Re: [colorforth] Understanding the x18 instruction jump table

["Helen Austin" <gwenhwyfaer@xxxxxxxxx>]

On 6/14/06, John Drake <jmdrake_98@xxxxxxxxx> wrote:
> I'm aware of the "32 words - one for every 5-bit
> opcode" part.  I guess that's why I found the
> repeated instructions a bit odd.  It's also
> different from what was reported in Chuck's
> paper on the subject.

> : inst ( n ) 1 slot +! 1f and jump ret nul call
> jmp nul nul jz jns n @+ @b @a !r !+ !b !a
> +* 2* 2/ - + and or drop dup over pop a nul
> b! push a!

Well... I'm going to hazard a guess here.

jmp and ret are the same operation; they just get their operand from
different places. jmp gets it inline and ret gets it from the return
stack. Perhaps this has been generalised across all control-flow words
now, to provide "conditional return" instructions as well as a
coroutine instruction?

That would also explain where ; went; and if an LSB of 0 means "find
the destination on the return stack", that would explain why ; becomes
"4 [ ip ] +".

Just some completely random, unfounded speculation :)

Regards
Helen

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