Re: Gathering entropy for PRNGs on MVS

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From: Bob
Date: Mon, 10 Mar 2003 21:37

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Hi Duke,

I haven't  played with IBM mainframes for quite some time, but I had a
similar problem many years ago.  The entity I settled on was derived from
the 8 byte TOD clock - but I reversed the BYTES not the bits.  As you know,
the high order byte of the TOD clock changes least quickly, while the low
order byte of the TOD clock changes most quickly.  From (dim) memory, the
(32 bit) code went something like

; get TOD into my memory
      SVC ???,TODF
      ICM R4,1,TODF
      ICM R4,2,TODF+1
      ICM R4,4,TODF+2
      ICM R4,8,TODF+3
      ICM R5,1,TODF+4      ; note adjacent register
      ICM R5,2,TODF+5
      ICM R5,4,TODF+6
      ICM R5,8,TODF+7
      STM R4,R5,TODR

TODF DS 1D    ; TOD Forwards
TODR DS 1D    ;TOD Reverse

I can't remember the exact format of the ICM instruction (does the mask come
first?).  I can't remember the exact SVC call to get the eight byte TOD
either.  What I do remember is that the results were more accurate than the
documentation described.

Bob

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From: "Bob" <me@privacy.net>
Newsgroups: sci.crypt.random-numbers,bit.listserv.ibm-main,sci.crypt
Subject: Re: Gathering entropy for PRNGs on MVS
Date: Tue, 11 Mar 2003 08:37:46 +1100
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