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Re: cc65 competition?

BruceMcF wrote:

On Apr 19, 2:32 am, "Michael J. Mahon" <mjma...@aol.com> wrote:


For a 6502, zero page *is* the "registers".  Nothing of interest to
a compiler--like pointers or integers--will fit in the processor's
registers.



Yes, but at the same time, much of low level interfacing with the
Kernal
from C involves treating zero page locations as either a global
integer
variable or global pointer variable.

I took it that he was referring to that kind of use of the zero
page ...
the zero page *used as* memory that is quicker to get to and quicker
to use when dereferencing a pointer.


And page 0 is the natural place for "register" variables, both
the declared ones and the ones needed by the compiler/runtime.


Using a 256-byte stack for both return addresses and parameters is
really pushing it.



Yeah, I can see a pointer to a software stack structure passed on
the hardware stack before making the 6502 call.


You don't even need to pass it, since it is a global page 0 variable.


With that, you only use the TSX register one time, at the start of
a called function, to set up the local variable pointer in the zp,
and possibly at the end if that slot in the hardware stack is used,
for ANSI C returns, as the pointer to the return value, or for K&R
returns, for the return value itself. So X would be free during
the routine, but Y would be in heavy use.


Every function call/entry/exit maintains the global procedure stack,
so it never needs to be passed, and is always implicitly in "sync" with
the hardware stack for return addresses.


The general point stands ... without the built in stack frame address
modes of the 65816, there's going to be one of the two index
registers
that is in use to reference local variables.


True--or indeed to reference *anthing* relative to a pointer.


And an option for inlining small functions can also be a big winner,
and a very effective time-space tradeoff.



Yes, I overlooked this entirely, but but there are a lot of
optimizations available for a zp routine that are not available
outside the zp.


If you take a "threaded code" approach for operations on larger
data types, then putting a bit of that code on page 0 can do
wonders for speed--at the cost of precious real estate, of course.

-michael

NadaPong: Network game demo for Apple II computers!
Home page:  http://members.aol.com/MJMahon/

"The wastebasket is our most important design
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