[SeaBIOS] How to add user-defined interrupt to SeaBIOS?

[Andreas H.]

Hi,

as part of a small exercise project for university I have to add a
custom software interrupt to the bios code which is then used by some
later booting stages. I already had a close look to the sourcefiles
and although I don't understand many of the stuff (I can read and
understand the individual parts but don't get the real big picture) I
have some anchorpoints which I think are necessary to complete my
work. I'd really appreciate if someone had a quick look on those
points and just confirmed or denied (maybe with other hints) them :)

1.) the IVT is set up in "post.c", to add my interrupt, I have to add
a line "SET_IVT(0x60, FUNC16(entry_60));" to init_ivt()
2.) romlayout.S contains the declarations for the IRQ handler, I have
to add a "DECL_IRQ_ENTRY_ARG 60" to the section "irqentryarg:"
3.) in my own .c-file I have to add a function "void VISIBLE16
handle_60(struct bregs *regs)" which receives the IRQ
4.) in this custom function I receive the set parameters from the
caller in the register-structure and also set my return values in this
5.) usable registers for arguments/return values are AX, BX, CX, DX,
DS, etc. I'd need DS:DX (analog to DOS) for passing a memory address
and CS:CX for a buffer size.
6.) the IRQ handler has to end with "set_success(regs);" or
"set_invalid(regs);" to return success/failure in CF (or should I use
AX for this and just call set_success?)

One last question: I chose IRQ 60 because it's stated as "user
defined" in "post.c". Should I use another one to not run into issues
with later stages of OS booting or will it just get overwritten at
some point?

Thanks in advance,
 Kosi