[ECOS] RE: Objloader question

["Anthony Tonizzo" <atonizzo at lycos dot com>]

Mark:

> In the myapp.o file, the diag_printf function is defined as a
> R_PPC_REL24  relocatable entry, I am running a PowerPC system which
> means that it is not possible to perform a branch over the 16MB
> boundary.

Not strictly correct, although it is might still not to solve your 
problem...
The branch instructions in the PowerPC ISA reserve 24 bits for the address
(bits 6 through 29, hence the 24 in R_PPC_REL24) and the 2 least significant 
bits are taken for different uses (LK and AA bits). And so the real 
addressability of the R_PPC_REL24 is actually 26 bits, or, in bytes, 
+/-32MB. I apologize, but the comment I put in the PPC relocator file 
of the OBJLOADER package is wrong, but I found out too late. 
Will correct soon.

As far as your problem is concerned, I think the "branch island"
would not be too difficult to implement. In assembler, you can define 
the "jump table":

start_of_table:
      diag_printf_vector        diag_printf
      sprintf_vector            sprintf
      ...
      ...
      ...

and then something like this:

FAR_JUMP:
      lwi     r4,start_of_table
      lwi     r3,vector(r4)
      mtlr    r3
      blr

where r3 can be passed as a parameter (remember to restore registers, 
blah, blah)

I might have forgotten something, but in principle it should work,
and the objloader would only have to resolve the FAR_JUMP call.

A word of caution about the OBJLOADER package...
To my knowledge only one intrepid soul (Andrew Lunn) has done any tinkering 
with it, and thus it might be too crude to use in sensitive application until 
it gets more mileage. Knowing first hand how paranoid you transportation 
engineers are about safe software, you might want to be very careful before 
you rely too heavily on it.

Regards
Tony

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