# Instruction format (ivss)

The **instruction format** describes the four bytes fetched for each machine instruction. ivss packs an operation code, modifier, three register fields, and a signed displacement:

$$
\underbrace{OC}_{4}\;
\underbrace{MOD}_{4}\;
\underbrace{A}_{4}\;
\underbrace{B}_{4}\;
\underbrace{C}_{4}\;
\underbrace{D}_{12}
\qquad (32\text{ bits})
$$

The numbers indicate field widths in bits. `Asm_PushInstr()` emits them in byte order:

```c
Elf_PushByte(elf, ((oc & 0xf) << 4) | ((mod & 0xf) << 0));
Elf_PushByte(elf, ((ra & 0xf) << 4) | ((rb & 0xf) << 0));
Elf_PushByte(elf, ((rc & 0xf) << 4) | (((disp >> 8) & 0xf) << 0));
Elf_PushByte(elf, disp & 0xff);
```

For example, `add %r2, %r1` encodes as bytes `50 11 20 00`. The operation adds registers B and C into A; here A and B both select `%r1`.

The emulator sign-extends the displacement, giving $-2048\leq D\leq2047$. Its meaning depends on the operation. Some assembly statements expand into several encoded instructions, so four-byte machine width does not imply four bytes per source statement.

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