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Preliminary Report on the Attachment of the Hough-Powell Device to the Chilton Atlas
J K Saggerson
The Rutherford High Energy Laboratory's main interest in Atlas was to analyse bubble chamber tracks. The hope that the scanning device (HPD) could be directly attached to Atlas. Orion had proved to be to too small for the processing load anticipated. The report was prepared initially by RHEL and commented on by Ferranti.
This report gives in outline a proposed method for the analysis of Bubble Chamber photographs using an Atlas computer. These proposals must be treated as preliminary ideas on the subject, but they attempt to solve some of the problems of store utilisation, and the organisation of programmes within the structure of the Supervisor, and it is hopes that the times shown in the final sheet are sufficiently accurate to assess the work load.
The Bubble Chamber photographs will be scanned by a Hough-Powell device which will be connected on-line to Atlas. The attachment will be via the V-store in the same way as a conventional slow peripheral device.
Each event in the Bubble Chamber is recorded on three separate photographs (or views) so that a three dimensional geometrical reconstruction of the event can be obtained. Only one film can be scanned by the HPD at a time, and collation of the separate views must therefore be achieved in the computer.
The HPD can be operated at a slow speed with a scan time of approximately 7.5 ms. or at high speed with a scan time of approximately 2ms. Both speeds would be acceptable to Atlas. The slow HPD will allow part of the filtering programme to reside normally on the drums, whereas the fast HPD will probably require part of the filtering programme to be kept permanently in the core store. This fact must be borne in mind when comparing storage requirements.
Against this, the total storage requirement would be approximately 6,000 words in each case and must be occupied throughout the time the HPD is attached to Atlas. This period is approximately three times as long when the slow HPD is used.
The period for which magnetic tape units must be reserved is similarly increased.
It has also been suggested that it would be possible to produce a rough filtering by means of a gate which uses an Atlas half word as a mask. This would mean that two thirds of the data was discarded even before it reached the HPD buffer.
The operations which have to be carried out on data produced from the HPD are:-
- Input to the computer via a 128 word buffer.
- Final Gating and Filtering. Filtering is necessary to obtain the best tracks from the recorded points on the photographs.
- A geometrical three dimensional reconstruction of the events.
- Analysis of the kinematics of the event and hypothesis testing.
- Output to a line printer and library magnetic tape.
2. Input and Gating
The HPD scans the photographs producing an average of 30 digitizings per line of scan. These digitizings are fed into a 128 word buffer, which is part of the HPD itself. Where external gating is used, only those digitizings which pass through the gate will be stored in the buffer. The buffer can be considered as divided into two 64 word blocks. When one block is full, output from the HPD is fed into the other block, and the full block is emptied into Atlas.
Input from the buffer store to Atlas is achieved using an interrupt routine which will form part of the Supervisor system. The suggested mechanism is as follows:-
The final gating and filtering programme will be treated as a normal object programme. When it requires further digitizings it calls for a new block by entering the Supervisor by means of a special extracode function. The main gating/filtering programme is then halted until a block of input is made available to it. (This process is very much like calling for a magnetic tape transfer.) The interrupt routine which deals with the HPD will simply fill a block of the core store with digitizings direct from the HPD buffer. The speed of the buffer store must be sufficient to cope simultaneously with the rate at which the scanner generates digitizings and with the interrupt routine.
Number of 24-bit words per view on input to Atlas (without rough gating) = 75,000
Number of 24-bit words per view (with rough gating) = say, 25,000
One view will take about 20 seconds to scan and wind on, at the slow scanning rate (7.5 ms. per scan line) or about 7 seconds at the fast scanning rate (2 ms. per scan line).
Let us now consider the amount of machine time used for this operation in each of the various cases:-
Slow HPD - with no rough gating
It is estimated that one scan line would require an interrupt routine lasting about 250 µs.
The utilisation of the arithmetic unit in this case = 3%
Slow HPD ~ with rough gating (by hardware)
Utilisation of arithmetic unit = 1.0%
Fast HPD - with no rough gating
Utilisation of arithmetic unit = 8.6%
Fast HPD - with rough gating (by hardware)
Utilisation of arithmetic unit = 2.9%
3. Input of Rough Road Data
There will probably be too much rough road data for it to be stored on the drum throughout the time the HPD is operating. It will therefore be necessary to put the data on a magnetic tape, which will require a tape unit throughout the time the HPD is in operation.
4. The Gating and Filtering Programme
This programme will be handled in the computer as a normal object programme, which may be held up for data in which case it will be time-shared with other object programmes.
The degree to which this programme will be held up for lack of data depends on the speed of the HPD scanner and whether or not rough gating is done.
It is reasonable to suppose that the Gating and Filtering programme will occupy about 6,000 words of store and that it will take about 0.8 seconds computing per view. (The storage will be used throughout the scanning time and one or two tape units will also be required.) Since it is estimated that the gating and filtering programme will run for 0.8 seconds per view, and there are about 75 blocks of data per view, (without rough gating) one can assume that the processing of one block will take about 11 ms. Since even at the fast rate the HPD cannot provide a block of data in so short a time, this means the gating and fi1tering programme will be HPD limited.
It will thus be appreciated that in all cases the main gating and fi1tering programme will be held up for lack of data. It will not therefore, be necessary to reserve a large area of store to receive data waiting to be filtered.
The utilisation of the central computer in the various cases will be as follows:-
- Fast HPD - no rough gating
- 0.8 seconds computing/view
7 seconds to scan a view (including wind-on times)
Utilisation = (0.8 × 100)/7 = 11.5%
- Fast HPD- with rough gating
If one assumes that rough gating eliminates two thirds of the digitizing, then
Mean Utilisation = 3.8%
- Slow HPD - no gating
- Mean Utilisation = 4.0% =
- Slow HPD with rough gating
- Mean Utilisation = 1.4%
5. Geometrical Reconstruction
A Fortran programme has been written for this job on the IBM 7090 and it occupies about 20K words of store, including 4K words of data. This programme will require two magnetic tape decks, one for input of the filtered data and the other for output data. It is estimated that approximately 30 hours of Atlas time will be required by this programme per year, for 100,000 events.
6. Analysis of Kinematics
It is estimated that approximately 32K words of store will be required by this programme and the data; and 140 hours of Atlas time will be used per year for 100,000 events. Two tape decks will be utilised, one for input of data, and the other to keep a permanent record of the output. Output will also be provided on a line printer, printing being carried out by the Supervisor from information stored on the permanent record tape.
SUMMARY OF REQUIREMENTS FOR PROCESSING 100,000 EVENTS PER YEAR
|Slow HPD - no rough gating|
|Gating and Filtering||33.0||4.0||1.3||12||396||2||66.0|
|Fast HPD - no rough gating|
|Gating and Filtering||11.0||11.5||1.3||12||132||2||22.0|
|Slow HPD - with gating|
|Gating and Filtering||33.0||1.4||0.5|
|Fast HPD - with gating|
|Gating and Filtering||11.0||3.8||0.3|