Jack,

"reference-z84c15,z80180,z80280,z84013/c13,z84015/c15,from their super integration products (embedded controllers)."

Zilog named none of these microcontrollers,
means none can work as a single chip,
since no ROM, OTP or Flash inside.


"no distinction between code/data memory "

On decoding M1 you can also use on the Z80
64kB code and 64kB data separate.
Also on ANDing /RD and PSEN on the 8051
you can use the same 64kB for data and code.


"had correct design (hardware) to eliminate the requirement to have
high at the port to read"

I like the intelligent way to implement
the IO ports on the 8051. I must never again decide ins and outs, I can read and write without thinking about the direction.
Especially the way of power saving was very nice:
- no pull ups on low level,
- reduced pull ups on low driven external,
- strong pull ups on changing from low to high.
Really clever the old Intel guys.
So its very easy to implement the I2C bus or any other bidirectional using.


"...
rst 10h
db xx ---this byte is used to define further rst 10
next instruction
..."

Sorry, I give up.
I'm not so familiar with Z80 programming tricks.
But Im sure, I can you also teach some tricks on the 8051.


My conclusion:

Its more and more the compiler, which do such low level thinking for me.

Today not the core architecture of a micro was important.
Only the pricing, availability,
power consumption, speed, hll support,
included peripherals, ... are the points to make the decision of a micro.



Peter