Scenario x64_1: Hello World!

Prerequisites
Be familiar with the “Programming in Assembler for x64” chapter, where an introduction to writing assembler programs for the Windows OS is presented. Have the programming tools configured and ready.

Scenario
First, a Hello World type program written purely in assembler.

Result
The “Hello World!” text is displayed in a console.

Start
Create a new assembler source file.

Step 1
It will not be very surprising that the first code example will be the “Hello world!”. This program uses three system functions:

GetStdHandle - returns the handle of the console window, which is the main window of our application.
WriteConsole - displays the text in the console.
ExitProcess - returns control to the operating system.

The functions are implemented in a library file, kernel32.lib, which, in our case, is statically linked. We use the “includelib” directive to inform the linker where to search for functions. To inform the assembler about the names of functions, we declare them with the set of “extern” directives.

Windows library functions can be used in a dynamic way by using dynamically linked libraries (DLLs). We will show this method in other scenarios.

The details of each statement of the program are explained in comments.

option casemap:none             ; recognising small and capital letters
 
includelib kernel32.lib         ; statically linked library with system functions
 
EXTERN GetStdHandle:PROC        ; declaration of system functions for use
EXTERN WriteConsoleA:PROC
EXTERN ExitProcess:PROC
 
STD_OUTPUT_HANDLE equ -11       ; STD_OUTPUT_HANDLE costant
 
; In the data section of our program, there is a string to be displayed
.data
    message db "Hello, World!", 13, 10
    msgLen  equ $ - message     ; constant calculation containing string length
 
; In the code section of our program, there are instructions for execution
.code
main PROC                       ; main function - entry point
    sub rsp, 28h                ; shadow space + align
 
; HANDLE hConsole = GetStdHandle(STD_OUTPUT_HANDLE)
    mov ecx, STD_OUTPUT_HANDLE
    call GetStdHandle           ; this function returns the handle of the console window
 
; WriteConsoleA(hConsole, message, msgLen, &written, NULL)
    mov rcx, rax                ; console window handle
    lea rdx, message            ; pointer to the buffer
    mov r8d, msgLen             ; length
    lea r9, written             ; pointer to a var with a real number of chars written
    mov qword ptr [rsp+20h], 0  ; 5th argument (lpReserved = NULL)
    call WriteConsoleA          ; this function displays text in the console
 
; ExitProcess(0)
    xor ecx, ecx                ; value to be returned
    call ExitProcess            ; return to operating system
main ENDP                       ; end of the main function
 
; In the uninitialised data section of our program, there is a "written" variable
.data?
    written dq ?                ; variable which holds the number of written chars
 
END                             ; end of source file

Step 2
Assemble the program and observe results.

Result validation
The program should print the “Hello World!” text in a console window.

If it does not assemble?
Observe the messages passed by the assembler and linker programs. Refer to the documentation and instruction set. Be sure that all library files are accessible in the environment.