Teaching X86 assembly language programming with MS visual studio

Transcription

1 Teaching X86 assembly language programming with MS visual studio Dr. S. S. Limaye Principal, Jhulelal Institute of Technology, Nagpur, India ; Abstract: In many colleges, 8086 assembly language practicals are still conducted using the outdated MASM on DOS platform. This paper describes how to use Microsoft Visual Studio for teaching assembly language programming of Intel 80X86 processor and its derivatives like Pentium. It also illustrates how to integrate assembly program into a C program. Keywords : Pentium, 80X86, learning assembly language, Visual studio, Microsoft 1 Introduction 1.1 Why learn assembly language? The UG syllabus for any electronics or computer science course usually includes X86 assembly language programming. This is important because it gives the student, a thorough understanding of the hardware. Moreover, programs written in assembly language are faster to execute and consume less memory space. Also, you have full control over the machine, which is needed for writing I/O drivers or boot loaders. Microsoft corp. itself writes critical sections of code in assembly. In general, the difficulty level of assembly level programming is high. Therefore, things like GUI (Graphical User Interface) should be better left to the high level languages and special cases should be dealt with assembly. Writing a console program in assembly is fairly simple as we shall see shortly. 1.2 Why Visual Studio? Earlier, the standard packages for X86 programming were MASM and DEBUG under DOS. Unfortunately Microsoft stopped supporting MASM after 1991 and proper documentation for advanced versions of processors is not available. However the visual studio can be used for assembly language development without any change. It is high time that we stop using the obsolete packages and switch over to modern IDEs. Today MS Visual Studio is a standard development tool useful for various programming languages. By using Visual Studio, we can not only re use the skills for other languages, but also make integration into other languages easier. We are able to call an assembly program from C and vice versa. Also, the disassembly mode in Visual Studio lets you to see how your C instructions are converted to assembly. MASM has now been renamed as ML and is shipped with visual studio. However there are user groups like MASM32 who have kept MASM alive and have created useful libraries.

2 2 Changes from DOS based assembler and debugger MASM was designed for the real mode 8086 having 64K sized segments. We used SEGMENT and ASSUME directives for defining segments. It does not work in the protected mode in which windows runs. Here, we need to use MODEL, CODE and DATA directives. The segment registers are initialized by the OS when the program starts and they should not be disturbed. We use the FLAT memory model in which all segments overlap and are of size 4 GB. In MASM, when the program was over, it could be terminated by calling INT 21 Function 0. It does not work in windows because the system calls are dynamically linked. A simple way to terminate a program is to use debug break, i.e. INT 3. It is not a clean way but it works. A better way is to invoke Exit Process call from the kernel32 library though it requires some work. We will study it in later examples. We need to remember that in FLAT model, all offsets are 32 bits and hence all base and index registers must be 32 bits, i.e. we should always use EBX, EBP, ESP, ESI, EDI rather than BX, BP, SP, SI, DI. The string oriented instructions MOVSB, MOVSW, MOVSD must use ESI, EDI and ECX rather than SI, DI and CX. 3 Assembly Program skeleton A typical Pentium program has following skeleton..686p ; Pentium Pro or later.model flat, stdcall ;Use windows API calling convention.data <Data declarations>.code <Program> The first line indicates Pentium Pro instruction set. The other choices are 386, 486 or 586. The second line uses a flat model, i.e. it uses all near pointers and overlapping segments of 4GB each. There exist some other models but they are complicated to use, and we will not discuss them here. The second parameter stdcall indicates that we will use windows API calling convention. i.e. it pushes the arguments on the stack from right to left and stack popping is done

3 by the called program. STDCALL is useful even if you are not calling windows API because otherwise the linker mangles the procedure names( start becomes _start). The third line reserves a 4K stack. The fourth line specifies no case mapping, i.e. upper case and lower case symbols are treated differently. This is necessary to avoid confusion when we are using windows API. The fifth line starts the data segment. Define your data elements with DB or DW directives here. The next line marks the beginning of code segment. Enter the program here. It starts with the start lable. The last line is end and it specifies start as the entry point. 4 Examples 4.1 Example 1: Stand alone assembly program - Addition of two numbers Start Visual studio. From main menu, select New>Project. In the project dialog box, select Visual C++ > Win32 in the left pane and Win32 console application in the right pane. Enter tut1 as project name and press OK. Press the NEXT button on the application wizard window and in the next screen, click the check box labelled Create empty project. Then click FINISH. In the explorer pane on the left side, right click on Source files. From pop up menu, select Add> New item. In the dialog box, click on C++ source file. Enter file name as main.asm. Click the Add button. Visual Studio does not know how to handle asm files, so, in the solution explorer pane, right click on the tut1 entry and in the POP up menu, select Custom build rules. In the dialog box, assert the checkbox for Microsoft Macro Assembler and press OK. Enter following program in the editor pane..686p ; Pentium Pro or later.model flat, stdcall ;Use windows API calling convention n1 dw 5 n2 dw 6 n3 dw? mov ax,n1 add ax,n2 mov n3,ax int 3 Click the build tool button. If there are errors, then correct them and build again till errors are removed. Place cursor on the line next to start label. Right click and in pop up menu, select Breakpoint > Insert Breakpoint. Note that we cannot set the break point on the start line. Press the run button. The program will halt at the break point. Invoke registers window, memory window and disassembly window through menu DEBUG>Windows. You can step through the program with the step button and watch how registers and memory change.

4 4.2 Example 2: Search a substring in a string The problem is to find the offset of the key string Computer in the given message and place it in the EBX register. If the key is absent in the message, then EBX should return 0FFFFFFFFH. The program uses CMPS instruction to compare the key string with various substrings of the message starting at an offset specified by the EBX register. If the key is not found till the offset reaches 12, then the program terminates with 0FFFFFFFFH in the EBX register. Enter the following program..686p ; Pentium Pro or later.model flat, stdcall ;Use windows API calling convention ;Program to find where "Computer" is located in message message db "Trash","Computer","Garbage" key db "Computer" mov ebx,0 ;Initially, offset =0 cmpr: mov esi,offset message;esi at start of message add esi,ebx ;ebx holds offset from start mov edi,offset key ;edi points to search key mov ecx,8 ;count of chars in key repe cmpsb ; compare 8 chars. Z is set if match found jz found ;match found inc ebx cmp ebx,12 js cmpr ;Not yet reached end of message mov ebx,0ffffffffh ;key not found in message found: int 3 Let us now learn how to create listing and map files. In the solution explorer, right click on the tut2 entry and select properties. The property pages dialog pops up. In its left pane, click on the + sign to the left of Microsoft Macro Assembler entry to expand the branch. Select listing File. In the right pane, click on the Assembled code Listing file and enter file name as $(InputName).lst. In the left pane, expand the Linker entry by clicking the + sign and select debugging. In the right pane, click on Generate map file option and change to yes. Click on map file name and enter it as $(InputName).map. Build and debug the project as before. 4.3 Example 3 Separate out even and odd numbers 20 numbers are stored at array called buffer. Copy all even numbers to array even and all odd numbers to array odd. The program is self explainatory and it is given below. Note that we have used a segment directive DATA? To define uninitialized data arrays odd and even. We could have included them in the DATA segment but it would have forced the loader to initialize them to 0, making the program inefficient.

5 .MODEL flat, stdcall ;Use windows API calling convention ;Program to sort odd and even numbers ExitProcess PROTO :DWORD buffer db 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20? evn db 20 dup(?) ;even array will be stored here odd db 20 dup(?) ;odd array will be stored here mov esi,offset evn ; esi at start of even mov edi,offset odd ;edi points to odd array mov ebx,offset buffer ;bx points to input array mov ecx,20 ;count of chars in buffer L1: mov al,[ebx] ;Get a byte from buffer ror al,1 ;Get a0 in Cy jc got_odd ;Cy is set for odd numbers rol al,1 ;restore al mov [esi],al ;Store in odd array inc bx ;increment source pointer inc esi ;increment even destination pointer jmp loop_end ; got_odd: rol al,1 ;restore al mov [edi],al ;Store in odd array inc bx inc edi loop_end: loop L1 ;Decrement ecx and go back over: invoke ExitProcess, Example 4: Using system calls. In the above program, replace int 3 with ExitProcess, 0 ExitProcess is an external system call. We need to define it using PROTO directive similar to the declaration of a function in C. Add following line before data segment declaration. ExitProcess PROTO :DWORD The linker binds it to the appropriate function in kernel32.dll. The program now gracefully exits with following message. The program '[0x808] tut3.exe: Native' has exited with code 0 (0x0). To read and write on the console, we will use functions StdIn and StdOut. But before using them, we first need to do some work. Create a directory C:\MASM 32 and download MASM32 package into it from This is a huge package. If you are not bothered about the disk space, then leave the entire package there but we need only two files from it - C:\MASM

6 32\lib\masm32.lib and C:\MASM 32\include\masm32.inc. Give following commmands before the DATA declaration. includelib C:\masm32\lib\masm32.lib include C:\masm32\include\masm32.inc The masm32.inc file contains the necessary PROTO directives for StdIn and StdOut. The includelib command causes the linker to search the masm32.lib for the object modules of StdIn and StdOut. The console message output can be achieved by following command. invoke StdOut,ADDR message; Where message is address of buffer (0 terminated) The console message input can be achieved by following command. invoke StdIn,ADDR buffer,100 ;Read upto 100 characters and store at buffer Following code illustrates the usage. prompt db "Enter something and press ENTER",13,10,0 Found db "Key found",0? buffer db 100 dup(?) invoke StdOut,ADDR prompt ;Send messsage to console invoke StdIn,ADDR buffer,100 ;Read upto 100 chars from keyboard and ;store in buffer How about invoking a windows style messagebox? It is quite simple using the MessageBoxA function in the user32.lib. Since this library is already in the search path of visual studio linker, we don t need to write a includelib statement. Define a proto like this. MessageBoxA PROTO :DWORD,:DWORD,:DWORD,:DWORD You can invoke it with the command invoke MessageBoxA, 0, addr message, addr Found, 0 The message string is printed in the body and the Found string is the title. 4.5 Example 5 Calling assembly program from C We will write a small program asm_add for adding two integers. The program will be called from a C program listed below. extern int add(int x, int y); main() { int a=5,b=3,c; c=asm_add(a,b); } The assembly program listing is given below..586 ; Use instructions for Pentium class machines.model FLAT, C ;Use the flat memory model. Use C calling conventions.stack ;Define a stack segment of 1KB (Not req for this example).data ;Create a near data segment.(not required for this example).code ;Indicates the start of a code segment. asm_add PROC ;Define procedure push ebp ;Save base pointer mov ebp,esp ;Copy ESP to EBP mov eax,[ebp+8] ;Get a add eax,[ebp+12] ;Add b

7 pop ebp ;Restore base pointer ret ;return asm_add ENDP END Conclusion A methodolgy for teaching 80X86 assembler using Microsoft Visual Studio has been presented. Integration of assembly module in a C program is also explained. This will make the students more upto date with latest technology.