When a sector is being read or written, the diskette controller searches the diskette track for the sector's ID, the essential part of which is R, the record or sector number. The 4 bytes are referred to as C for cylinder, H for head, R for record (or sector number), and N for number of bytes per sector (also called the size code). The 4 bytes of data associated with a sector on the disk are known as address marks and are used by the disk controller to identify individual sectors during the read, write, and verify operations. When you format a disk track, the 4-byte groups are written to the diskette immediately in front of the individual sectors in the track. You specify these 4 bytes for each sector to be formatted by creating a table of 4-byte groups and passing the table's address in the register pair ES:BX.

In the words of Peter Norton, from The New Programmer's Guide to the IBM PC and PS/2:Įvery sector on a diskette track has 4 descriptive bytes associated with it. If CF is set, then an error occurred and the AH register contains the error code.

If the interrupt returns with the carry flag ( CF) clear, then no error occurred and the AH register will contain 0.

sector number: CL (not used for format!).

Here's a list of the required parameters for floppy-disk services: The parameters for service 05h are basically the same as those for every other disk read/write service, except that you don't need to specify a sector number (normally passed in CL), since you cannot format individual sectors. The nice thing about this is that it allows you to specify different characteristics for each track-and even each sector on a track (some old-school copy-protection schemes used this by creating tracks with oddball formatting).

You have to call this service multiple times to format an entire disk. (The service number goes in the AH register when you invoke the interrupt.) Note that this service formats a single track, not the entire disk at once. Specifically, int 13h, service 05h formats a disk track. An application program generally leaves such low-level manipulation of the FAT to the operating system, using only the OS-provided services to do high-level read/write operations. I don't think DOS provides any service to format a disk. Int 13h is the appropriate way to do it, but rather than calling a DOS service, you're actually using it to call a ROM BIOS service.

Formatting floppy disk