INT 13h AH=02h

INT 13h AH=02h copies whole sectors from the disk into memory at ES:BX: AL sectors, 512 bytes each, starting at the address given by CH, CL and DH on drive DL (see INT 13h for how the cylinder is packed into CH and CL). The buffer must be big enough for all of them. On success CF = 0 and AL says how many were read.

The first sector of a hard disk, cylinder 0, head 0, sector 1, is the master boot record: boot code, then from offset 1BEh four 16-byte partition entries, and at 1FEh the signature bytes 55h AAh that mark the sector as bootable. The example reads it and prints those last two bytes:

PUSH    CS
POP     ES
MOV     BX, OFFSET SECT1  ; ES:BX = a 512-byte buffer
MOV     AX, 0201h         ; AH = 02h read, AL = 1 sector
MOV     CX, 0001h         ; CH = cylinder 0, CL = sector 1 (sectors count from 1)
MOV     DX, 0080h         ; DH = head 0, DL = 80h, the first hard disk
INT     13h
JC      FAIL1             ; CF = 1: AH = what went wrong
MOV     DX, OFFSET OK1
MOV     AH, 09h           ; DOS: print the $-string at DS:DX
INT     21h
MOV     AL, SECT1+1FEh    ; the boot signature: 55h AAh on a real disk
CALL    PHEX2
MOV     DL, " "
MOV     AH, 02h           ; DOS: print the character in DL
INT     21h
MOV     AL, SECT1+1FFh
CALL    PHEX2
JMP     DONE1
FAIL1:
MOV     BL, AH            ; keep the status
MOV     DX, OFFSET BAD1
MOV     AH, 09h           ; DOS: print the $-string at DS:DX
INT     21h
MOV     AL, BL
CALL    PHEX2
DONE1:
 
OK1     DB      "read ok, bytes at 1FEh: $"
BAD1    DB      "read failed, AH = $"
SECT1   DB      512 DUP (0)
PHEX2   PROC    NEAR      ; print AL as two hex digits; keeps every register
PUSH    AX
PUSH    CX
PUSH    DX
MOV     DH, AL            ; keep the byte
MOV     CL, 4
SHR     AL, CL            ; the high nibble
CALL    HEXN1
MOV     AL, DH
AND     AL, 0Fh           ; the low nibble
CALL    HEXN1
POP     DX
POP     CX
POP     AX
RET
HEXN1:
ADD     AL, "0"
CMP     AL, "9"
JBE     HEXN2
ADD     AL, 7             ; past "9": "A".."F"
HEXN2:
MOV     DL, AL
MOV     AH, 02h
INT     21h
RET
PHEX2   ENDP

Limits

The buffer must not cross a 64 KB physical boundary (an address that is a multiple of 10000h): the PC's DMA controller, used for floppies, cannot carry over into the next 64 KB page, and the BIOS reports error 09h. Sector-sized buffers in a small program are usually safe, but a big one should be checked. Reads of several sectors should stay within one track on old BIOSes; reading one sector at a time is the safe choice. And floppy reads fail now and then for harmless reasons: retry three times with INT 13h AH=00h in between.

In DOSBox-X

Only drives attached as images (IMGMOUNT, or a booted disk) have sectors. For a mounted folder DOSBox-X answers most reads with CF = 1 and AH = 01h; the one exception is a read of cylinder 0, head 0, sector 1 of a hard-disk number, which some old installers use to check that a disk exists. That read "succeeds" and writes only a partition-entry marker (80h at 1BEh, 06h at 1C2h) into the buffer, leaving the rest as it was, which is why the example clears its buffer first and then sees 00 00 where the signature would be.