Re: [f-cpu] Smooth Register backup issues...

[Michael Riepe <michael@stud.uni-hannover.de>]

On Thu, Nov 13, 2003 at 10:47:53PM +0100, Beat Steiner wrote:
> 
> Some thoughts about SRB:
> 
> Expensive approach: On initiation of SRB, all registers are backed
> up to a mirror set of registers. Every register is directly connected
> to its mirror partner (makes 64x63 connections!). Mirroring can
> take place in one clock cycle this way (much like a snapshot backup
> of a journaling filesystem). Writing out the backup to the RAM is
> performed as described in the SRB section of the manual.
> 
> Cheap approach: The compiler shuffles up registers in a random way,
> reducing the probability that r1 has always to be backed up and is the
> first register used by the new context (programmers tend to use r1
> first).

r1...r15 hold the arguments of a function (or system call).  That is,
at least some of them will be in use most of the time.  Well-optimized
code should use as many registers as possible, so most other registers
will be busy as well (and unused argument registers can be reclaimed
and used as temporaries).

I prefer a double-buffering approach: The "shadow" register is loaded
from the new ("incoming") CMB, while the "foreground" register is
saved to the old ("outgoing") CMB.  As soon as the shadow is loaded,
register and shadow can be swapped (by flipping an address or enable
bit).  That way, a direct connection between a register and its
shadow counterpart is not required since you never have to transfer
data between them.  In addition to that, it enables you to process
interrupts very quickly -- just swap the registers on entry and exit
of the interrupt service routine.

Did anybody say "Zilog Z80"? ;)

-- 
 Michael "Tired" Riepe <Michael.Riepe@stud.uni-hannover.de>
 "All I wanna do is have a little fun before I die"
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