On 01/02/2019 02:31 AM, Paul Birkel via cctalk wrote:
I'm curious as to why you make this claim that microcode is no-go in "modern"
designs. Could you please elaborate on this point? I don't see why the alternative
random control logic would be a better proposition.
Random logic instruction decode was a REAL issue in about
1960 - 1965, when computers were built with discrete
transistors. The IBM 7092, for instance, had 55,000
transistors on 11,000 circuit boards. I don't know how much
of that was instruction decode, but I'll guess that a fair
bit was. The IBM 360's benefited from microcode, allowing
them to have a much more complex and orthogonal instruction
set with less logic.
But, once ICs were available, the control logic was less of
a problem. But, microcode still made sense, as memory was
so slow that performance was dictated by memory cycle time,
and the microced did not slow the system down. Once fast
cache became standard, then eliminating performance
bottlenecks became important. And, once we went from lots
of SSI chips to implement a CPU to one big chip, then it was
possible to implement the control logic within the CPU chip
efficiently.
Jon
