[A92] Re: Forth-92
[A92] Re: Forth-92
Hey there are a forth implementation over 68k in the motorola BBS, well they put
all those files in an ftp somewhere at www.sps.mot.com.
i have the file *somewhere* and i can look for it.
"Cliff L. Biffle" wrote:
> I've found an implementation of Forth on the 68000 (actually, for the
> Macintosh, but it's 68K generation) written by a guy about ten miles from
> me, and I'm going to see if I can sweet-talk some source out of him, if
> only as an example.
> One of the problems I'm dealing with is that the dictionary is likely to
> move around in memory quite a bit. We can't put it at the high end of RAM
> and lock it (the traditional way of fixing addresses without hurting the
> heap block compression routines), because then we can't grow the dictionary
> without manually moving it down in RAM--which changes addresses
> anyway. The issue here is that most Forth words are identified by their
> code addresses, which we don't want to recalculate--every single one--when
> the dictionary is moved.
> The 68K has a 32-bit-address-plus-16-bit-signed-offset addressing mode, the
> name of which I can't remember (it's been ages since I've written
> asm). This allows one to reference a 64K block around an address, but only
> 32K in either direction. So, my solution is this:
> --Obtain the address of the start of the dictionary (using heap handle
> --Add 32768 (the negative range of a signed word) to it. (Let's call that D)
> --Treat the first cell in the dict as -32768(D), and the last cell (if the
> dictionary is the full 64K) as 32767(D).
> This offset address D may not necessarily even be a valid RAM address,
> because the actual RAM available in the TI-whatever is a tiny little sliver
> of the 68000's address space. The offsets from D into the file would be
> constant, and as long as we don't write any really funky execution
> addresses, we won't be grabbing random bytes from somewhere past the end of
> I don't know if this would work like I intend it to, or if it's a good
> idea. The -other- 68K Forth implementor (creator of Pocket Forth for the
> Mac) imposed a 32K dictionary size limit, as that was the positive offset
> range of the pointers. Either he didn't think of this rather strange
> solution, or he ruled it out as unwise.
> Cliff Biffle