add “urgent” or other similar phrases to my question, in order to obtain faster answers? - the summary is that this is not an ideal way to address volunteers, and is probably counterproductive to obtaining answers. Please refrain from adding this to your questions.
– halfer
Nov 20 '18 at 17:55