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i am reading CS:APP 3rd edition (Ch3. pic 3.7 GAS assembly code)

long exchange(long* xp, long y)

 long x = *xp;
 *xp = y;
 return x;


exchange:
 movq (%rdi), %rax
 movq %rsi, (%rdi)
 ret

i wonder why below assembly code(1. asm : which is converted to nasm) does not work?

i had diassembled working c function to nasm assembly source code using c2nasm. it is quite different to original assembly.

main.cpp :

int main()

 long a = 4;
 // long b = exchange(&a, 3);
 long b = exchange2(&a, 3);

 printf("[a: %ld] [b: %ld]n", a, b);

 return 0;

1.asm :

BITS 64

; default rel
default abs 

global exchange2 
section .text

exchange2:
 ;;; this code does not works
 ;;; program output -> [a: 4] [b: 0]
 mov rax, [rdi]
 mov [rdi], rsi 
 ret

 ;;; this code works, generated by c2nasm.
 ;;; program output -> [a: 3] [b: 4]
 ; push rbp
 ; mov rbp, rsp
 ; sub rsp, 16
 ; mov qword [rbp+10H], rcx
 ; mov dword [rbp+18H], edx
 ; mov rax, qword [rbp+10H]
 ; mov eax, dword [rax]
 ; mov dword [rbp-4H], eax
 ; mov rax, qword [rbp+10H]
 ; mov edx, dword [rbp+18H]
 ; mov dword [rax], edx
 ; mov eax, dword [rbp-4H]
 ; leave
 ; ret

EDIT: thanks!

working version for Windows x64 long long exchange(long long*, long long):

BITS 64

default rel

global _exchange2 ; Windows name-mangling prepends _ to C names
section .text

_exchange2:
 mov rax, [rcx]
 mov [rcx], rdx 
 ret

edited Mar 22 at 2:01

asked Mar 21 at 6:22

sailfish009

722816

1

Windows uses a different calling convention than x86-64 System V ABI (used on BSD and Linux).Your version of exchange2 doesn't work because it is not compatible with Windows.64-bit calling convention. In that convention the first 4 integer class parameters are passed through RCX, RDX, R8, R9. Try mov eax, [rcx] mov [rcx], edx ret .On win64 longs are 32-bit so you can use the 32-bit registers instead of RAX and RDX

– Michael Petch
Mar 21 at 6:38

1

Your "working" version does a 64-bit copy, so it's only correct for long long, not long. Your current version writes 4 extra bytes outside the long destination. Also, there's no reason to use default abs, but it has no effect anyway because you aren't using any [symbol] addressing modes.

– Peter Cordes
Mar 21 at 14:02

1

default rel is preferred, I'd recommend always using that. IDK why you commented it out and used default abs (which is the default anyway). If c2nasm includes that by default, then it's just reminding you that you probably want to change the NASM default.

– Peter Cordes
Mar 22 at 1:50

add a comment |

i am reading CS:APP 3rd edition (Ch3. pic 3.7 GAS assembly code)

long exchange(long* xp, long y)

 long x = *xp;
 *xp = y;
 return x;


exchange:
 movq (%rdi), %rax
 movq %rsi, (%rdi)
 ret

i wonder why below assembly code(1. asm : which is converted to nasm) does not work?

i had diassembled working c function to nasm assembly source code using c2nasm. it is quite different to original assembly.

main.cpp :

int main()

 long a = 4;
 // long b = exchange(&a, 3);
 long b = exchange2(&a, 3);

 printf("[a: %ld] [b: %ld]n", a, b);

 return 0;

1.asm :

BITS 64

; default rel
default abs 

global exchange2 
section .text

exchange2:
 ;;; this code does not works
 ;;; program output -> [a: 4] [b: 0]
 mov rax, [rdi]
 mov [rdi], rsi 
 ret

 ;;; this code works, generated by c2nasm.
 ;;; program output -> [a: 3] [b: 4]
 ; push rbp
 ; mov rbp, rsp
 ; sub rsp, 16
 ; mov qword [rbp+10H], rcx
 ; mov dword [rbp+18H], edx
 ; mov rax, qword [rbp+10H]
 ; mov eax, dword [rax]
 ; mov dword [rbp-4H], eax
 ; mov rax, qword [rbp+10H]
 ; mov edx, dword [rbp+18H]
 ; mov dword [rax], edx
 ; mov eax, dword [rbp-4H]
 ; leave
 ; ret

EDIT: thanks!

working version for Windows x64 long long exchange(long long*, long long):

BITS 64

default rel

global _exchange2 ; Windows name-mangling prepends _ to C names
section .text

_exchange2:
 mov rax, [rcx]
 mov [rcx], rdx 
 ret

edited Mar 22 at 2:01

asked Mar 21 at 6:22

sailfish009

722816

1

Windows uses a different calling convention than x86-64 System V ABI (used on BSD and Linux).Your version of exchange2 doesn't work because it is not compatible with Windows.64-bit calling convention. In that convention the first 4 integer class parameters are passed through RCX, RDX, R8, R9. Try mov eax, [rcx] mov [rcx], edx ret .On win64 longs are 32-bit so you can use the 32-bit registers instead of RAX and RDX

– Michael Petch
Mar 21 at 6:38

1

Your "working" version does a 64-bit copy, so it's only correct for long long, not long. Your current version writes 4 extra bytes outside the long destination. Also, there's no reason to use default abs, but it has no effect anyway because you aren't using any [symbol] addressing modes.

– Peter Cordes
Mar 21 at 14:02

1

default rel is preferred, I'd recommend always using that. IDK why you commented it out and used default abs (which is the default anyway). If c2nasm includes that by default, then it's just reminding you that you probably want to change the NASM default.

– Peter Cordes
Mar 22 at 1:50

add a comment |

i am reading CS:APP 3rd edition (Ch3. pic 3.7 GAS assembly code)

long exchange(long* xp, long y)

 long x = *xp;
 *xp = y;
 return x;


exchange:
 movq (%rdi), %rax
 movq %rsi, (%rdi)
 ret

i wonder why below assembly code(1. asm : which is converted to nasm) does not work?

i had diassembled working c function to nasm assembly source code using c2nasm. it is quite different to original assembly.

main.cpp :

int main()

 long a = 4;
 // long b = exchange(&a, 3);
 long b = exchange2(&a, 3);

 printf("[a: %ld] [b: %ld]n", a, b);

 return 0;

1.asm :

BITS 64

; default rel
default abs 

global exchange2 
section .text

exchange2:
 ;;; this code does not works
 ;;; program output -> [a: 4] [b: 0]
 mov rax, [rdi]
 mov [rdi], rsi 
 ret

 ;;; this code works, generated by c2nasm.
 ;;; program output -> [a: 3] [b: 4]
 ; push rbp
 ; mov rbp, rsp
 ; sub rsp, 16
 ; mov qword [rbp+10H], rcx
 ; mov dword [rbp+18H], edx
 ; mov rax, qword [rbp+10H]
 ; mov eax, dword [rax]
 ; mov dword [rbp-4H], eax
 ; mov rax, qword [rbp+10H]
 ; mov edx, dword [rbp+18H]
 ; mov dword [rax], edx
 ; mov eax, dword [rbp-4H]
 ; leave
 ; ret

EDIT: thanks!

working version for Windows x64 long long exchange(long long*, long long):

BITS 64

default rel

global _exchange2 ; Windows name-mangling prepends _ to C names
section .text

_exchange2:
 mov rax, [rcx]
 mov [rcx], rdx 
 ret

edited Mar 22 at 2:01

asked Mar 21 at 6:22

sailfish009

722816

i am reading CS:APP 3rd edition (Ch3. pic 3.7 GAS assembly code)

long exchange(long* xp, long y)

 long x = *xp;
 *xp = y;
 return x;


exchange:
 movq (%rdi), %rax
 movq %rsi, (%rdi)
 ret

i wonder why below assembly code(1. asm : which is converted to nasm) does not work?

i had diassembled working c function to nasm assembly source code using c2nasm. it is quite different to original assembly.

main.cpp :

int main()

 long a = 4;
 // long b = exchange(&a, 3);
 long b = exchange2(&a, 3);

 printf("[a: %ld] [b: %ld]n", a, b);

 return 0;

1.asm :

BITS 64

; default rel
default abs 

global exchange2 
section .text

exchange2:
 ;;; this code does not works
 ;;; program output -> [a: 4] [b: 0]
 mov rax, [rdi]
 mov [rdi], rsi 
 ret

 ;;; this code works, generated by c2nasm.
 ;;; program output -> [a: 3] [b: 4]
 ; push rbp
 ; mov rbp, rsp
 ; sub rsp, 16
 ; mov qword [rbp+10H], rcx
 ; mov dword [rbp+18H], edx
 ; mov rax, qword [rbp+10H]
 ; mov eax, dword [rax]
 ; mov dword [rbp-4H], eax
 ; mov rax, qword [rbp+10H]
 ; mov edx, dword [rbp+18H]
 ; mov dword [rax], edx
 ; mov eax, dword [rbp-4H]
 ; leave
 ; ret

EDIT: thanks!

working version for Windows x64 long long exchange(long long*, long long):

BITS 64

default rel

global _exchange2 ; Windows name-mangling prepends _ to C names
section .text

_exchange2:
 mov rax, [rcx]
 mov [rcx], rdx 
 ret

assembly x86-64 nasm calling-convention win64

edited Mar 22 at 2:01

asked Mar 21 at 6:22

sailfish009

722816

edited Mar 22 at 2:01

asked Mar 21 at 6:22

sailfish009

722816

edited Mar 22 at 2:01

asked Mar 21 at 6:22

sailfish009

722816

asked Mar 21 at 6:22

sailfish009

722816

asked Mar 21 at 6:22

sailfish009

722816

1

Windows uses a different calling convention than x86-64 System V ABI (used on BSD and Linux).Your version of exchange2 doesn't work because it is not compatible with Windows.64-bit calling convention. In that convention the first 4 integer class parameters are passed through RCX, RDX, R8, R9. Try mov eax, [rcx] mov [rcx], edx ret .On win64 longs are 32-bit so you can use the 32-bit registers instead of RAX and RDX

– Michael Petch
Mar 21 at 6:38

1

Your "working" version does a 64-bit copy, so it's only correct for long long, not long. Your current version writes 4 extra bytes outside the long destination. Also, there's no reason to use default abs, but it has no effect anyway because you aren't using any [symbol] addressing modes.

– Peter Cordes
Mar 21 at 14:02

1

default rel is preferred, I'd recommend always using that. IDK why you commented it out and used default abs (which is the default anyway). If c2nasm includes that by default, then it's just reminding you that you probably want to change the NASM default.

– Peter Cordes
Mar 22 at 1:50

add a comment |

1

Windows uses a different calling convention than x86-64 System V ABI (used on BSD and Linux).Your version of exchange2 doesn't work because it is not compatible with Windows.64-bit calling convention. In that convention the first 4 integer class parameters are passed through RCX, RDX, R8, R9. Try mov eax, [rcx] mov [rcx], edx ret .On win64 longs are 32-bit so you can use the 32-bit registers instead of RAX and RDX

– Michael Petch
Mar 21 at 6:38

1

Your "working" version does a 64-bit copy, so it's only correct for long long, not long. Your current version writes 4 extra bytes outside the long destination. Also, there's no reason to use default abs, but it has no effect anyway because you aren't using any [symbol] addressing modes.

– Peter Cordes
Mar 21 at 14:02

1

default rel is preferred, I'd recommend always using that. IDK why you commented it out and used default abs (which is the default anyway). If c2nasm includes that by default, then it's just reminding you that you probably want to change the NASM default.

– Peter Cordes
Mar 22 at 1:50

Windows uses a different calling convention than x86-64 System V ABI (used on BSD and Linux).Your version of exchange2 doesn't work because it is not compatible with Windows.64-bit calling convention. In that convention the first 4 integer class parameters are passed through RCX, RDX, R8, R9. Try mov eax, [rcx] mov [rcx], edx ret .On win64 longs are 32-bit so you can use the 32-bit registers instead of RAX and RDX

– Michael Petch
Mar 21 at 6:38

Your "working" version does a 64-bit copy, so it's only correct for long long, not long. Your current version writes 4 extra bytes outside the long destination. Also, there's no reason to use default abs, but it has no effect anyway because you aren't using any [symbol] addressing modes.

– Peter Cordes
Mar 21 at 14:02

default rel is preferred, I'd recommend always using that. IDK why you commented it out and used default abs (which is the default anyway). If c2nasm includes that by default, then it's just reminding you that you probably want to change the NASM default.

– Peter Cordes
Mar 22 at 1:50

add a comment |

1 Answer
1

active

oldest

votes

Your example is for the x86-64 System V ABI, used on Linux, OS X, and other non-Windows platforms (64-bit long, args in RDI, RSI, ...)

You compiled for Windows x64, which uses a different calling convention (args in RCX, RDX), and long is a 32-bit type.

Also you compiled with optimization disabled so the asm is full of store/reload noise. With optimization it would be basically the same but with different registers.

BTW, you can use gcc -O3 -masm=intel to get Intel-syntax assembly which is a lot closer to NASM syntax. (It's not NASM syntax though; it's MASM-like for addressing modes, and still uses GAS directives.)

answered Mar 21 at 6:32

Peter Cordes

133k18203341

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Your example is for the x86-64 System V ABI, used on Linux, OS X, and other non-Windows platforms (64-bit long, args in RDI, RSI, ...)

You compiled for Windows x64, which uses a different calling convention (args in RCX, RDX), and long is a 32-bit type.

Also you compiled with optimization disabled so the asm is full of store/reload noise. With optimization it would be basically the same but with different registers.

answered Mar 21 at 6:32

Peter Cordes

133k18203341

add a comment |

Your example is for the x86-64 System V ABI, used on Linux, OS X, and other non-Windows platforms (64-bit long, args in RDI, RSI, ...)

You compiled for Windows x64, which uses a different calling convention (args in RCX, RDX), and long is a 32-bit type.

Also you compiled with optimization disabled so the asm is full of store/reload noise. With optimization it would be basically the same but with different registers.

answered Mar 21 at 6:32

Peter Cordes

133k18203341

add a comment |

Your example is for the x86-64 System V ABI, used on Linux, OS X, and other non-Windows platforms (64-bit long, args in RDI, RSI, ...)

You compiled for Windows x64, which uses a different calling convention (args in RCX, RDX), and long is a 32-bit type.

Also you compiled with optimization disabled so the asm is full of store/reload noise. With optimization it would be basically the same but with different registers.

answered Mar 21 at 6:32

Peter Cordes

133k18203341

Your example is for the x86-64 System V ABI, used on Linux, OS X, and other non-Windows platforms (64-bit long, args in RDI, RSI, ...)

You compiled for Windows x64, which uses a different calling convention (args in RCX, RDX), and long is a 32-bit type.

Also you compiled with optimization disabled so the asm is full of store/reload noise. With optimization it would be basically the same but with different registers.

answered Mar 21 at 6:32

Peter Cordes

133k18203341

answered Mar 21 at 6:32

Peter Cordes

133k18203341

answered Mar 21 at 6:32

Peter Cordes

133k18203341

answered Mar 21 at 6:32

Peter Cordes

133k18203341

add a comment |

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