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.