Every AmigaOS library exposes its functions via a **JMP table** at negative offsets from the library base. This document covers the structure of the table, how LVOs are assigned, and how to create or patch one programmatically.
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## JMP Table Structure
```
Address Content Description
lib_base - N×6: 4EF9 XXXXXXXX JMP <absoluteaddress> ← function N
...
lib_base - 24: 4EF9 XXXXXXXX JMP Reserved
lib_base - 18: 4EF9 XXXXXXXX JMP Expunge
lib_base - 12: 4EF9 XXXXXXXX JMP Close
lib_base - 6: 4EF9 XXXXXXXX JMP Open
lib_base + 0: struct Library ← pointer returned by OpenLibrary
```
Each slot is exactly **6 bytes**: opcode `$4EF9` (JMP abs.l) + 4-byte target address.
---
## LVO Formula
```
LVO = −6 × slot_index
where slot_index counts from 1 (Open) upward:
Open = slot 1 → LVO = −6
Close = slot 2 → LVO = −12
Expunge = slot 3 → LVO = −18
Reserved = slot 4 → LVO = −24
First user fn = slot 5 → LVO = −30
Second user fn = slot 6 → LVO = −36
...
```
The `.fd` file `##bias` value is the positive LVO: `bias 30` → LVO `−30`.
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## MakeFunctions — Building a JMP Table
`exec.library MakeFunctions()` fills in the JMP table from a function pointer array:
After `MakeFunctions` or `SetFunction`, exec updates `lib_Sum` via `SumLibrary`:
```c
SumLibrary((struct Library *)myLib);
```
If `LIBF_SUMUSED` is set, exec verifies the checksum at `CloseLibrary` time. Patching the JMP table without calling `SumLibrary` will trigger a checksum failure (alert box or guru).
---
## Viewing Vectors in IDA Pro
1. Navigate to `lib_base − 6` (first standard vector)
2. Each 6-byte group: opcode `4EF9` + 4-byte address
3. Press `C` to disassemble if not auto-detected
4. The 4-byte value is the actual function address — press `G` (Go to) to navigate
5. Name each function with the `.fd` file as reference
