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From: woolsey@umn-cs.UUCP (Jeff Woolsey)
Newsgroups: net.arch
Subject: Re: uninitialized data (on 6000/Cyber)
Message-ID: <818@umn-cs.UUCP>
Date: Mon, 7-Oct-85 01:26:34 EDT
Article-I.D.: umn-cs.818
Posted: Mon Oct  7 01:26:34 1985
Date-Received: Sat, 12-Oct-85 05:43:53 EDT
References: <436@uvm-cs.UUCP> <164@tulane.UUCP> <92@opus.UUCP>
Reply-To: woolsey@umn-cs.UUCP (Jeff Woolsey)
Organization: Computer Science Dept., U of Minn, Mpls, MN
Lines: 30

This was known as PRESETA=DEBUG (a loader option) , which consisted of 
filling the field length with 60000460000400400000B+address, which 
1) fell into the class of floating point numbers known as Negative Indefinite,
2) contained the address of its original location, and
3) disassembles as
	SB0	A0+46000B	A No-op of sorts, since B0 is hardwired to 0
	EQ	*+400000B	which produces an address range error.
As I recall, this was a rather ugly integer, as well.
I suppose the reason for the 15-bit NO-OP (46000B) as the operand of the Set B0
instruction was that on the 7600 and its descendants, 30-bit instructions
could start in the last parcel of a word, meaning that the 60000B was the
operand of the previous instruction.

All manner of bizarre things were done on Cybers:

o memory was cleared by doing out-of-range reads of ECS (external core storage)
  which returned blocks of zeros quite rapidly (faster than a loop).
o Some folks would do 'display code math' in which two words containing the
  display code representations of 10 digits were added together (with the
  machine's integer add instruction) and then unbiased with '0000000000',
  having propgating carry. 
o In order to get anything done in a PPU (Peripheral Processing Unit), you HAD
  to do code-modification.
-- 
-- 
Even the ghosts will have settled down and raised families by now.

				Jeff Woolsey
				...ihnp4{!stolaf}!umn-cs!woolsey
				woolsey@umn-cs.csnet