30 Aug
2006
30 Aug
'06
12:35 p.m.
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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