Programming in Assembler for x64

In this section, we will show some examples of programs written purely in assembler or in connection with other programming languages, including C++ and C#. We assume that the reader is familiar with the coursebook, instructions and directives used to write the assembler programs. We will describe the use of the integrated development environment (Visual Studio) and methods to assemble programs with the command line only.We will also show how to create the static and dynamic library written in assembler for use in assembler or in other compilers.

In the following chapter, we explain how to write, assemble, link, and execute assembly-language programs for x64 processors. We assume that the reader is familiar with the most important processor instructions and MASM directives.

To write a program in assembler, it is convenient to use a Visual Studio IDE: either commercial or free (Community Edition). The following section presents the installation of Visual Studio Community Edition on Windows.

Installation requires the following simple steps:

1. Download installer executable from: https://visualstudio.microsoft.com/free-developer-offers/ - Mind to choose Community Edition (purple), not Code (blue)! as in figure 1.
2. Run the installer, let it download and install.
3. Configure components - a minimal set of development platforms for our laboratory exercises requires:
   1. full install of C++ development platform in native code for Windows (remember to click additional components as in the figure 2),
   2. default install of the C# development platform in managed code for Windows.

This scenario concerns the implementation of a command-line Windows x64 application written in pure Assembler. Assembling, debugging, disassembly window, register view, memory view - data section,

[piotr] TO BE DONE

It is possible to use command-line MASM tools to assemble, link, and create libraries written in assembly language. You can use any editor to create the assembler source code and translate it into machine code. The tools required are integral elements of the Visual Studio Community installation, installed with the option “Desktop development with C++”. For the default VS installation, you can find them in the following folder (it can change due to different version numbers).

C:\Program Files\Microsoft Visual Studio\18\Community\VS\Tools\MSVC\14.50.35717\bin\Hostx64\x64

To use statically included Windows libraries, you need lib files. The essential library is kernel32.lib, but for other Windows functions, you will also need some additional libraries. All are available in the following folder (it can change due to different version numbers).

C:\Program FIles (x86)\Windows Kits\10\Lib\10.0.26100.0\um\x64

For assembling the source file, the ML64.exe program is used. This program has many options, which you can see executing:

ML64.exe /?

After assembling, ML64 can call the linker automatically. An exemplary MASM execution command to assemble and link the file named source.asm can look like this:

ml64 /Fl /Zi /Zd source.asm /link /entry:main

The options used explanation:

• /Fl - generate listing file. MASM will output the source.lst file with the report on the assembling process.
• /Zi - add symbolic debug info. MASM will add to the object file names of symbols defined in the program. It will allow debuggers to name user-defined symbols during debugging.
• /Zd - add line number debug info. MASM will add to the object file source code line numbers.
• /link - MASM will call the linker.
• /entry:main - option for the linker, which informs about the entry point of the program.

To create the static library, the assembler module shouldn't have the main procedure defined. All other procedures will be made available for other programs by default. If there is a need to hide a procedure from visibility, it is possible to mark it as PRIVATE. The first step is to assemble the source file with MASM.

ml64 /c source.asm

• /c - assemble without linking.

The second step is to create the lib file with the lib tool.

lib source.obj

This will create the source.lib file, which can be imported into the program, where we can use all available procedures.

The example for the library will be the program containing the function “int_to_ascii”, which converts the integer number into a text representation. Let's begin with the function itself. The function accepts two arguments: the number to be converted passed by RCX and the pointer to the buffer for the resulting text passed by RDX. It converts a signed 64-bit number and returns the updated pointer in RDX and the length of the resulting string in RAX. We can use the results in the WinAPI function WriteConsoleA to display the ASCII representation of a number in the console.

option casemap:none
 
.code
; ----------------------------------------
; int_to_ascii
; input:   RCX = signed 64-bit number
; output:  updated string at address in RDX
;          RAX = length of the resulting string
; ----------------------------------------
int_to_ascii PROC
    push rbx              ; rbx is nonvolatile
    push rdi              ; rdi is nonvolatile
    sub rsp, 24           ; shadow space
    mov [rsp+8],  rcx
    mov [rsp+16], rdx
    mov rax, rcx          ; mov imput number to rax
 
; point rdi into the buffer end
    mov rdi, rdx          ; pointer to a string
    add rdi, 31
    mov byte ptr [rdi], 0 ; mark string end with terminator
 
    mov rbx, 10
 
; test if the numer is positive or negative
    xor r8d, r8d	  ; r8 = 0 → positive flag
    test rax, rax	  ; test the sign
    jge convert		  ; jump if rax positive
 
    neg rax	          ; change the sign of rax
    mov r8d, 1		  ; r8 = 1 → negative flag
 
; conversion loop
convert:
    dec rdi		  ; starting from the end of the text (least significant digit)
    xor rdx, rdx	  ; prepare to divide rdx:rax by rbx
 
    div rbx		  ; rax / 10 → remainder in rdx
    add dl, "0"		  ; convert remainder into ASCII
    mov [rdi], dl	  ; write character of a digit to buffer
    test rax, rax	  ; test if there is still a value for conversion
    jne convert
 
; add minus if needed
    cmp r8d, 0            ; r8 = 1 → negative flag
    je write
    dec rdi               ; add minus character
    mov byte ptr [rdi], '-'
 
write:
; calculate length of the text (end – rdi)
    mov rax, [rsp+16]     ; get pointer to an original buffer
    add rax, 31
    sub rax, rdi          ; resulting number length in rax
    mov rdx, rdi          ; adjusted pointer to string in a buffer
 
    add rsp, 24           ; restore stack pointer 
    pop rdi
    pop rbx
    ret
int_to_ascii ENDP
 
END

This library can be imported into the assembler program or a program written in another programming language. Assembly program can look as follows:

option casemap:none
 
; include the system library and our convert library
includelib kernel32.lib
includelib convert.lib
 
; declare function we use in our program
EXTERN GetStdHandle:PROC
EXTERN WriteConsoleA:PROC
EXTERN ExitProcess:PROC
EXTERN int_to_ascii:PROC
 
; costant required by GetStdHandle system function
STD_OUTPUT_HANDLE equ -11
 
; data section
.data
    buffer db 32 dup(0) ; buffer for a string
    hOut   dq ?         ; placeholder for console handle
    dummy  dq ?         ; place for dummy parameter
 
;code section
.code
 
; -------------------------------------------
; main function of the program - entry point
; -------------------------------------------
main PROC
; shadow space
    sub rsp, 40
 
; get the handle of stdout
    mov ecx, STD_OUTPUT_HANDLE ; console output
    call GetStdHandle
    mov hOut, rax       ; store the handle
 
; call conversion function
    mov rcx, 33550336   ; number for displaying
    lea rdx, buffer     ; pointer to a buffer
    call int_to_ascii
 
; prepare agruments for WriteConsoleA(hOut, rdi, len, ...)
    mov rcx, hOut       ; console handle 
                        ; pointer to the beginning of a string is in rdx
    mov r8, rax         ; nNumberOfCharsToWrite is in rax  
    lea r9, dummy       ; dummy for lpNumberOfCharsWritten
    mov qword ptr [rsp+20h], 0  ; lpReserved (must be NULL)
 
    call WriteConsoleA  ; displaying function
 
    xor ecx, ecx        ; return value of a program
    call ExitProcess    ; go back to Windows OS
main ENDP
 
END