🚀 go-pugleaf

mac@coos.dartmouth.edu (Alex Colvin) wrote:

>>] In fact, the situation with Alpha is very similar to the current situation
>>] with Macintosh and 16-bit Windows programs, with the sizes doubled of course.
>>] You will run into the same types of problems around 2 GB on the Alpha as we
>>] currently run into at 32 KB with Mac and Windows programs.  Of course these

>32KB? I see, 16 bit (int). However, 68K pointers have always been 32 bits.
>Nowadays, (int) usually is too. Still, embedded 68K systems sometimes try to
>get by with halfwords. And really small ones store halfword pointers, knowing
>that they don't have more than, e.g., 24K of memory.

The register+displacement addressing mode in the original 68K only
offered a 16 bit signed offset.  This is the origin of the Mac 32K
data limitation (under what contexts I'm not sure, I've never done any
Mac programming).  That limit should be gone now that the 68020 and
later CPUs allow 32 bit offsets.

And as for 16 bit addresses... Well, the 68K has a 16 bit absolute
addressing mode.  With this you can address the first 32K and the last
32K of memory.  On the original 68000 it's much faster than the 32 bit
absolute mode.  And when you load a 16 bit word into an address
register it's automatically sign-extended to 32 bits.

--
Richard Krehbiel, Kastle Systems, Arlington VA USA
rich@kastle.com (work) or richk@cais.com (personal)