Creating calling conventions and system call disassembly

[Stovent]

Hi, I'm not sure if this is the right place to ask but I'm struggling with the poor documentation I have, especially regarding SLEIGH and the files around.

Basically I'm making an extension to analyze OS-9 programs but there are some things I don't understand how to do.

First, OS-9 system calls have nearly all their parameters in registers, however error handling is not a single register and I struggle to make Ghidra understand it. When executing a system call:

• If it is successful, the result are in the output registers and the carry flag is cleared.
• If an error occurred, the carry flag is set and D1.W contains the error number.
  How can I indicate that ? The .cspec file of the compiler can't have it.

Second is regarding system call disassembly, what is the correct way of doing it?
Os-9 system calls are with a TRAP #0 (0x4E40) and the system call number immediately after the instruction.
For now I modified the Motorola 68000 SLEIGH to recognize this instruction as a system call, but it's just a generic system call. To make it recognize the arguments and return value (mostly stored in registers and conditional flags), I have two choices:

• manually define every system call and its parameters in the SLEIGH so the disassembler knows right aways how to process the syscall and the parameters.
• Or I could keep the single generic system call and make a Ghidra analyzer class to remap syscall with their arguments in Java.
  What is the most correct way from Ghidra's architecture to do?

Thanks for the help!