(This tutorial was originally written in 2004 and featured in http://asm.inightmare.org/)

mov ah, 0x1A
mov dx, DTA
int 21h

Simple, DX stores pointer to new DTA table. How does that DTA thing look like? Like that:

OFFSET	SIZE	MEANING
0	23 bytes	Data that only DOS needs
0x15	Byte	File attribute
0x16	Word	File creation/modification time
0x18	Word	File creation/modification date
0x1A	DWord	File size
0x1E	13 Bytes	File name terminated with zero

Now, we have our table set up, we can find the first file.

mov ah, 0x4E
mov dx, query
int 21h

It’s pretty simple. DX stores pointer to search query (DOS dir argument) terminated with zero. Like:

db “*.*”, 0x0
db “*.com”, 0x0

Carry flag is set if file wasn’t found. If file was found DTA table is filled with files attributes. For all other files we want to find we use other call:

mov ah, 0x4F
mov dx, query
int 21h

If no more files found Carry Flag is set after the call, if file is found DTA table is filled with it’s attributes. That’s it. Grab the source to see how everything looks. Have fun!

NASM Source code - here