Path: utzoo!utgpu!attcan!uunet!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.cbm
Subject: Re: Questions about 128 Reliability (Considering upgrade from C64)
Message-ID: <4971@cbmvax.UUCP>
Date: 10 Oct 88 18:03:39 GMT
References: <1576@nunki.usc.edu>
Organization: Commodore Technology, West Chester, PA
Lines: 41

in article <1576@nunki.usc.edu>, aliu@sal8.usc.edu (Alejandro Liu) says:
> 
> In article <39729@yale-celray.yale.UUCP> sekora-jay@CS.YALE.EDU (Jay Sekora) writes:
>>My understanding is that this was true of the CP/M add-on card for the C64,
>>but that on the 128 the Z80 does all it's own i/o and doesn't have to rely
>>on the 8502 for anything.  

> 	I read in the Compute's! introduction to the C-128 (I think that's
> what the book was called) that the Z80 DO surrender control to the 8502 for
> I/O control, actually for handling the screen, the User and serial ports and
> some other housekeeping procedures.  

Compute!?!?  When there's a perfectly good C128 Programmer's Reference Manual,
written mainly by the folks who designed the hardware and software for the
C128.  No accounting for taste :-) 

> The Z80, can also access this resources by itself using the IN and OUT commands.

The trick is that it uses the not-documented-in-every Z-80 Manual 16 bit IN and
OUT commands.  As I recall, the instructions that claim to use the C register
for the I/O address actually use the BC pair to give you a full 16 bit address.
The Z-80 even has an advantage here, in that it doesn't have to map I/O in and
out when using both I/O and other resources that sit in the same place.

As I recall, the Z-80 is using 8502 routines to do more high level stuff, like
talk to disks and printers, while it's going directly to the hardware for modems.
Fred Bowen could probably explain all the details.

> Anyway, is interesting to note how Commodore engineers save some programming 
> by using the C-128 8502 routines to handle those chores.  Since the 8502 in 
> native mode always is doing that!

And the 8502 at 1.02MHz does run about twice as fast as the Z-80 at it's C128
effective 2.04MHz.  Which is why the 8502 at 2.04MHz really makes sense.

>                     aliu@nunki.usc.edu (Alejandro Liu) 
> (Simple .signature, $CHEAP$) 
-- 
Dave Haynie  "The 32 Bit Guy"     Commodore-Amiga  "The Crew That Never Rests"
   {ihnp4|uunet|rutgers}!cbmvax!daveh      PLINK: D-DAVE H     BIX: hazy
		"I can't relax, 'cause I'm a Boinger!"