DATIP from clr always? (obscure unibus & pdp-11 question)

Henk Gooijen henk.gooijen at hotmail.com
Wed Aug 30 14:35:56 CDT 2006


>From: Brad Parker <brad at heeltoe.com>
>Reply-To: "General Discussion: On-Topic and Off-Topic 
>Posts"<cctalk at classiccmp.org>
>To: "General Discussion: On-Topic and Off-Topic Posts" 
><cctalk at classiccmp.org>
>Subject: DATIP from clr always? (obscure unibus & pdp-11 question)
>Date: Wed, 30 Aug 2006 13:35:57 -0400
>
>An obscure unibus & pdp-11 question:
>
>Does "clr" always do a DATIP (i.e. read-modify-write) on all 11's?
>
>It does on my 11/44.  Something tells me I've seen threads (long ago) on
>this and the perils of using clr as opposed to "mov #0," when talking to
>hardware.
>
>The rl01 bootstrap does it talking to rl11 registers, which tripped me
>up for a bit.
>
>-brad

The only difference between CLR and MOV #0 that I think of immediately
is that CLR clears the Carry flag too, whereas MOV #0 does not affect the
Carry flag. That is needed to do multi-word ADD or SUB.

- Henk, PA8PDP.

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