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Different asm on Windows x64 from a CS:APP x86-64 Linux example function for swapping a long


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0















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









share|improve this question



















  • 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
















0















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









share|improve this question



















  • 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














0












0








0








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









share|improve this question
















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






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Mar 22 at 2:01







sailfish009

















asked Mar 21 at 6:22









sailfish009sailfish009

722816




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













  • 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








1




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






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




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





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




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






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













1 Answer
1






active

oldest

votes


















2














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.)






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    1 Answer
    1






    active

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    active

    oldest

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    active

    oldest

    votes









    2














    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.)






    share|improve this answer



























      2














      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.)






      share|improve this answer

























        2












        2








        2







        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.)






        share|improve this answer













        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.)







        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered Mar 21 at 6:32









        Peter CordesPeter Cordes

        133k18203341




        133k18203341





























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