Skip to main content

Loading the code workspace…

LC

Write Custom Interrupt Handlers in Real Mode

hard8/8

Use x86 String Instructions (MOVSB, STOSB, LODSB)Next

Language

Compiler

2.0s Limit 256MB

Submit and pass all tests to enable saving to GitHub.

Implementation

Implement custom interrupt handlers in real mode using C simulation.

Requirements:

Define IVT structure (256 entries of segment:offset)
Implement install_handler(vector, segment, offset)
Implement remove_handler(vector) that restores default
Implement a simple timer tick counter handler
Implement a custom software interrupt handler (e.g., INT 0x80)
Show IRET vs RET difference
Show EOI being sent to PIC
Demonstrate interrupt chaining
Show register save/restore
Implement a simple interrupt dispatcher

Test:

Show handler installed
Show timer ticks counting
Show software interrupt firing
Show EOI sent
Show registers preserved

Why This Matters

This exact IVT-hooking technique (save old vector, write new segment:offset, chain back when done) is how real DOS TSR utilities like early antivirus tools and screen savers embedded themselves into a running system, and it is also how DOS-era games hooked the IRQ0 timer for custom-rate game loops. Using RET instead of IRET in a handler is a classic real bug — it fails to restore FLAGS, silently corrupting the interrupt-enable state of the entire system.

Hints

0 / 3 revealed

Validation Suite

main.c

Workspace 1.0

← Use x86 String Instructions (MOVSB, STOSB, LODSB)