Path: utzoo!attcan!uunet!husc6!rutgers!mcnc!rti!bcw
From: bcw@rti.UUCP (Bruce Wright)
Newsgroups: comp.sys.dec
Subject: Re: PDP-11 movb instruction
Summary: movb to PDP-11 device space
Message-ID: <3054@rti.UUCP>
Date: 30 Jun 89 02:15:38 GMT
References: <377@mountn.dec.com>
Organization: Research Triangle Institute, RTP, NC
Lines: 13

In article <377@mountn.dec.com>, coburn@clo.dec.com (John T. Coburn) writes:
> In article <2592@blake.acs.washington.edu>, erf@blake.acs.washington.edu (John Firestone) writes...
> >If a MOVB instruction writes the low byte of a word-oriented device
> >register, what does the high byte get set to?
> 
> The high byte would be unchanged if the device register allows byte access. 
> Otherwise the result would be undefined.

I think officially this is true - but in practice, essentially all 
implementations of the PDP-11 architecture will sign extend the byte to 
either all 0's or all 1's depending on the setting of bit 7.

						Bruce C. WrighT