osx - Storing keyboard Input in x64 assembly (Mac OS/X) -

i have been trying time number keyboard , comparing value on stack. if correct print "hello world!" , if incorrect, should print out "nope!". however, happens no matter input "jne" called, nope printed, , segfault. perhaps 1 of lend hand.

.section __data,__data     str:       .asciz "hello world!\n"  sto:      .asciz "nope!\n"  .section __text,__text .globl _main _main:     push %rbp     mov %rsp,%rbp     sub $0x20, %rsp     movl $0x0, -0x4(%rbp)     movl $0x2, -0x8(%rbp)      movl $0x2000003, %eax     mov $0, %edi     subq $0x4, %rsi     movq %rsi, %rcx     syscall      cmp -0x8(%rbp), %edx     je l1     jne l2     xor %rbx, %rbx     xor %rax, %rax     movl $0x2000001, %eax               syscall  l1:     xor %rax, %rax     movl $0x2000004, %eax              movl $1, %edi                      movq str@gotpcrel(%rip), %rsi      movq $14, %rdx                    syscall     ret  l2:     xor %eax, %eax     movl $0x2000004, %eax               movl $1, %edi                        movq sto@gotpcrel(%rip), %rsi        movq $6, %rdx                      syscall     ret 

i start os/x syscall tutorial (the 64-bit part in case). written nasm syntax important information text , links syscall calling convention. syscall table found on apple webpage. additional information on standard calling convention 64-bit os/x can found in system v 64-bit abi.

of importance syscall convention:

• arguments passed in order via these registers rdi, rsi, rdx, r10, r8 , r9
• syscall number in rax register
• the call done via syscall instruction
• what os x contributes mix have add 0x20000000 syscall number (still have figure out why)

you have many issues with sys_read system call. syscall table says this:

3 aue_null    { user_ssize_t read(int fd, user_addr_t cbuf, user_size_t nbyte); }  

so given calling convention, int fd in rdi, user_addr_t cbuf (pointer character buffer hold return data) in rsi, , user_size_t nbyte (maximum bytes buffer can contain) in rdx.

your program seg faulted on ret because didn't have proper function epilogue match function prologue @ top:

push %rbp                 # mov  %rsp,%rbp            # function prologue 

you need reverse @ bottom, set result code in rax , ret. like:

mov %rbp,%rsp             # \ function epilogue pop %rbp                  # / xor %eax, %eax            # return value = 0 ret                       # return c runtime exit                           #     gracefully , return os 

i did other minor cleanup, tried keep structure of code similar. have learn more assembly better understand code sets rsi address sys_read syscall . should try find tutorial/book on x86-64 assembly language programming in general. writing primer on subject beyond scope of answer.

code might closer looking takes above account:

.section __data,__data str:     .asciz "hello world!\n"  sto:     .asciz "nope!\n"  .section __text,__text  .globl _main _main:     push %rbp                 #     mov  %rsp,%rbp            # function prologue      sub  $0x20, %rsp          # allocate 32 bytes of space on stack                               #     temp local variables      movl $0x2, -4(%rbp)       # number comparison                                # 16-bytes -20(%rbp) -5(%rbp)                               #     char input buffer     movl $0x2000003, %eax     mov $0, %edi              # 0 stdin     lea -20(%rbp), %rsi       # address of temporary buffer on stack     mov $16, %edx             # read 16 character maximum     syscall      movb (%rsi), %r10b        # rsi = pointer buffer on stack                               # first byte     subb $48, %r10b           # convert first character number 0-9     cmpb -4(%rbp), %r10b      # did find magic number (2)?     jne l2                    #     if no exit error message  l1:                           # if magic number matched print                               #     hello world     xor %rax, %rax     movl $0x2000004, %eax     movl $1, %edi     movq str@gotpcrel(%rip), %rsi     movq $14, %rdx     syscall     jmp l0                    # jump exit code  l2:                           # print "nope"     xor %eax, %eax     movl $0x2000004, %eax     movl $1, %edi     movq sto@gotpcrel(%rip), %rsi     movq $6, %rdx     syscall  l0:                           # code exit main     mov %rbp,%rsp             # \ function epilogue     pop %rbp                  # /     xor %eax, %eax            # return value = 0     ret                       # return c runtime exit                               #     gracefully , return os