Microcode, which is a no-go for modern designs

[Paul Koning]

> On Jan 2, 2019, at 2:31 PM, Chuck Guzis via cctalk <cctalk at classiccmp.org> wrote:
> 
> On 1/2/19 10:44 AM, Guy Sotomayor Jr wrote:
> 
>> Also, recall that there are different forms of micro-code: horizontal
>> and vertical.  I think that IBM (in the S/360, S/370, S/390, z/Series)
>> uses the term micro-code for horizontal micro-code and millicode
>> for vertical microcode.
> 
> On the CDC STAR-100, "microcode" as such was a relatively recent concept
> and the designers went overboard, mostly because of an ill-defined
> customer base (hence, BCD and other commerical-class instructions, like
> translate, edit and mark, etc.).  The STAR is basically a RISC-type
> vector architecture with a pile of microcoded instructions bolted on.
> ...
> For a compiler writer, or even an assembly coder, this was more of a
> problem--which combination of instructions could be used to the greatest
> effect?  And why do I have to have the hardware manual on my desk to
> look up instructions?

That reminds me of the Motorola 68040.  I used that at DEC in a high speed switch (DECswitch 900 -- FDDI to 6 Ethernet ports).  When studying the instruction timings, I realized there is a "RISC subset" of the instructions that run fast, a cycle or so per instruction.  But the more complex instructions are much slower.  So the conclusion for a fastpath writer is to use the RISC subset and pretend the fancy addressing mode instructions do not exist.

	paul