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Path: utzoo!watmath!clyde!caip!cbmvax!daveh
From: daveh@cbmvax.cbm.UUCP (Dave Haynie)
Newsgroups: net.micro.cbm
Subject: Re: Z80A chip
Message-ID: <495@cbmvax.cbmvax.cbm.UUCP>
Date: Mon, 7-Jul-86 13:25:51 EDT
Article-I.D.: cbmvax.495
Posted: Mon Jul  7 13:25:51 1986
Date-Received: Tue, 8-Jul-86 06:44:37 EDT
References: <271@sunybcs.UUCP>
Organization: Commodore Technology, West Chester, PA
Lines: 65

> 
> First : 
> 
> To the person who posted about the Vt100 emulator. Please send me the address
> of the supplier. I'm in desparate need of one. I tried replying by Email, but
> we had some trouble with the net recently and my reply won't go through.
> Thanks a million!!!!
> 
> 
> Second :
> 
> To all you people at Commodore (and any others that can help). I learned ML on
> a Z80 and was thrilled to hear of one in the 128, now I'm wondering if there is
> any way you can switch in a bank of RAM and I/O and then turn on the Z80 so
> that all ML can be Z80 code. I see no reason why this couldn't be but I haven't
> seen any meantion of it in the reference guide. I know the Z80 can be activated
> with a bit in the MMU, but I haven't tried it because once I activate the Z80
> I have no idea where to put my Z80 code so that it can find it!! I much prefer
> the Z80 to the 8502 and I'd love to use it extensively (with the VIC, SID, and
> MMU of course). By the way, why did you guys do the CP/M BIOS in 8502? I'm
> still struggling with that one, too. Thanks for any help...
> 
>                                         Devon E Bowen
>                                         University of Buffalo

I don't know the exact details (my PRG is at home), but I can tell you a bit
about the Z-80 interface on the C128.  When you power up, the Z-80 takes 
control immediately, checks for a few C64 mode things (C= key or a cartridge
in place), then initializes anything else important to the Z-80 in C128
mode, and then writes to the MMU.  The Z-80 is left frozen at the next
instruction.  The 8502 and the Z-80 communicate via a sort of RAM based
mailbox.  When you set the MMU next, the Z-80 should check this area for
a command (which includes, I believe, jumping to a machine language
program).  Once you get to this program, you could certainly stay on the
Z-80 side forever if you desire.  

There were several reasons for doing some of the CP/M BIOS with the 8502.
First of all, only part of the CP/M BIOS is done by 8502; mainly the serial
bus driving for disk and printer, etc.  There's alot of software involved
in this, and it already exists for the 8502 side.  The Z-80 does its own
screen display, keyboard I/O and user port I/O.  The 8502 at 1.02MHz 
(SLOW for you BASIC fans) is almost twice as fast as the Z-80, which runs
at an effective speed of 2.04MHz.  The 8502 in FAST mode (not used during
CP/M) is nearly four times as fast.  So in addition to saving lots of code
on the Z-80 side, doing disk I/O with the 8502 is also faster.

For talking to I/O devices from the Z-80 side, don't set the MMU's I/O bit,
this could really confuse the C128.  Instead, turn off MMU I/O and use the
Z-80's 16 bit I/O instructions.  What, you say, you didn't know that the 
Z-80 HAD 16 bit I/O instructions?  Yup, IN (C) and OUT (C) instructions both
actually generate an address from the BC pair.  And all of your favorite
I/O devices will be at $D000 (or 0D000H if you prefer) on up, just like
in C128 mode (except that you don't bank stuff around so much).  I believe
the CP/M section in the PRG should cover the Z-80/8502 interface in more
detail.


-- 
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Dave Haynie    {caip,ihnp4,allegra,seismo}!cbmvax!daveh

   A quote usually goes here, but its currently being rennovated.

	These opinions are my own, though for a small fee they be yours too.
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