Monitor for iSBC 8024 - test-drb@ccmp.vtda.org

4 Nov 2000

From: Eric Smith &lt;eric(a)brouhaha.com&gt;

...
 Allison wrote:
  To be exact the z80 has [list of registers
deleted]

You forgot IFF2.  :-) 

I know that but consider who most of this is aimed at.

No question the z80 has many more bits than 6502,
8080, 8085, 9900, and a pot load more.  Only the 1802
has more stoarge (though simpler elsewhere).

...
 And both the Z-80 and 6502 have at least one more
bit that's "sort of" programmer visible, the NMI
edge-detect flop. 

I tried to limit it to the programmer accessable.  If
we add temp registers and whole knows what in the
state or microcode portion of the machine it surely
would grow more.

The 6502 distinguished itself in the same fashon as PDP-8.
That is it was simple, cheap to manufacture and had an
adaquate instruction set to do a lot of tasks.  it's cheapness
was due to the ease of implementation in silicon.

...

  6502
     SP            9BIT  (HIGH BIT =1) 
It's either 8 bits, or 16 bits with the high 8 being stuck at
"00000001".
...
 There's no meaningful reason to consider it a 9-bit
register.  I personally
...
 only find it sensible to count the bits that change, so
I call it 8. 

Agreed.  The 9bit comes from the Osborne view of microprocessors.

...

      STATUS    8BIT 
7 bits, or 8 with one bit stuck high.. 
same deal.  We can consider the unused bit as "reserved".

Allison