Re: Infinity (to the nearest integer)

It's noteworthy that the tag bits in AS/400 were ultimately removed. The
initial conversion of the hardware to PowerPC used a variant processor with
tagged registers, but dropped tags on the memory. A later iteration shifted
to the conventional PowerPC implementation, eliminating the need for a
non-commodity processor. I'm not aware of any performance loss resulting
from this. Frank Soltis could be asked and would probably be willing to
comment.

This was possible because the AS/400 architecture is, in effect, a low-level
language-style virtual machine specification, where compilation is performed
at segment load time.

The case for tags in the integer unit is much weaker than the case in the
floating point unit. The side computations necessary to maintain the tags
are relatively more straightforward and can be performed in the ALU if the
architecture has suitable conditional move instructions (most importantly
conditional move if register is [not] zero) and/or multiple condition codes.

Andy Glew (and others) have observed that tag bits can be implemented using
commodity DRAMs if the memory controller is tweaked. Basically, the bits can
be stored in a side bank. The caches still need to be modified to add the
extra bit, but caches tend to be full-custom logic anyway, so concerns about
commodity-based design aren't as intensive there.


shap

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