Re: Z80 CPU Speeds? - Upgrading

[pausch@saaf.se (Paul Schlyter)]

In article <3D9BA11A.8040502@adelphia.net>,
Steven N. Hirsch <shirsch@adelphia.net> wrote:
 
> One of the reasons that the Applicards were so fast was that
> they made efficient use of the 6502 for I/O.  Unlike the original
> Microsoft Softcard, both CPUs are running at the same time
> (the Softcard is an either/or proposition.)
 
Even though, on the AppliCard, both CPU's ran concurrently, they
rarely did something useful concurrently: when the Z80 executed CP/M
programs, the 6502 waited for commands from the Z80.  And when the
6502 executed commands form the Z80, the Z80 waited for the 6502 to
complete these commands.  In addition, after the 6502 had done e.g.
a read on some I/O device, the data resided on the 6502 RAM, which
was inaccessible to the AppliCard's Z80.  So before the Z80 could use
it, the data had first to be transferred, byte by byte, from the
6502's RAM to the Z80's RAM - this was accomplished through a few Z80
ports (which were accessible as memory mapped I/O ports to the 6502)
combined with an internal protocol between the 6502 and the Z80.
 
No, the reason the AppliCard was so fast was that the AppliCard's Z80
had its own RAM: this RAM was independent of the main RAM, and there
was no video circuits accessing it either.  So the Z80 could run, and
use this RAM, at full speed (4 or 6 MHz, depending on whether you
used the Z80A or Z80B).  And the Appli-Card was significantly faster
only on CPU-bound programs (where it ran 2 or 3 times faster than the
SoftCard).  But on I/O bound programs, the Appli-Card was probably a
little slower than the SoftCard, simply because of the extra overhead
'due to the need to transfer data between the Z80's RAM and the
6502's RAM.
 
-- 
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