INT 13h

INT 13h is the lowest level a DOS program normally reaches for disks. It knows nothing about files, directories or FAT: it reads and writes sectors, 512-byte blocks addressed by their physical position. DOS itself uses it to reach the disks; a program uses it to read a boot sector or partition table, copy a whole floppy, or run before DOS is there (boot loaders do nothing else).

Drive numbers

DL selects the drive by BIOS number, not by DOS letter: 00h is the first floppy (A:), 01h the second, 80h the first hard disk, 81h the second. Bit 7 is the hard-disk bit. The BIOS keeps the number of hard disks it found at 0040:0075h. DOS letters come later: C: is usually the first primary partition on 80h, but it can be anything, and network or substituted drives have no BIOS number at all.

Cylinder, head, sector

A sector is addressed by CHS: the cylinder (the track position of the heads), the head (which surface) and the sector within the track. Sectors count from 1, cylinders and heads from 0. The registers pack this tightly: CH holds the low 8 bits of the cylinder, CL bits 0-5 the sector (1..63) and CL bits 6-7 the two high bits of the cylinder (so cylinders go up to 1023), DH the head. A 1.44 MB floppy has 80 cylinders, 2 heads and 18 sectors per track; the first sector of a disk, its boot sector or partition table, is always cylinder 0, head 0, sector 1.

; cylinder 300 (12Ch), head 2, sector 5:
MOV     CH, 2Ch         ; low 8 bits of the cylinder
MOV     CL, 05h OR 40h  ; sector 5, plus cylinder bits 8-9 (01b) in bits 6-7
MOV     DH, 2           ; head

CHS caps a disk at 1024 × 255 × 63 sectors, about 8 GB, and older BIOSes at 504 MB. The extensions (AH=42h read, 43h write) take a 64-bit linear block address in a packet instead.

Errors

Every function returns CF = 0 on success and CF = 1 with a status in AH on failure: 01h bad command or drive, 02h address mark not found, 03h write protected, 04h sector not found, 06h disk changed, 80h timeout (no disk in the drive). Floppies fail often for harmless reasons, a motor not up to speed or a just-changed disk, so the rule for floppies is three tries with a reset (INT 13h AH=00h) in between before reporting an error.

The example asks the BIOS data area how many hard disks the BIOS found:

MOV     AX, 40h
MOV     ES, AX        ; the BIOS data area
MOV     AL, ES:[75h]  ; number of hard disks
XOR     AH, AH
CALL    PDEC
MOV     DX, OFFSET DISKS1
MOV     AH, 09h       ; DOS: print the $-string at DS:DX
INT     21h
 
DISKS1  DB      " hard disk(s) known to the BIOS", 13, 10, "$"
PDEC    PROC    NEAR  ; print AX in decimal; keeps every register
PUSH    AX
PUSH    BX
PUSH    CX
PUSH    DX
MOV     BX, 10
XOR     CX, CX        ; digits on the stack
DECD1:
XOR     DX, DX
DIV     BX            ; AX = AX / 10, DX = the last digit
PUSH    DX
INC     CX
TEST    AX, AX
JNZ     DECD1
DECD2:
POP     DX            ; the digits come back first to last
ADD     DL, "0"
MOV     AH, 02h
INT     21h
LOOP    DECD2
POP     DX
POP     CX
POP     BX
POP     AX
RET
PDEC    ENDP

In DOSBox-X

DOSBox-X has two kinds of drives. A folder of the host mounted with MOUNT is a DOS drive only: DOS sees files, but there are no sectors, and the BIOS has no disk behind the letter. A disk image attached with IMGMOUNT is a real BIOS disk with sectors, and INT 13h works on it as on a PC. The workbench's drive C is a mounted folder, so INT 13h on drive 80h does not reach a real disk there (see INT 13h AH=02h for what it returns instead), even though the BIOS data area claims two hard disks: DOSBox-X writes that count unconditionally, for programs that check it before looking further.