Use BIOS INT 0x16 for Keyboard Input

Topic Overview

INT 0x16 keyboard input

Keyboard BIOS routines buffer scan codes from the 8042 controller. AH=0x00 blocks until a key is available; AH=0x01 polls with ZF indicating emptiness. For example, after INT 0x16 with AH=0x00, AL holds ASCII (or zero for function keys) and AH holds the scan code.

Typical usage

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• Extended keys: Prefix 0xE0 in scan stream (AH may be 0x00 with AL zero)
• Shift state: BIOS flags at 0x0040:0x17 track Ctrl/Alt/Shift
• Flush buffer: AH=0x00 in a loop or use AH=0x03 to reset
• Combine with INT 0x10 teletype for echo programs

Buffer and modifier behavior

The BIOS maintains a ring buffer at 0x0040:0x001A/0x001C. Polling with AH=0x01 avoids blocking when you only want to redraw a status line. Function keys and arrow keys often return AL=0 with a non-zero scan code in AH, so your echo logic must treat zero ASCII as a special case. Caps Lock and Num Lock update the byte at 0x0040:0x17; reading it helps implement case-correct echo.

Why for this exercise

You will read keystrokes and echo them to the screen with INT 0x10. This exercise requires blocking reads, optional polling, and handling non-printing keys gracefully.

Write a C program that implements the US keyboard translation table the BIOS INT 16h handler uses: make scan code + shift state -> character.

Input: whitespace-separated tokens until EOF.

• A bare token is a hex scan code (e.g. "1E"): look it up in the US layout table and print "SC=ss:X" where X is the unshifted character, or the key's name (ESC, ENTER, SPACE, TAB).
• "S:ss" is the same scan code with Shift pressed: letters become uppercase and punctuation becomes its shifted symbol (1->!, 2->@, ...).
• The token "E0" is the extended-key prefix: print "E0:SPECIAL" and skip the next token (the actual extended scan code).
• Scan codes without an entry (0x0E backspace, F-keys, arrow codes, ...) print "SC=ss:NONE".