This note sets out a method by which I.B.M. Magnetic Tape of the types handled by the I.B.M. 729 IV Magnetic Tape Unit can be read into ATLAS and can be written by ATLAS. The system described here is not intended to be the principal method of using magnetic tape on ATLAS - the Ampex tape system described elsewhere is intended for more general use. It is, however, felt to be an adequate way of reading and writing I.B.M. Magnetic Tape.
As with all peripheral equipments on ATLAS, the control of the mechanism (an I.B.M. 729 Magnetic Tape UnitJ) is governed by bits held in the V-store, these bits being changed as required by fixed store programs.
Associated with the unit there will be a small buffer store of four 24-bit words (or half-words in ATLAS terms). Transfers between the main core store and this buffer store are organised by a fixed store program that is called in on an interrupt basis whenever the buffer signals, by means of a look-at-me indicator, that it requires attention (re-filling in the case of writing to tape or emptying when reading from tape).
For high density tape (i.e. 556 bits/inch) four 24-bit words would need to be transferred to the core store every 256 microseconds. The interrupt program to deal with such a transfer will take approximately 55 microseconds. Thus, approximately 22% of computer time would be occupied in dealing with one I.B.M, unit handling high density tape through a 4 × 24-bit buffer. A fast printer and six Ampex tape units could operate simultaneously with the I.B.M. unit. Thus, it is felt that such a buffer size is suitable. A larger buffer would reduce the percentage of machine time occupied with the transfers but, because the less frequent interrupts are individually longer, fewer other peripheral equipments would be able to operate at the same time.
For the low density tape (200 bits/inch) 14% of machine time would be occupied with the transfers and virtually no restriction would apply to the number of other peripheral equipments that would operate at the same time.
In reading from tape the contents of the buffer will be transferred to the core store by the fixed store program that is called in whenever the look-at-me indicator signals that the buffer is full. However, when the end of a block is reached the buffer may not be full, but an end-of -block interrupt will occur and a count of the number of characters currently held in the buffer will be available in the V-store. The fixed store program called in when this interrupt occurs will thus be able to complete the storage of the block of information in the core store, and will also be able to store the length of the block that has been read.
This will be read as a block of length one character, and it will be recognised by the fixed store program dealing with the end-of-block interrupt, so that the tape movement can be halted.
When reading I.B.M. tape, the lateral and longitudinal parity bits will be checked automatically by hardware, and an indicator in the V-store will be set when an error is detected. (A bit in the V-store will indicate whether the mode of recording is binary or BOD). This indicator will be examined by the end-of-block routine and repeat reading organised.
On writing, parity bits will be automatically generated and recorded on the tape. The information recorded will be checked by the trailing head and appropriate action organised if an error is detected.
The following bits will be required in the V-store to control the I.B.M. deck and the associated information transfers:
ENGAGE - DISENGAGE START - STOP FORWARD - REVERSE READ - WRITE REWIND CHARACTER COUNT (4 bits) FAILURES Lateral parity . Longitudinal parity General mechanical failures DENSITY OF RECORDING (High or low) MODE OF OPERATION (Binary or BOD) LOOK-AT-ME Buffer full End of block INFORMATION (24 bits)
