I need to implement an assembler and interpreter for a simple
instruction set. For example, this code
\begin{code}
compute112 = do
{ zero r1
; zero r2
; addi r2 3 -- loop three times
;topLoop <- label
; addi r1 4
; addi r2 (-1) -- decrement loop counter
; brPositive r2 topLoop
; add r1 100
}
\end{code}
would result in register r1 having a value 112. Without forward
branches, the implementation is reasonably straightforward. The
assembler can use a Monad with state and writer capabilities. The
interpreter can use a continuation monad to implement branches and a
state monad for the machine state.
However, forward branches make things harder. I'd like to be able to
write something along the lines of:
\begin{code}
compute12 = mdo
{ zero r1
; zero r2
; addi r2 3 -- loop three times
;topLoop <- label
; addi r1 4
; addi r2 (-1) -- decrement loop counter
; brPositive r2 topLoop
; mov r3 r1
; addi r3 (-10)
-- !!! Forward branch:
; brPositive r3 out -- if r1 > 10 don't add 100
; add r1 100
;out <- label
}
\end{code}
but it's not clear to me how to implement it. I'd be willing to
accept a somewhat less clean version (for instance, with more
cumbersome forward references.)
Any help would be greatly appreciated.
thanks,
mike
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