[OpenBIOS] [PATCH 1/3] bootstrap.fs: add pseudo r-stack implementation for interpret mode

[Mark Cave-Ayland]

On 17/07/16 18:29, Segher Boessenkool wrote:

> On Sun, Jul 17, 2016 at 11:44:06AM -0500, Segher Boessenkool wrote:
>> On Sun, Jul 17, 2016 at 11:20:15AM -0500, Segher Boessenkool wrote:
>>>> +: >r state @ if ['] >r , exit then r> swap prstack-push >r ; immediate
>>>> +: r> state @ if ['] r> , exit then r> prstack-pop swap >r ; immediate
>>>> +: r@ state @ if ['] r@ , exit then r> prstack-pop dup prstack-push swap >r ; immediate
>>
>> Btw, why do you need those  r> ... smth ... >r  gymnastics?
> 
> Hrm, okay, coffee helps thinking...  Could use some documenting ;-)

As you've already worked out, the reason is that : calls the primitive
docol which always pushes the caller onto the r-stack when calling
another Forth word. Hence there's an extra parameter on the r-stack that
needs to be juggled to make sure the >r primword and the resulting
parameter are placed back correctly on the r-stack.

Regarding the documentation, I wasn't too worried about this since it's
fairly obvious that calling another word is going to involve another
r-stack push from the caller (and indeed it was the first thing that
stood out when I tried your code in the debugger).

If you would like to suggest something suitable in a maximum of a couple
of lines you think would help then I don't mind adding it in a v2,
however compared to other undocumented gotchas in the Forth code I've
come across whilst working on OpenBIOS this was very easy to figure out
so I can't get too excited about it.


ATB,

Mark.