I’m pretty new to assembly and am experimenting with calling C standard library functions from my assembly code. I’m on 64 bit windows 11, and use nasm syntax.

I have three files which call putchar and pass 77 (the ascii code for capital M).
The first and last work, but not the second.

The three files are called test1.asm, test2.asm, and test3.asm:

test1.asm

bits    64
default rel

extern ExitProcess
extern putchar
global main

section .text
main:
    ; sub 32 to allocate shadow space
    sub  rsp, 32

    ; call putchar with ascii character M

    mov  rcx, 77
    call putchar

    ; call ExitProcess with exit code 0

    xor  rcx, rcx
    call ExitProcess

test2.asm

bits    64
default rel

extern ExitProcess
extern putchar
global main

section .text
main:
    ; sub 8  to allocate memory for a qword
    ; sub 32 to allocate shadow space
    sub  rsp, 40

    ; copy 77 into rbx
    ; copy rbx (77) into [rbp-08]

    mov  rbx, 77
    mov  [rbp-08], rbx

    ; call putchar passing the value at [rbp-08] (77)

    mov  rcx, [rbp-08]
    call putchar

    ; call ExitProcess with exit code 0

    xor  rcx, rcx
    call ExitProcess

test3.asm

bits    64
default rel

extern ExitProcess
extern putchar
global main

section .text
main:
    ; sub 32 to allocate shadow space
    sub  rsp, 32

    call secondary

    ; zero rcx to call ExitProcess

    xor  rcx, rcx
    call ExitProcess
secondary:
    push rbp
    mov  rbp, rsp
    ; sub 8  to allocate memory for a qword
    ; sub 8  so that the final total is 16-byte aligned
    ; sub 32 to allocate shadow space
    sub  rsp, 48

    ; save rbx because its volatile
    ; (although technically this shouldnt be needed bc main doesnt use rbx)
    push rbx

    ; copy 77 into rbx
    ; copy rbx (77) into [rbp-08]

    mov  rbx, 77
    mov  [rbp-08], rbx

    ; call putchar passing the value at [rbp-08] (77)

    mov  rcx, [rbp-08]
    call putchar

    ; restore rbx
    pop rbx

    add  rsp, 48
    pop  rbp
    ret

I assembled and linked them using the following commands in the cmd command prompt:

D:Desktoptemp>nasm -fwin64 test1.asm 

D:Desktoptemp>nasm -fwin64 test2.asm 

D:Desktoptemp>nasm -fwin64 test3.asm 

D:Desktoptemp>ld C:WindowsSystem32kernel32.dll C:WindowsSystem32msvcrt.dll test1.obj -o a1.exe

D:Desktoptemp>ld C:WindowsSystem32kernel32.dll C:WindowsSystem32msvcrt.dll test2.obj -o a2.exe 

D:Desktoptemp>ld C:WindowsSystem32kernel32.dll C:WindowsSystem32msvcrt.dll test3.obj -o a3.exe 

And I got this as output when I ran each of the executables (also in the cmd command prompt):

D:Desktoptemp>a1.exe
M
D:Desktoptemp>a2.exe

D:Desktoptemp>a3.exe
M
D:Desktoptemp>

I can’t figure out why test2.asm doesnt print out M but is instead blank.

In my mind, test1 does the same thing as test2 but in less steps by moving 77 directly into rcx, and test3 does the same thing as test2, only in a seperate function.

Both test1 and test3 work, so why doesn’t test2?

This question is similar to mine:
Can’t call `putchar` in assembly
But does not help me to understand why test1 and test3 do happen to work correctly, whereas test2 does not (assuming the fact that I don’t see any output from test2 is indeed because “the libc’s IO package performs buffered IO and you do not flush the buffer of stdout before terminating your program”).