Question

I am using visual c++ express compiler to compile asm32:

; Example assembly language program -- adds 158 to number in memory
; Author:  R. Detmer
; Date:    1/2008

.586
.MODEL FLAT

.STACK  4096            ; reserve 4096-byte stack

.DATA                   ; reserve storage for data
number  DWORD   -105
sum     DWORD   ?

.CODE                           ; start of main program code
main    PROC
        mov     eax, number     ; first number to EAX
        add     eax, 158        ; add 158
        mov     sum, eax        ; sum to memory

        mov   eax, 0            ; exit with return code 0
        ret
main    ENDP

END                             ; end of source code

the above gives me a linking error, whereas this:

..
.CODE                           ; start of main program code
main:nop    
        mov     eax, number     ; first number to EAX
        add     eax, 158        ; add 158
        mov     sum, eax        ; sum to memory

        mov   eax, 0            ; exit with return code 0
        ret
end main                          ; end of source code
..

works greatly!

the only difference is the main:nop vs main proc

what is the difference between these two and why is one closed by end main and the other is main endp main ?

here’s the error i get:

1>------ Build started: Project: asm1, Configuration: Release Win32 ------
1>  Assembling [Inputs]...
1>LINK : error LNK2001: unresolved external symbol _WinMainCRTStartup
1>C:\Users\...\Downloads\asm1\Release\asm1.exe : fatal error LNK1120: 1 unresolved externals

Answer 1

It doesn’t have anything to do with the PROC directive. Your original code is missing the ‘main’ operand on the END directive. Which specifies the entrypoint for the program. Without one, the linker is going to try to find the default entrypoint for an executable, _WinMainCRTStartup. And that fails because you don’t have that and don’t link the CRT.

Fix:

main    PROC
        ; etc...
main    ENDP

END     main