[avr-gcc-list] Fixed, external function "pointers"

[Erik Walthinsen]

I've got a bootloader under development (started in C, converted to ASM 
blind, now am validating ASM piece by piece) that should fit in 512 
bytes, uses I2C for communication, and provides the application with an 
I2C register-style interface, as well as a function to write to PROGMEM 
arrays.  And of course application-code uploads when in bootloader mode.

The trick is that I need to find an efficient way for the application to 
call functions that will be at a fixed *known* address in the final 
bootloader.  Obviously I can use function pointers defined in a header, 
but that takes up SRAM and each call is quite bulky.

The next method that works is to put stub *inline* functions in the 
header, that load the address and 'icall'.  The problem with this is 
that it's rather inflexible: if the CPU in question has 'call', it'd be 
better to use that.  More specifically, if the calling code is within 
the first 1.5KB-2KB of Flash, it should be using 'rjmp' instead...

A third method is to create __naked__ stubs of these functions, each one 
in its own section.  Relocate each section with something like 
"-Wl,--section-start=.bootloader_enter=0x1840" on the compiler 
commandline, then remove them with "--remove-section=.bootloader_enter" 
with objcopy.  This lets the compiler produce the most appropriate call 
code, but has a *severe* disadvantage in that it's an absolute beast to 
compile/link/etc. because of all these "ghost" sections being relocated 
and then removed.

This last method even works for the interrupt handler, if I define 
__vector_17 (SIG_TWI on the mega8) the same way as the others.  I get a 
negative rjmp in the vector table pointing "backwards" into the bootloader.

My question is: is there some more elegant way of achieving this same 
result without having to put all this mess on the compiler and objcopy 
commandlines?  Can I define a "fixed function pointer" in C (const 
doesn't do anything) that will tell it where to go without the section 
stupidities?  Maybe some assembler constructs that I can pass through 
the C?  Or even some kind of magic .o I can link into the app?  If any 
of these are possible, will they also work for interrupt vectors? (I 
really don't want to have to double-jump for the TWI IRQ).

TIA,
   Omega
   aka Erik Walthinsen
   address@hidden