On Fri, 18 Feb 2000, Tony Nixon wrote:

> As far as I know the F series do not have auto context save and stack
> manipulation, although 'I think' the 18C series do???

You think correctly. In fact, there are two stacks. There's the one we're
all familiar with that saves the return address following a call and
there's the 'fast stack' that can save the status, W, and bsr (bank
select) registers in a single cycle. The 'normal stack' is 32 words deep
and you have access to it via some special function registers.
Additionally, there are push and pop instructions for placing moving data
onto and off of the stack. The 'fast stack' is only one level deep and is
useful for saving the cpu context through interrupts.

You could also argue that there are three more stacks available with the
indirect addressing modules. These would be useful for implementing a data
stack for passing parameter to subroutines. (The call stack could be used
for this, but it's much more difficult to read and write data to it).


This little monitor thingy you guys have been discussing could be
implemented quite easily in the 18cxxx. I'm not sure if there's a config
parameter for trapping single cycle executions, but you could modify the
program memory to achieve the same behavior (e.g. by writing:
   bra  monitor
immediately after the next instruction (s) that supposed to be executed).


Scott