<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/64847>64847</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [M68k] Exception Vector Table
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          thankfulmachine
      </td>
    </tr>
</table>

<pre>
    ## Description

The exception vector table exists in the first 1024 bytes of the address space, 256 long words in total. The first two are predefined to the the initial supervisor stack pointer and the initial program counter respectively. [Clements, p. 448](https://archive.org/embed/microprocessor-systems-design-68000-hardware-software-and-interfacing-3e)

Some are unassigned or reserved so, for forward compatibility, they should not be used as they can be defined in later 680x0 generations. See below for more information.

## Tying Declarations to EVT Slots

Clang currently requires a single numeric argument to the `interrupt` attribute, presumably for the interrupt vector number, but this may not necessarily be required. It also arbitrarily limits the number to around 30, from what I can tell.

Two usability points:

1. It would be nice if the user did not have to know the exact interrupt *number*, but maybe this isn't possible without a linker script.
1. The user should be able to reuse the same function in multiple slots in the table.

GCC has dealt with this via linker script in the past, as shown in this [example linker script from m68k_bare_metal](https://github.com/tomstorey/m68k_bare_metal/blob/master/standalone/platform.ld). I like this solution pretty well.

What can we do better?

## Default Exception Implementation(s)

Some of these slots must be populated. What is the default implementation for an exception? `rte`? `stop`? It might be good to let the user decide somehow (well known decl name?). FWIW, the bare metal project mentioned above uses:

```
stop #2700
bra .
```

## Example exception vector table (68000)

|Vector Number|Address|Description|
|-|-|-|
|0 |000|Initial supervisor stack pointer|
|- |004|Initial program counter|
|2 |008|Bus error|
|3 |00C|Address error|
|4 |010|Illegal instruction|
|5 |014|Divide by zero|
|6 |018|CHK instruction|
|7 |01C|TRAPV instruction|
|8 |020|Privilege violation|
|9 |024|Trace|
|10|028|Line A (1010) emulator|
|11|02C|Line F (1111) emulator|
|12|030|Unassigned, reserved|
|13|034|Unassigned, reserved|
|14|038|Unassigned, reserved|
|15|03C|Uninitialized interrupt vector|
|16|040|Unassigned, reserved|
|17|044|Unassigned, reserved|
|18|048|Unassigned, reserved|
|19|04C|Unassigned, reserved|
|20|050|Unassigned, reserved|
|21|054|Unassigned, reserved|
|22|058|Unassigned, reserved|
|23|05C|Unassigned, reserved|
|24|060|Spurious interrupt|
|25|064|Level 1 interrupt autovector|
|26|068|Level 2 interrupt autovector|
|27|06C|Level 3 interrupt autovector|
|28|070|Level 4 interrupt autovector|
|29|074|Level 5 interrupt autovector|
|30|078|Level 6 interrupt autovector|
|31|07C|Level 7 interrupt autovector|
|32-47|080-0BC|TRAP 0-15 instruction vector|
|48-63|0C0-0FC|*Unassigned, reserved* ⭐️|
|64-255|100-3FC|User interrupt vectors|

⭐️ These are filled with [FPU and MMU conditions](https://github.com/tomstorey/m68k_bare_metal/blob/master/standalone/platform.ld#L107-L117) for later 680x0.

</pre>
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