3 May
2004
3 May
'04
3:17 p.m.
One other peculiarity I do remember about the BPC instruction set was that
as it had byte addressing, the byte select was the _most_ significant
bit of a
16-bit address, so that the indirect addresses were only one level -- that
is, you could do indirect addressing through one target word -- this is in
contrast to the 2100 instructions where the most significant bit was used as
an indirection bit indicating a chain of indirect addresses...
The 98x0 proxesor also does that -- you can indirect as many times as
you like (and, indeed, tie the processor in a tight-ish microcode loop if
you get an indirect address that tries to indirect through itself). AFAIK
the 98x0 porcessor has a different instruction set to the 2100 series,
but it's similar in concept.
-tony