[Coco] 6809 CC Reg Overflow bit

[William Astle]

On 2018-11-08 9:54 p.m., James Jones wrote:
> The 6809 (or 6309) is the best 8/16 bit processor there ever was, but like
> other processors of the time, the register set is small and non-orthogonal.
> It might be tough to make a 6809 compiler play "as if", though if you could
> do it it would very much worthwhile--though to take full advantage of it
> you'd also want to bring in function prototypes--passing parameters is
> essentially assignment, and would thus provide more opportunity.

gcc6809 seems to do a pretty good job avoiding all the sign extension 
business. It does so by the simple expedient of doing arithmetic on 8 
bit values using 8 bit operations where possible. It's also pretty good 
at optimizing useless stuff out of the final binary. At least if you 
enable optimization, anyway.

I only mention that because it does show that a compiler can do better, 
even with a 6809 target. Also, even if K&R demands that 8 bit operations 
always be treated as 16 bit operations, even when the destination of the 
result is known to be 8 bits, I would feel justified in treating that as 
a bug in the specification and ignoring that provision (you're not going 
to see a difference in the result if you do). That said, dealing with 
different sizes of operation depending on context requires more semantic 
analysis and, thus, more code complexity and more memory usage. As with 
everything, it's a trade-off.