Directory Listing Program
Let’s write a short program that clears the screen, displays the current disk directory, and asks
the user to enter a filename. (You might want to extend this program so it opens and displays the
selected file.)
C++ Stub Module The C++ module contains only a call to asm_main, so we can call it a
stub module:
// main.cpp // stub module: launches assembly language program extern "C" void asm_main(); // asm startup proc void main() { asm_main(); }
ASM Module The assembly language module contains the function prototypes, several strings,
and a fileName variable. It calls the system function twice, passing it “cls” and “dir” commands.
Then printf is called, displaying a prompt for a filename, and scanf is called so the user can input
the name. It does not make any calls to the Irvine32 library, so we can set the .MODEL directive to
the C language convention:
; ASM program launched from C++ (asmMain.asm) .586 .MODEL flat,C ; Standard C library functions: system PROTO, pCommand:PTR BYTE printf PROTO, pString:PTR BYTE, args:VARARG scanf PROTO, pFormat:PTR BYTE,pBuffer:PTR BYTE, args:VARARG fopen PROTO, mode:PTR BYTE, filename:PTR BYTE fclose PROTO, pFile:DWORD BUFFER_SIZE = 5000 .data str1 BYTE "cls",0 str2 BYTE "dir/w",0 str3 BYTE "Enter the name of a file:",0 str4 BYTE "%s",0 str5 BYTE "cannot open file",0dh,0ah,0 str6 BYTE "The file has been opened",0dh,0ah,0 modeStr BYTE "r",0 fileName BYTE 60 DUP(0) pBuf DWORD ? pFile DWORD ? .code asm_main PROC ; clear the screen, display disk directory INVOKE system,ADDR str1 INVOKE system,ADDR str2 ; ask for a filename INVOKE printf,ADDR str3 INVOKE scanf, ADDR str4, ADDR filename ; try to open the file INVOKE fopen, ADDR fileName, ADDR modeStr mov pFile,eax .IF eax == 0 ; cannot open file? INVOKE printf,ADDR str5 jmp quit .ELSE INVOKE printf,ADDR str6 .ENDIF ; Close the file INVOKE fclose, pFile quit: ret ; return to C++ main asm_main ENDP END
The scanf function requires two arguments: the first is a pointer to a format string (“%s”), and
the second is a pointer to the input string variable (fileName). We will not take the time to
explain standard C functions because there is ample documentation on the Web. An excellent
reference is Brian W. Kernighan and Dennis M. Ritchie, The C Programming Language, 2nd
Ed., Prentice Hall, 1988.