Implement a System Call Interface

User/kernel Gateway

Your teaching kernel exposes syscalls via int 0x80 or sysenter with a stable ABI: number in register, args in others, return in eax. User test programs call write and exit without libc.

Dispatch Table

Kernel side:

• syscall_table[nr] function pointers
• Argument copy from user stacks with validation
• Return via saved register frame

For example, SYS_WRITE might be 1, taking fd in ebx, buffer in ecx, count in edx for 32-bit teaching ABI.

Why This Matters in Production

The int 0x80 syscall table you're dispatching through here is the literal mechanism Linux used for two decades before sysenter and the AMD64 syscall/sysret instructions replaced it for speed, and the EAX-as-syscall-number convention is still how strace decodes every process's kernel requests. Returning a positive value for an error instead of a negative errno is a classic bug that makes user-space code think a failed syscall actually succeeded.

Debugging Checklist

Before you call the implementation done, walk failure modes on purpose. Test empty structures, single-element edge cases, maximum concurrency, and errno paths that must not crash the program. OS code usually fails in production when happy-path tests pass but invariants break under contention or memory pressure.

Implementation Notes

Keep structures small and name fields after kernel counterparts when possible. That lets you read man pages and kernel source side by side while you work. Print observable events during development; remove noisy logs once tests pass reliably.

For This Exercise

You will implement syscall entry stub, table, and two handlers (write to VGA/serial, exit to zombie path). This exercise requires refusing pointers above user memory limit with EFAULT.