Date   : Wed, 14 Aug 1985 11:08:50-MDT
From   : Rick Conn <RCONN@SIMTEL20.ARPA>
Subject: Re: HD64180 boards

Hi, Ron,

I'm afraid your rumors are true.  The 64180 has an on-chip MMU
which provides the memory management.  This is accessed via 
on-chip I/O ports and the special set of 64180 I/O instructions.

I haven't found this to be a problem yet.  SW-wise, you init the
MMU, defining the 64K memory banks to have 0, 1, or 2 common
banks (common to all memory 64K memory banks) and 1 base
bank (unique to each 64K memory bank).  Switching banks is like loading
a segment register, wherein you output any of the standard registers or
(HL) to the MMU for the selection.  The resolution of the common and base
banks is to 4K, but that does not stop you from having a 63K+ TPA.

I really like the 64180, but a problem came up last night which really
bothered me.  One of the two UARTS onchip (which I am using to connect
to a Smartmodem) pays attention to the DCD line.  You can sense this
line internally thru a port.  However, if DCD drops off (no carrier),
then the Transmitter section of the UART is disabled!  Ban news!
No more comm to the Smartmodem for dialing the phone, answering the
phone, hanging up the phone, etc.  You can, of course, set the switch
on the Smartmodem to keep DCD true (reverse logic, so 0 actually), but
then you can't use DCD to detect loss of user signal.

There is a way around this problem for the Ciarcia board, but I can't
talk until Nov or so for reasons of confidence.  Ciarcia
will have a project on this option in Dec Byte.  If Steve says I can
talk earlier, I will.

       Rick