How does INT 10, 13h work with attributes?

Question

Hey Im trying to understand the INT 10h, 13h (19) that writes a Character String with the BIOS interruption 10 with 13h in ah. I have found the info below about the different flags to put in the the different registers. One thing I still don't get is what is supposed to go in BL, if i simply want to write a string with this function what should be in BL as attribute? Right now it writes out strange blinking symbols that don't make sense. Thank you, beforehand

Writes a string of characters with specified attributes to any display
page.

   On entry:      AH         13h
                  AL         Subservice (0-3)
                  BH         Display page number
                  BL         Attribute (Subservices 0 and 1)
                  CX         Length of string
                  DH         Row position where string is to be written
                  DL         Column position where string is to be written
                  ES:BP      Pointer to string to write

   Returns:       None

   Notes:         This service is available only for XTs dated 1/19/86
                  and later, ATs, EGAs, and PC Convertibles.

                  The service has four subservices, as follows:

                  AL=00h: Assign all characters the attribute in BL;
                  do not update cursor
                  AL=01h: Assign all characters the attribute in BL;
                  update cursor
                  AL=02h: Use attributes in string; do not update
                  cursor
                  AL=03h: Use attributes in string; update cursor

                  In Subservices 0 and 1, all characters in the string
                  are written to the screen with the same attribute--
                  the attribute specified in BL.

                  In Subservices 2 and 3, the attribute byte for each
                  character is found in the string itself. The string
                  itself consists of a character followed by its
                  attribute, another character followed by its
                  attribute, and so on. The string is copied directly
                  to the video buffer as is.

                  In Subservices 0 and 2, the cursor position is not
                  updated after the string is written.

                  In Subservices 1 and 3, the cursor is moved to the
                  first position following the last character in the
                  string.

                  Like Service 0Eh, Service 13h responds appropriately
                  to ASCII 07h (bell), 08h (backspace), 10h (line
                  feed), and 0Dh (carriage return). All other
                  characters are printed.

Answer 1

With int 10h , the BL register is used for the colour attributes.

Unless you're dealing with CGA, where the BL value is the palette number, the BL value is a number representing the foreground color (4 bits - low part) and the background color (4 bits - high part).

For instance, if you want a blue (0x01) background with red (0x04) text colour, you'll need to put 0x14 into the BL register - in binary that is 00010100 .

      0001             0100
|_ Background _| |_ Foreground _|

Colours are usually:

 - Black            0x00
 - Blue             0x01
 - Green            0x02
 - Cyan             0x03
 - Red              0x04
 - Magenta          0x05
 - Brown            0x06
 - LightGray        0x07
 - DarkGray         0x08
 - LightBlue        0x09
 - LightGreen       0x0A
 - LightCyan        0x0B
 - LightRed         0x0C
 - LightMagenta     0x0D
 - LightBrown       0x0E
 - White            0x0F