Use BIOS Interrupts to Read Keyboard Input and Display Text

Topic Overview

BIOS interrupt services

Before an OS provides drivers, software interrupts reach firmware routines for video, keyboard, disk, and more. The calling convention puts the function number in AH and passes extra arguments in other registers. For example, INT 0x10 with AH=0x0E and AL='A' prints one teletype character and advances the cursor.

Core interrupts for boot code

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• INT 0x10: Video (modes, cursor, teletype output)
• INT 0x16: Keyboard (read or poll keystroke)
• INT 0x13: Disk (CHS and extended LBA reads)
• INT 0x15: Memory map (E820), A20 control, ACPI tables

BIOS data areas at 0x0040:0000 hold equipment flags and keyboard buffer pointers. Interrupts work only in real mode unless you build v8086 thunks.

Real-mode-only constraint

These interrupts trap through the IVT; once CR0.PE is set without virtual 8086, BIOS services are unsafe. Teletype output preserves cursor state across calls, which is why boot messages rarely manually track column numbers. Testing with QEMU -fda still routes INT 0x10 to the emulated VGA BIOS, so behavior matches physical hardware closely enough for learning.

Why for this exercise

You will use INT 0x10 and INT 0x16 to print text and echo keyboard input. This exercise requires following the register-based BIOS calling convention without any C library or OS syscall layer.

Your Task

In real mode, INT n is an indirect far call through the Interrupt Vector Table: 256 four-byte entries at physical address 0. Entry n is at 0000:n*4 and holds the handler's offset, then its segment, both little-endian. The BIOS installs handlers for its services, such as INT 10h (video), and a program can hook any vector by overwriting its entry. Build a small emulator of this: an IVT you can hook and inspect, and the three INT 10h functions a boot loader uses to print, driving an 80×25 text screen.

Input

Hex values are exact-width: 2 digits for vectors and 8-bit registers, 4 for 16-bit registers, segments and offsets.

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Registers keep their values between commands (all start at 0). The assignments on an int line are applied only if all of them are valid.

Semantics

• A hooked vector always wins. int prints the jump and does nothing else.
• Unhooked vectors reach the BIOS at F000:FF53:
  • INT 10h AH=0Eh (teletype) writes AL at the cursor and advances it. 0D returns to column 0, 0A moves down one row, 08 moves left (not past column 0), and 07 (bell) does nothing visible. Column 80 wraps to the next row. Moving below row 24 scrolls the screen up one row.
  • INT 10h AH=02h sets the cursor to row DH, column DL. It is ignored, with a message, if that position is off screen.
  • INT 10h AH=03h returns the cursor in DH/DL.
  • INT 12h returns the conventional memory size in AX (640 KiB = 0280).
  • Any other INT 10h function is not emulated. Any other vector has no effect.

Output

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• Teletype and set-cursor print nothing.
• screen prints the rows from 0 to the last non-blank row, with trailing spaces removed.
• Errors:
  • error: usage: hook NN SSSS:OOOO;
  • error: usage: vector NN;
  • error: usage: int NN [REG=VAL...];
  • error: bad register assignment REG=VAL;
  • error: unknown command CMD.

Example

Input:

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Output:

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Requirements

1. Compute IVT addresses and linear addresses (segment × 16 + offset) exactly. Real-mode addressing is the point.
2. Implement teletype control characters, wrapping and scrolling as described. Boot loaders print through exactly this path.

Hidden tests cover hooking over the video vector, 80-column wrap, scrolling past row 24, backspace at column 0, AX-style assignments, and malformed commands.