osx - Mac OS x86 Assembly: Why does the initialized memory amount change? -


i started learning assembly week or ago, , when debugging program, came across strange memory usage. following code (see end of post) broken 2 files reason.

if compile , run with

gcc main.s ./a.out

with code block 1 running (code block 2 commented out), program prints "8", meaning right when program starts, mac os automatically puts 8 bytes worth of stuff on stack, leaves program thing.

however, if compile , run with

gcc main.s print.s ./a.out

with code block 2 running (code block 1 commented out), program prints "16", meaning mac os putting 16 bytes on stack instead of 8. when happens, offsets applied rsp achieve 16-byte alignment remain same, meaning start of stack being offset 8 bytes whenever outside function called.

i tried putting _printnum function in same file main.s, discrepancy persisted. thing tried add format string , use later on in program see if format string using memory, made no difference.

what think going on mac os pushing instruction pointer next instruction execute when program terminates onto stack, pushing old base stack pointer onto stack, both 32-bit, total of 8 bytes. when include function call (either local or external main file), seems assembler decides use 64-bit addresses instead of 32-bit addresses, doubling memory used, , hence 16 bytes used.

why happening, , if wrong, mac os doing stack? of stack used of value me? computer doing else instead of switching 32-bit 64-bit addressing? thanks.

main program (main.s):

.cstring     _format: .asciz "%d\n"  .text .globl _main  _main:     movq   %rbp, %rax   # put stack base pointer in rax     subq   %rsp, %rax   # subtract stack pointer total memory used     subq   $8, %rsp     # 16-byte alignment      #---------------------------------------------------------     # code block 1 - prints rax manually     #---------------------------------------------------------      movq   %rax, %rsi           # value print needs in rsi     lea    _format(%rip), %rdi  # address of format string goes in rdi                                 # don't know "_format(%rip)" does,                                 # works (any info handy)     call   _printf      #---------------------------------------------------------     # code block 2 - prints rax via function call     #---------------------------------------------------------      call   _printnum    # prints value of rax      #---------------------------------------------------------     # stack cleanup , return     #---------------------------------------------------------      addq   $8, %rsp     # account previous -8 rsp     ret                 # end program

printing function (print.s):

.cstring     _format: .asciz "%d\n"  .text .globl _printnum  # assumes 16-byte aligned when called # prints value of rax register _printnum:     push   %rbp         # save %rbp - previous stack base     movq   %rsp, %rbp   # update stack base     push   %rsi         # save %rsi - register     push   %rdi         # save %rdi - register      # print - 16 byte aligned (rip , 3 values 32 bytes)     movq   %rax, %rsi            # load value print     lea    _format(%rip), %rdi   # load format string     call   _printf      # restore registers     popq   %rdi     popq   %rsi     popq   %rbp      # return     ret


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