Why isn't my interrupt handler firing?

Question

I'm trying to install an interrupt handler in Bochs, but for some reason my interrupt handler isn't firing. First I set a breakpoint: ``` 00036440222i[CPU0 ] [36440222] Stopped on MAGIC BREAKPOINT (0) [0x0000000000703044] 0008:00000000c0003044 (unk. ctxt): int3 ``` Then I try to execute `int 3`. When I do, though, the error says `int_trap_gate(): selector null`, which seems to be telling me a selector in some table is null. However, the GDT is shown below: ``` info gdt Global Descriptor Table (base=0x0000000000007c08, limit=23): GDT[0x00]=??? descriptor hi=0x00000000, lo=0x00000000 GDT[0x01]=Code segment, base=0x00000000, limit=0xffffffff, Execute/Read, Accessed, 32-bit GDT[0x02]=Data segment, base=0x00000000, limit=0xffffffff, Read/Write, Accessed ``` and so is the IDT: ``` info idt Interrupt Descriptor Table (base=0x00000000c0101020, limit=2047): IDT[0x00]=32-Bit Interrupt Gate target=0x0001:0xc0004148, DPL=0 IDT[0x01]=32-Bit Interrupt Gate target=0x0001:0xc00041…

Accepted Answer

Here:

```
IDT[0x03]=32-Bit Interrupt Gate target=0x0001:0xc000416c, DPL=0
```

0x0001 is a selector value. Its 2 least significant bits are `RPL` bits. The rest of the bits are used as an index into a segment descriptor table (GDT or LDT). And these bits are 0. For all intents and purposes that's a NULL selector.

Most likely you want 8 there, not 1.

See the CPU documentation for how exactly selectors are used.

Why isn't my interrupt handler firing?

Solution

Problem