Re: [Simulavr-devel] anacomp getting reserved access messages

[Joerg Wunsch]

As Joel Sherrill wrote:

> FYI the avr .h file has this:
> 
> io4433.h:/* 0x3D..0x3E SP */
> 
> Is this by convention?

Yes, that's GCC's frame pointer management.  We know this is slightly
incorrect, but GCC classifies AVRs into a few rather coarse CPU models
for its code generation, and the actual size of the accessible SRAM
(and thus the stack pointer width) is not part of that classification,
so the SPH register is always manipulated.  So far, none of the
(non-Xmega) AVRs has broken the model of using fixed addresses for
SPL, SPH, and SREG.  Those devices that only feature SRAM addresses
below 0x100 (and thus don't need SPH) have IO location 0x3E as a
reserved IO register where writing to it does not harm, and reading
from it yields anything which actually nobody cares about (except of
writing it back later on).

So in light of this technique, simulavrxx should probably by default
ignore access attempts to SPH even on CPUs that don't have it.  (I
know VMlab always complains about it, it can be a little annoying.)

-- 
cheers, J"org               .-.-.   --... ...--   -.. .  DL8DTL

http://www.sax.de/~joerg/                        NIC: JW11-RIPE
Never trust an operating system you don't have sources for. ;-)