Function 08h looks at the cell under the cursor of page BH and returns its character in AL and its attribute in AH. In a text mode that is a plain read of video memory. A graphics mode has no characters, only pixels, so the BIOS compares the dots at the cursor with its font and returns the character they match, or 0 if none does; AH means nothing there.

It serves for saving what a popup will cover, for reading what another program left on the screen, and for simple screen-reading tools. It reads one cell; for many, move the cursor each time or, much faster, read video memory at B800h directly. Here a Q printed by DOS is read back:

Prints "Q: read back Q, attribute 07h" (07h, light grey on black, is the usual colour at the DOS prompt; a screen painted in other colours gives another value).
MOV     DL, "Q"
MOV     AH, 02h                 ; DOS: print a Q
INT     21h
MOV     AH, 03h                 ; the cursor is now just after it
MOV     BH, 0
INT     10h                     ; DH = row, DL = column
DEC     DL                      ; one column back: onto the Q
MOV     AH, 02h
INT     10h
MOV     AH, 08h                 ; read the character and attribute there
INT     10h                     ; AL = character, AH = attribute
MOV     [CELL1], AX
INC     DL                      ; DX still holds the position: after the Q again
MOV     AH, 02h
INT     10h
MOV     AH, 09h
MOV     DX, OFFSET MSG1
INT     21h
MOV     DL, BYTE PTR [CELL1]    ; the character
MOV     AH, 02h
INT     21h
MOV     AH, 09h
MOV     DX, OFFSET MSG2
INT     21h
MOV     AL, BYTE PTR [CELL1+1]  ; the attribute
CALL    PHEX2
MOV     AH, 09h
MOV     DX, OFFSET MSG3
INT     21h
 
CELL1   DW      0
MSG1    DB      ": read back $"
MSG2    DB      ", attribute $"
MSG3    DB      "h", 13, 10, "$"
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

Pitfalls

The cursor does not move, so a loop over a line must step it with function 02h. In graphics modes the matching works only for characters drawn with the same font, in one colour, on a background of colour 0; text drawn over a picture, in another font or with XOR reads as 0. In text modes the result is exact.