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Thu Jul 16 10:47:10 CEST 2009

## PIC18 decoder

The way to approach this is to start with the smallest opcode field.
For PIC18 this is the top 4 bits of the 16 bit instruction word.
(Which becomes the top 3 in my representation, as bcf/bsf are modeled
as a single parameterized instruction to ease the optimization of
logic complement).
;; p: 0/1
(bpf (p f b a) "100p bbba ffff ffff") ;; bsf/bcf
(btfsp (p f b a) "101p bbba ffff ffff") ;; btfss/btfsc
(btg (f b a) "0111 bbba ffff ffff")
The question is then: how to specify this as an equation
(bidirectional function) and how to attach semantics to the fields to
build the simulator.
Currently the letters used as parameter names have a semantics: this
should be defined better as connections to machine state (i.e. R means
PC-relative word addressed). These should all be equations.
This really hints at the proper solution though: all this is so
intimately related that it should be solved as a whole. It is really
not much more than a single morphism between the two spaces:
( machine-statevector, binary-instructions, computation-logic )
( msv-rep, highlevel-instruction-syntax, simulator-primitives)
The problem is representing all this in a form that is accessible for
metacomputation. Maybe it's best to start with a ball-of-mud and
then disentangle it piece by piece?

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