INT 10h AH=13h: write a string — 8086 assembly guide

Write a whole string at a row and column in one call, in one attribute or with an attribute per character, and optionally leave the cursor at its end.

Function 13h, which arrived with the PC/AT and the EGA, is teletype for a whole string with colours. It puts the cursor of page BH at row DH, column DL, writes CX characters from ES:BP, and acts on CR, LF, backspace and bell as function 0Eh does, scrolling at the bottom. The write mode in AL decides two things:

AL	The string	The cursor afterwards
00h	characters, all in attribute BL	back where it was
01h	characters, all in attribute BL	after the string
02h	character, attribute, character, attribute...	back where it was
03h	character, attribute pairs	after the string

The string is addressed by ES:BP, not DS:DX or DS:SI as one might expect. In a .COM program ES already equals DS, so MOV BP, OFFSET TEXT1 is all it takes; an .EXE must set ES first. CX counts characters, so in modes 02h and 03h the string is 2 × CX bytes long, and no terminator is needed or recognised.

Prints "Two lines above from INT 10h AH=13h"; on the screen above it: "Yellow on blue" in yellow on blue, and a line "RGB" in light red, light green and light blue.

MOV     AH, 03h          ; start at the cursor: DH = row, DL = column
MOV     BH, 0
INT     10h
MOV     AX, 1301h        ; write a string: one attribute, move the cursor
MOV     BX, 001Eh        ; page 0, yellow on blue
MOV     CX, 16           ; 14 characters, then CR and LF
MOV     BP, OFFSET TXT1  ; ES:BP = the string (ES = DS in a .COM)
INT     10h
MOV     AH, 03h          ; the cursor is now on the next line
MOV     BH, 0
INT     10h
MOV     AX, 1303h        ; character, attribute pairs; move the cursor
MOV     BX, 0000h        ; page 0 (BL is not used)
MOV     CX, 5            ; 5 characters: CX does not count the attributes
MOV     BP, OFFSET TXT2
INT     10h
MOV     AH, 09h
MOV     DX, OFFSET MSG1
INT     21h
 
TXT1    DB      "Yellow on blue", 13, 10
TXT2    DB      "R", 0Ch, "G", 0Ah, "B", 09h, 13, 07h, 10, 07h
MSG1    DB      "Two lines above from INT 10h AH=13h", 13, 10, "$"

Pitfalls

BP holds the string pointer, so code that keeps a stack frame in BP must save it around the call. The original PC and XT BIOS lack the function and return without doing anything. The cursor position comes from DH and DL, not from the current cursor, so to write "at the cursor" read it first with function 03h, as the example does. In modes 00h and 01h a CR or LF in the string moves the cursor without being drawn, and the attribute applies only to the characters drawn.