# Interrupt return (ivss)

**Interrupt return** restores the frame created when ivss delivers an interrupt. The assembler expands `iret` into four machine loads. The first restores the saved helper register thirteen:

```c
Elf_PushByte(elf, PACK(OC_LOAD, MOD_LOAD_3));
Elf_PushByte(elf, PACK(IDX, SP));
Elf_PushByte(elf, 0x00);
Elf_PushByte(elf, 0x04);
```

That encoding loads from the current stack pointer and increments it by four. The remaining generated instructions restore register twelve, `%status`, and `%pc` in that order.

At handler entry, the automatic frame has this layout:

```text
sp +  0: saved r13
sp +  4: saved r12
sp +  8: saved status
sp + 12: saved next pc
```

The complete expansion consumes sixteen bytes. Any registers pushed explicitly by the handler must be popped first, so the stack pointer again identifies this frame.

`ret` restores only a return address and therefore cannot replace `iret`. The actual ivss expansion is several instructions rather than one atomic machine operation; its final load transfers execution back to the saved program counter.

Source: [asembler.c](https://github.com/yanevskiv/ivss/blob/main/src/asembler.c).