Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!bloom-beacon!gatech!ncar!ames!pasteur!ucbvax!hplabs!hpl-opus!hpccc!samc
From: samc@hpccc.HP.COM (Samuel Chau)
Newsgroups: comp.sys.hp
Subject: Re: HP-71B : Thanks and more questions
Message-ID: <5950006@hpccc.HP.COM>
Date: 9 May 88 17:04:12 GMT
References: <351@usl.usl.edu>
Organization: HP Corporate Computing Center
Lines: 29

Reddy,

The HP-71B bus is a 4-bit multiplexed address/data/instruction bus with 20 bits
of address, i.e. 1Meg nibbles, or 512K bytes total.  This bus is available at
each of the four front module ports.  RAM or ROM modules plugged into this
bus have their own program counters and stack pointers, and so each device
'knows' if it is being addressed by the CPU.  The operating system firmware 
performs soft configuration for plugged-in RAM and ROM modules.  This means that
the actual address space occupied by a particular module is determined by
the '71B OS code, and is dependent on what you've plugged into the machine.
So in a way, the entire 512K is accessible at each module port.  The 64K
OS in ROM is, however, hardwired to the lowest 128K-nibbles of address.
Also, the largest RAMs and ROMs I've seen for the '71B are 64K bytes in size
(or, more correctly, 128K-nibs).

HP's 4K RAM module does not have a battery built in, and will forget
everything you put in it the moment you unplug the module from the '71B.  It
is, however, possible to install a flat 3V lithium battery inside the module
to keeps its contents alive outside the machine.  You will also need to put
a Schottky diode in series with the battery to prevent the '71B from
charging the battery when the module is plugged in.  I have made such a
modification to one of my 4K RAMs.  I use it to exchange files with friends
who don't have either the card reader or the HP-IL module.

I hope this helps.

Sam Chau
...hplabs!hpccc!samc
(408) 447-0238