jimomura@lsuc.UUCP (Jim Omura) (02/10/86)
A while back someone mentioned the limit of the 68000 relative
addressing mode (+/-32K and *longer* with index registers).
First, I think the person who posted it misunderstands the index
register displacement addressing system. It can use the whole 32 bits
of a register as an address (granted with the real limit in practice
being the pinouts of the DIP package). He seems to think it's limited
to +/-128 bytes. That figure is for an additional 8 bit variable
taken from the instruction. It's a very complex, but I suspect slow,
form of addressing. Nevertheless, there *is* full +/-2 Gigabyte
relative addressing in a 68k system subject to the above.
The 68020 adds a full 32 bit Long Branch set of instructions whic
should execute much faster. In either case though, there should be
no problem with the OS-9 design philosophy. I am guessing here, but
I should think that a top notch C compiler system should be able to
choose the appropriate addressing mode to minimize object code (and
speed) for these situations.
Cheers! -- Jim O.
--
James Omura, Barrister & Solicitor, Toronto
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