Hector Martin wrote:

>> The one thing that wouldn't work with my scheme is if there is a jump to
>> one of the first 8 bytes in the application (except 0). But I think this is
>> quite rare.
> 
> What about a relative branch to the user code instead of an absolute goto?
> 
> 	ORG	0
> 	bra	START

You're right, I didn't consider a relative branch. I think this is quite
rare with compilers, but I don't know this for a fact.


> 	<copied 8 bytes from user code>
> 	goto 8
> 	goto 10 ; this is in case there's a skip instruction at the end of the 
> user code
> 
> 	goto <aaa>
> 	goto <bbb>
> 	goto <ccc>
> 	goto <ddd>

This looks like it would work and solve the issue, though.


>> Hi-Tech puts some basic context saving code at the interrupt vectors,
>> followed by a goto. If no interrupt functions are defined, it doesn't
>> generate any interrupt vector code. The goto that is at the reset vector is
>> preceded by a nop in some cases (seems to be required by some erratas).
> 
> Do you have an example handy? (hex or asm)

                         	psect	intcode
000008  CFD8 FF5F          	movff	status,saved_regsh+30
00000C  CFFA FF5E          	movff	pclath,saved_regsh+29
000010  CFFB FF5D          	movff	pclatu,saved_regsh+28
000014  EF9B F057         	goto	save
                         
                         	psect	intcodelo
000018  CFD8 FF40          	movff	status,saved_regsl+3
00001C  CFE0 F010          	movff	bsr,saved_bsrl
000020  EFE9 F058         	goto	savelo

save and savelo are the locations where the rest of the context is saved
and the user interrupt routine started.

The reset vector is simple:

powerup:
	nop	; Suggested Microchip errata workaround
	goto	start


>> This looks like it might complicate things. Do you really have to redirect
>> the interrupts? Isn't it enough to check at boot?
 
> I'm considering this feature experimental, and it's optional (the user 
> code has to include a magic word somewhere indicating it wants this 
> patching done, otherwise it just proceeds normally). The idea is to let 
> people write really simple programs (with no interrupts or only basic 
> interrupts, and no UART use) and automatically patch them to support 
> bootloader resetting via UART (which amounts to hijacking the interrupt 
> vector, just like the reset vector). 

Makes sense in your case.

Gerhard

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