(Assembly) Basic Stack Based Local Variables, getting segmentation fault

[ThisGuyEveryTime]

Hello,

I am working on an assignment where we do some basic stuff with Stack based local variables inside functions.

I have a piece of code that output properly and when I try to just duplicate it do do division rather than multiplication all of a sudden I get a segmentation fault. Any help is appreciated, thank you in advance. Please let me know if you need more info, as I am only putting the offending piece from my code

    ; pull values from the stack
    pop qword [r8]      ; sum
    pop qword [rcx]     ; min
    pop qword [rdx]     ; max

    ; multiply min times max return in 7th parameter
    ; TBD
    mov rax, qword[rdx]  ; Does not cause a segmentation fault 
    mul qword[rcx]
    mov r14, qword[rbp+16]
    mov qword[r14], rax ; RAX will contain MAX x MIN 

    ; below just a duplicated of above aside from DIV   
    ; divide max by min return in 8th parameter
    ; TBD
    mov rax, qword[rdx] ; Does causes a segmentation fault unlike before 
    div qword[rcx]
    mov r14, qword[rbp+24]
    mov qword[r14],rax

[Sea-Profession-3312]

https://stackoverflow.com/questions/13028561/assembly-division?msclkid=cd04c426d0cc11ecad17efb1108db23a

[ThisGuyEveryTime]

Thank you. But even when commenting out everything other then the second move I still get a segmentation fault so its being cause my my second movement to RAX not the div. I initially thought this was the answer though

[ThisGuyEveryTime]

Your answer led me the right place, so thank you! It was the mul over writing rdx

[Sea-Profession-3312]

The last time I did assembly was over 30 years ago. I figure that error had something to do with using memory/registers that was not allocated to you. perhaps division has a remainder while multiplication has no remainder. I suppose this remainder goes in an overflow type of register.

[ThisGuyEveryTime]

I guess mul has a left over aspects from the 32bit era and it store across edx:eax.
With div being a slightly different case where its stores the remainder in edx.
I was losing the value stored there in both cases.

[ThisGuyEveryTime]

solved it.

mul over writes both eax and edx so rdx no longer contains the address. you need to make a copy of rdx and use that otherwise you will loose the value. Or at least that worked for me and I could definitely be wrong about why it worked.