Function 0Fh reads the BIOS data area and returns three things at once: AL is the current mode (the one function 00h last set), AH the number of character columns, and BH the active display page. The column count is 80 in modes 02h, 03h and 07h and in the 640-pixel-wide graphics modes, 40 in modes 00h and 01h and in the 320-pixel modes, where the BIOS draws 8-pixel characters.

It is the first call of any program that changes the mode (so it can restore it at the end) or writes video memory directly (B000h for mode 07h, B800h for the colour text modes, and the right row length for the offsets).

Prints "mode 03h, 80 columns, page 0" at the usual DOS prompt.
MOV     AH, 0Fh          ; get the video mode
INT     10h              ; AL = mode, AH = columns, BH = active page
MOV     [COLS1], AH
MOV     [PAGE1], BH
PUSH    AX               ; keep the mode
MOV     AH, 09h
MOV     DX, OFFSET TXT1  ; "mode "
INT     21h
POP     AX
CALL    PHEX2            ; the mode in hex
MOV     AH, 09h
MOV     DX, OFFSET TXT2  ; "h, "
INT     21h
MOV     AL, [COLS1]
CALL    PDEC2            ; the columns in decimal
MOV     AH, 09h
MOV     DX, OFFSET TXT3  ; " columns, page "
INT     21h
MOV     DL, [PAGE1]
ADD     DL, "0"          ; the page: one digit
MOV     AH, 02h
INT     21h
MOV     AH, 09h
MOV     DX, OFFSET CRLF1
INT     21h
 
COLS1   DB      0
PAGE1   DB      0
TXT1    DB      "mode $"
TXT2    DB      "h, $"
TXT3    DB      " columns, page $"
CRLF1   DB      13, 10, "$"
PHEX2:                   ; print AL as two hex digits; changes AX, CX, DX
PUSH    AX
MOV     CL, 4
SHR     AL, CL           ; the high nibble first
CALL    PDIG1
POP     AX
AND     AL, 0Fh          ; then the low one, falling into PDIG1
PDIG1:
ADD     AL, "0"
CMP     AL, "9"
JBE     PDIG2
ADD     AL, 7            ; "A".."F"
PDIG2:
MOV     DL, AL
MOV     AH, 02h          ; DOS: print DL
INT     21h
RET
PDEC2:                   ; print AL (0..99) as two decimal digits
AAM                      ; AH = AL / 10, AL = AL mod 10
ADD     AX, 3030h        ; both into "0".."9"
PUSH    AX
MOV     DL, AH
MOV     AH, 02h
INT     21h
POP     AX
MOV     DL, AL
MOV     AH, 02h
INT     21h
RET

The number of rows is not returned. On an EGA or VGA it is at 40h:84h in the BIOS data area (rows minus one), and INT 10h AH=11h with AL = 30h returns it in DL; on older cards it is always 25.

Pitfalls

If the mode was set with bit 7 of AL on (the "do not clear" request of function 00h), many BIOSes return it with bit 7 set here too; AND AL, 7Fh before comparing. BH is the page being shown, not necessarily the page a program writes to. In VESA modes the answer is not meaningful: VESA has its own function for that, with AX = 4F03h.