general ISR for BareMetal

[platatomi]

I been working on expanding BareMetal OS, starting with installing ISR's for the built-in functions that come with BareMetal (like screen output, string length, etc). I decide to use interrupt 0x90 (sounds like a good number) and the service routines are working fine, but I cannot find an efficient way to call the correct function based on the function number:

this is the structure the int 0x90 handler now:

Code: Select all

;RAX = function number

general_int:

cmp rax, 0
je some_function

cmp rax, 1
je another_function

;and so forth

iretq

How can do this more efficiently, without tons of comparisons?

[gerryg400]

The normal way is to use a jump table. An array of addresses to jump to indexed by the function number.

[Nessphoro]

call [jumpTable+rax*8]

[platatomi]

Oh yeah, just like an array of function pointers in C. Thanks.

[Kazinsal]

call [jumpTable+rax*8]

This, but make sure you test to see if RAX is in the valid range for ISR functions.

So, if you have five functions, with RAX being 0-4, do a cmp rax, 4 ; ja .out_of_range (or something like that) so you don't execute garbage if someone calls your ISR with RAX being, say, 5.

[IanSeyler]

Are there any advantages to this over the existing call table setup? It's nice to see people using the OS by the way!

-Ian

[platatomi]

Well, not really. It's probably more flexible if applications can use interrupts instead of depending on an exact address (in case things need to be moved in the future) to call system functions, however it works great either way. I just thought it would be fun to write ISRs, as im not quite at the full-fledged multitasking OS level yet.

I have to say, BareMetal is great! It's well comented and a great learning tool.

[bubach]

Your first ISR could be to get the calltable address then, to remove any assumtion. And I have to agree, the OS looks great.