Last Time NOP Brain Teaser

I'm sure we've all seen a "First Time NOP", typically:

NOP  SOMEWHER
OI      *-3,X'F0'

On first execution, the NOP falls through to the OI, which changes the NOP t
o a
B. On subsequent executions, it branches to SOMEWHER. Note that the code
doesn't test any condition or storage external to itself to "know" it's the
first time, nor is it modified by anything outside of itself. This code can 
be
used in any program, regardless of the specific application. (Let's ignore R
ENT
considerations for purposes of this discussion).

There's also the  FIRST TIME BRANCH:

NOP  *+12
OI      *-3,X'F0'
B       SOMEWHER

Where the branch to SOMEWHER occurs only the first time.


I want to see someone design a LAST TIME NOP (or branch). The rules are that
you can't "cheat" by testing any condition or storage external to the actual
code itself, nor can the code be modified by anything outside of itself. It
should be useable in any application (again, forget RENT stuff). The action
(NOP or BRANCH) can only be taken the LAST TIME the code is executed.

On 23 Nov 2003 18:50:33 GMT rfcommsys@aol.com (RFCOMMSYS) wrote:

:>I want to see someone design a LAST TIME NOP (or branch). The rules are th
at
:>you can't "cheat" by testing any condition or storage external to the actu
al
:>code itself, nor can the code be modified by anything outside of itself. I
t
:>should be useable in any application (again, forget RENT stuff). The actio
n
:>(NOP or BRANCH) can only be taken the LAST TIME the code is executed.

Other than the trivial cases of clearing storage or loading a wait state PSW
,
i.e., the routine setting conditions that it can never be re-executed, it is
impossible without knowledge of what causes the routine to be executed.

--
Binyamin Dissen <bdissen@dissensoftware.com>
http://www.dissensoftware.com

Director, Dissen Software, Bar & Grill - Israel

Exactly. You cannot predict the future. The examples listed show knowledge o
f
the past, not the future.

"Binyamin Dissen" <postingid@dissensoftware.com> wrote in message
 news:0v02svce1ortusdpds4iivvtq9qsl3kj8t@
4ax.com...
> On 23 Nov 2003 18:50:33 GMT rfcommsys@aol.com (RFCOMMSYS) wrote:
>
> :>I want to see someone design a LAST TIME NOP (or branch). The rules are 
that
> :>you can't "cheat" by testing any condition or storage external to the ac
tual
> :>code itself, nor can the code be modified by anything outside of itself.
 It
> :>should be useable in any application (again, forget RENT stuff). The act
ion
> :>(NOP or BRANCH) can only be taken the LAST TIME the code is executed.
>
> Other than the trivial cases of clearing storage or loading a wait state P
SW,
> i.e., the routine setting conditions that it can never be re-executed, it 
is
> impossible without knowledge of what causes the routine to be executed.
>
> --
> Binyamin Dissen <bdissen@dissensoftware.com>
> http://www.dissensoftware.com
>
> Director, Dissen Software, Bar & Grill - Israel

xarax wrote:

> Exactly. You cannot predict the future. The examples listed show knowledge
 of
> the past, not the future.

Consider phase conjugate optics, where given an optical signal a device
will create the time reversed version of the signal.

Also, consider compilers with a COME FROM statement, that does the
opposite of the GO TO statement.

-- glen

"RFCOMMSYS" <rfcommsys@aol.com> wrote in message
news:20031123135033.02030.00000736@mb-m16.aol.com...
> I'm sure we've all seen a "First Time NOP", typically:
>
>           NOP  SOMEWHER
>           OI      *-3,X'F0'
>
> On first execution, the NOP falls through to the OI, which changes the NOP
to a
> B. On subsequent executions, it branches to SOMEWHER. Note that the code
> doesn't test any condition or storage external to itself to "know" it's
the
> first time, nor is it modified by anything outside of itself. This code
can be
> used in any program, regardless of the specific application. (Let's ignore
RENT
> considerations for purposes of this discussion).
>
> There's also the  FIRST TIME BRANCH:
>
>           NOP  *+12
>           OI      *-3,X'F0'
>           B       SOMEWHER
>
> Where the branch to SOMEWHER occurs only the first time.
>
>
> I want to see someone design a LAST TIME NOP (or branch). The rules are
that
> you can't "cheat" by testing any condition or storage external to the
actual
> code itself, nor can the code be modified by anything outside of itself.
It
> should be useable in any application (again, forget RENT stuff). The
action
> (NOP or BRANCH) can only be taken the LAST TIME the code is executed.
>
>
>

Just how do you know that it is the last time?

Sven

"glen herrmannsfeldt" <gah@ugcs.caltech.edu> wrote in message
news:Cgawb.217404$275.810476@attbi_s53...
> xarax wrote:
> 
knowledge of 
>
> Consider phase conjugate optics, where given an optical signal a device
> will create the time reversed version of the signal.
>
> Also, consider compilers with a COME FROM statement, that does the
> opposite of the GO TO statement.
>
> -- glen
>

COME FROM is no problem, any decent trouble diagnostics program includes
that for a traceback of the calling sequence leading up to the error.

Reversing time to see what the future will bring seems, well Startrek
technology
some 500 years ahead of us?

Sven

Sven Pran said ---

>
>
>Just how do you know that it is the last time?
>
>

Ah, how indeed ...

On Tue, 25 Nov 2003 05:41:12 GMT glen herrmannsfeldt <gah@ugcs.caltech.edu>
wrote:

:>RFCOMMSYS wrote:

:>> Sven Pran said ---

:>>>Just how do you know that it is the last time?

:>> Ah, how indeed ...

:>There is a question in another newsgroup about how to find the value of
:>the fifth from the last element of a linked list with one pass through
:>the list.

Well.......

You can save the last five values.

Obviously there are more constraints.

--
Binyamin Dissen <bdissen@dissensoftware.com>
http://www.dissensoftware.com

Director, Dissen Software, Bar & Grill - Israel

It's a standard M$ interview question. It simply means managing two pointers
that represent a specified span of elements in the linked-list. Just walk th
e
first pointer for 5 elements, then continue walking both elements. When the
first pointer gets to the end of the list, the other pointer is 5 elements f
rom
the end.

A variation of this technique is used for walking damaged linked-lists to fi
nd
the broken element (the one that points incorrectly back into the previous
elements somewhere).

"glen herrmannsfeldt" <gah@ugcs.caltech.edu> wrote in message
news:YXBwb.221104$mZ5.1690987@attbi_s54...
> RFCOMMSYS wrote:
> 
> 
> 
>
> There is a question in another newsgroup about how to find the value of
> the fifth from the last element of a linked list with one pass through
> the list.
>
> -- glen
>

rfcommsys@aol.com (RFCOMMSYS) wrote
> I want to see someone design a LAST TIME NOP (or branch). The rules are th
at
> you can't "cheat" by testing any condition or storage external to the actu
al
> code itself, nor can the code be modified by anything outside of itself. I
t
> should be useable in any application (again, forget RENT stuff). The actio
n
> (NOP or BRANCH) can only be taken the LAST TIME the code is executed.

If this were possible, what path would the following code take:


TOP_LOOP
...
... some looping process code
...

*
* Use of the grand-spankin new macro - LASTTIME_BRANCH_TO label
*
LAST_TIME_CHECK DS 0H
LASTTIME_BRANCH_TO  PARADOX_CODE
B   TOP_LOOP
*
PARADOX_CODE DS 0H
B   LAST_TIME_CHECK

So, does the macro LASTTIME_BRANCH_TO ever branch? Is the label PARADOX_CODE
ever reached?  Is this an infinite loop?

Tim