Microcode, which is a no-go for modern designs

[Jon Elson]

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