INT 10h AH=11h: fonts: the character generator — 8086 assembly guide

Load fonts into an EGA or VGA: the 8×8 font gives 43 or 50 text lines, a font of your own redraws characters. AL = 30h tells where the ROM fonts are.

In text modes the EGA and VGA draw characters from fonts held in the card's own memory, not from a fixed ROM as the CGA and MDA did. That makes the font replaceable: a program can load the ROM's other fonts or one of its own, redraw single glyphs (box drawing, national letters, a mouse arrow made from characters), and by loading a lower font fit more lines on the screen. Function 11h does all of this; AL selects the subfunction.

AL	What it does
00h	load CX glyphs of a user font from ES:BP, from character DX on, BH bytes each, into font block BL
01h	load the ROM 8×14 font
02h	load the ROM 8×8 font
03h	choose which font blocks are in use (BL)
04h	load the ROM 8×16 font (VGA)
10h, 11h, 12h, 14h	as 00h, 01h, 02h, 04h, and set the character height and the number of rows to match
20h–24h	fonts for graphics modes (through vectors 1Fh and 43h)
30h	get information: BH chooses the font, ES:BP returns its address

The 0xh forms only replace glyphs, for changing a few characters. The 1xh forms also reprogram the character height and recompute the rows, as a mode set would; they are meant to be called right after function 00h. 12h in mode 03h gives 400 / 8 = 50 lines on a VGA, 350 / 8 = 43 on an EGA:

Prints "rows on the screen: 50"; on the screen the text switches to the small 8×8 font, 50 lines, for 2 seconds, then mode 03h clears the screen back to 25 lines.

MOV     AX, 1112h  ; load the 8x8 ROM font, recompute the rows
MOV     BL, 0      ; into font block 0, the one in use
INT     10h
MOV     AX, 1130h  ; font information: DL = rows - 1
MOV     BH, 0
INT     10h
INC     DL
MOV     [ROWS1], DL
MOV     CX, 001Eh  ; wait 2 s: CX:DX = 1E8480h microseconds
MOV     DX, 8480h
MOV     AH, 86h    ; BIOS: wait
INT     15h
MOV     AX, 0003h  ; mode 03h: the 8x16 font and 25 rows again
INT     10h
MOV     AH, 09h
MOV     DX, OFFSET TXT1
INT     21h
MOV     AL, [ROWS1]
CALL    PDEC2
MOV     AH, 09h
MOV     DX, OFFSET CRLF1
INT     21h
 
ROWS1   DB      0
TXT1    DB      "rows on the screen: $"
CRLF1   DB      13, 10, "$"
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

Font information

With AL = 30h the BIOS returns in ES:BP the address of the font chosen by BH, in CX the height of the font in use (16 in mode 03h on a VGA) and in DL the number of rows minus one. The fonts are laid out simply: one byte per row of dots, bit 7 on the left, the rows of a character one after the other, so in the 8×16 font character n starts at n × 16. Here the BIOS's "A" is printed with DOS, a # for each dot:

Prints "16 scan lines, 25 rows", then the letter A of the 8×16 ROM font as 16 lines of eight characters, # for a dot and . for none; the first two and the last four lines are all dots.

MOV     AX, 1130h     ; get font information
MOV     BH, 06h       ; which font: the ROM 8x16
INT     10h           ; ES:BP = font, CX = height in use, DL = rows - 1
MOV     [FH1], CL
INC     DL
MOV     [ROWS1], DL
MOV     AL, [FH1]
CALL    PDEC2
MOV     AH, 09h
MOV     DX, OFFSET TXT1
INT     21h
MOV     AL, [ROWS1]
CALL    PDEC2
MOV     AH, 09h
MOV     DX, OFFSET TXT2
INT     21h
ADD     BP, 41h * 16  ; "A" is character 41h: 16 bytes each
MOV     SI, 16        ; 16 rows in the glyph
GROW1:
MOV     BL, ES:[BP]   ; one row: a bit per dot, bit 7 on the left
INC     BP
MOV     CX, 8
GBIT1:
MOV     DL, "."
SHL     BL, 1         ; the next dot into CF
JNC     GDOT1
MOV     DL, "#"
GDOT1:
MOV     AH, 02h
INT     21h
LOOP    GBIT1
MOV     AH, 09h
MOV     DX, OFFSET CRLF1
INT     21h
DEC     SI
JNZ     GROW1
 
FH1     DB      0
ROWS1   DB      0
TXT1    DB      " scan lines, $"
TXT2    DB      " rows", 13, 10, "$"
CRLF1   DB      13, 10, "$"
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

BH	Font returned by AL = 30h
00h	the one vector 1Fh points to (characters 80h–FFh in CGA graphics modes)
01h	the one vector 43h points to (graphics modes)
02h	ROM 8×14
03h	ROM 8×8, characters 00h–7Fh
04h	ROM 8×8, characters 80h–FFh
05h	ROM 9×14 replacement glyphs
06h	ROM 8×16 (VGA)
07h	ROM 9×16 replacement glyphs (VGA)

Pitfalls

A program that changes the font or the number of lines must put it back, and setting mode 03h again is the simple way: it reloads the 8×16 font and 25 lines. Many programs assume 25 lines and misbehave otherwise. CX from AL = 30h is the height of the font in use, not of the font BH asked for. A loaded font only lives in the card's memory; the next mode set replaces it.