EXTERNIAL INTERRUPT FOR (Sanitized) PLOTTER

• Document Type: CREST
• Collection: NGA Records (Formerly NIMA)
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78B04747A001600020009-4
• Release Decision: RIPPUB
• Original Classification: C
• Document Page Count: 3
• Document Creation Date: December 28, 2016
• Document Release Date: April 20, 2000
• Sequence Number: 9
• Publication Date: January 22, 1965
• Content Type: MF

Approved For Release 2000/06/0-bR&-gpik7 4 82p009-4
CONFIDENTIAL
22 January 1965
MEMORANDUM FOR: Assistant for Plans and Development
25X1A
THROUGH: Chief, Information Processing Division
SUBJECT: External Interrupt fo 25X1A
25X1A
1. It is requested that the capability to generate an external
interrupt signal be added to th? plotter to indicate that a
contingency condition has occurred. This change can either be done
at the factory or after installation as a field change. The status
word that some peripheral devices set up on the output lines (referred
to i need not be considered. 25X1 A
25X1A
2. An external interrupt is required because the computer has
no other means of being informed that the r is unable to execute
any further commands. The operation of th Allllllllllllllllll plotter from the 490
is controlled by a routine existing within the executive control
routine. This routine, called a handler, activates the channel hardware
to send a buffer of commands to the plotter. The plotter accepts the
buffer, one word at a time, and performs the commands contained in the
buffer. When the 490 has no more words to send, the handler marks
the operation complete and informs the plotting program of this fact.
While the plotter is functioning normally, this scheme is entirely
adequate. When a contingency condition occurs (e.g. power failure)
the executive routine and the associated handler must recover to allow
continuation of other programs which may be in progress. The usual
recovery procedure with respect to other peripheral equipment is
initiated by receiving an external interrupt. Certain software alter-
natives may exist to "get around" the lack of an external interrupt,
but these are unsatisfactory at best. It is my feeling that an
external interrupt is a necessary addition to the plotter in view of
our programming environment.
25X1A 3. discussed this specific problem with some of his
engineers. The consensus was that the change can be readily accomplished.
We did not discuss whether this could be an on site change or whether
it had to be done at the factory. However, did not specifi-
cally rule out the possibility of making it an on site change. It should
25X1A
Rr JP t
Lxcludeo traraaItamatiu
downgrading and
declass "" oved For Release 2000/06/07 CIA-RDP78BO4747AO01600020009-4
 Approved For Release 2000/06/07 : CIA-RDP78BO4747AO01600020009-4
CONFIDENTIAL
25X1A
SUBJECT: External interrupt fo~ Plotter
be noted that the handler cannot be written until the Programming
Branch/IPD is informed as to whether we will have an external
interrupt or not. Further, if a field change is to be done the
REX handler and the on-line operation of the plotter cannot be
completely checked out until the change has been made.
25X1A
Chief, Programming Branch/IPD
Distribution:
Original & 1 - Asst for Plans & Dev
1 - C/IPD
2 - C/PRB/IPD
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Approved For Release 2000/06/07 : CIA-RDP78BO4747AO01600020009-4
 Approved For Release 2000/06/07 : CIA-RDP78B04747A001600020009-4
NPIC ROUTING SLIP
FROM: C/Programming Branch/IPD
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DATE: 22 Jan 1965
TO INITIALS DATE
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Approved For Release 2000/06/07 : CIA-RDP78B04747A001600020009-4