Use BIOS INT 0x10 for Video Output

Topic Overview

INT 0x10 video services

BIOS video services handle mode setting, cursor control, and character output in real mode. Teletype output (AH=0x0E) is the workhorse for boot messages: put ASCII in AL, page in BH, call INT 0x10. For example, printing \\r\\n uses bytes 0x0D and 0x0A in sequence to return the carriage and feed the line.

Common functions

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• AH=0x00: Set video mode
• AH=0x02: Set cursor (DH=row, DL=column)
• AH=0x09: Write character with attribute (CX count)
• AH=0x0E: Teletype write (handles control characters)

Color text lives at 0xB8000; each cell is character plus attribute byte.

Attribute bytes

Direct writes to 0xB8000 interleave character and attribute bytes: even index character, odd index color. Foreground is the low nibble, background the high nibble of the attribute byte. Function 0x09 writes without advancing the cursor; use it for bulk screen fills. Function 0x0E interprets control characters, which is why boot messages rarely need manual newline handling beyond emitting CR LF.

Why for this exercise

You will drive INT 0x10 to clear the screen, position the cursor, and print strings. This exercise requires choosing the right AH function for teletype versus attributed writes.

Write a C program that simulates two BIOS INT 0x10 services in 80x25 text mode and the matching 0xB8000 VGA memory layout.

Input: three hex numbers cmd a b.

• cmd = 0x02 (set cursor, DH=a=row, DL=b=column): print "CURSOR=nnnn" (the linear cell offset row80+col in decimal) and "VGA=xxxxx", the 5-digit hex physical address 0xB8000 + offset2 where the character byte lives.
• cmd = 0x03 (read attribute, AL=a=attribute byte, BX=b=cell index 0..1999): print "FG=f BG=b BLINK=k" (foreground = low nibble in hex, background = bits 4-6 in hex, blink = bit 7) and "VGA=xxxxx", the physical address of that cell's attribute byte (0xB8000 + cell*2 + 1).
• Any other cmd: print only UNSUPPORTED. Values out of range (cmd/a over 0xFF, b over 0xFFFF): print only INVALID.