Re: [9fans] Google search of the day

["Eris Discordia" <eris.discordia@gmail.com>]

On Fri, 15 Feb 2008 19:05:28 -0000, maht <mattmobile@proweb.co.uk> wrote:

> Forth doesn't use CALL it uses JMP
>
> Similar Threaded Interpretive Languages run an inner loop that does this  
> pseudo code :
> (from Threaded Interpretive Languages: Their Design and Implementation  
> by R. G. Loeliger)
>
> COLON:
>     PSH I -> RS
>     WA -> I
>     JMP NEXT
> SEMI:
>     POP RS -> I
> NEXT:       @I -> WA
>     I += 2
> RUN:       @WA -> CA
>     WA += 2
>     CA -> PC
>

I know nothing about interpreted/interpretive languages but the above  
pseudo-code is, to my own surprise, quite readable to me.

The "indirection pointer" seems to funcionally match the (stack) base  
pointer on x86, with the word address equalling the stack pointer. So, the  
above says:

- Save the stack base address (PSH is PUSH, I assume).
- Set the stack base address to the beginning of the current frame.
- Jump to NEXT.
- Put the return address on top of the stack.
- Reserve space on the stack for one 16-bit (2-byte) address by  
incerementing the base pointer. On x86 one would decrement the base  
because the stack grows from higher addresses towards 0x0.
- Set the the call address to the beginning of the procedure.
- Set the program counter to the procedure ("call" it, in other words).

One strange thing is that code and (stack) data seem to both grow from 0x0  
towards 0xFFFF unlike x86 where the code sits in low address and the stack  
grows downwards. You have re-implemented the calling mechanism. You still  
have a call stack and the pertaining problems, but you have lost the  
convenience of a simple call instruction.

By the way, now I see what Erik Quanstrom meant by "alternative" calling  
conventions. Your pseudo-code is an example.

> that's all very well but you then extend the assembler parts with new  
> words made from the WAs  (colon is the type marker for this) SEMI is  
> where it JMPs to at the end
>
> moddiv:
>     colon
>     divmod
>     swop
>     semi

I am genuinely lost, do not understand a word of it.

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