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Further reading □ Overview □ Brochures □ ICT 665 printer (1960) □ Ampex TM-2 tape deck (1961) □ Anelex printer (1961) □ Creed 300 tape punch (1961) □ Papers: 1960 □ Users' specification □ Punched card codes □ Description of the Ferranti Atlas □ IBM magnetic tape □ 1961 □ Peripheral equipment □ Punched tape codes □ Magnetic drum □ Operating System □ IBM tape control logic □ Paper tape input □ Operator's I/O □ 1962 □ Memo on extracodes (April) □ Memo on extracodes (May) □ Primary Supervisor □ Intermediate Supervisor □ Notes on Extracodes □ Monitor program □ 1963 □ Processing commercial data □ Magnetic tape □ Intermediate supervisor □ Alternative monitor action □ Non-standard peripherals
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Further reading ⇓

□ Overview
Brochures
□ ICT 665 printer (1960) □ Ampex TM-2 tape deck (1961) □ Anelex printer (1961) □ Creed 300 tape punch (1961)
Papers: 1960
□ Users' specification □ Punched card codes □ Description of the Ferranti Atlas □ IBM magnetic tape
1961
□ Peripheral equipment □ Punched tape codes □ Magnetic drum □ Operating System □ IBM tape control logic □ Paper tape input □ Operator's I/O
1962
□ Memo on extracodes (April) □ Memo on extracodes (May) □ Primary Supervisor □ Intermediate Supervisor □ Notes on Extracodes □ Monitor program
1963
□ Processing commercial data □ Magnetic tape □ Intermediate supervisor □ Alternative monitor action □ Non-standard peripherals

Magnetic Drums for Atlas

9 May 1961

Capacity

Drum transfers are performed on a band of 24 tracks of digits in parallel using fixed block lengths of 512 words called sectors. Six sectors are stored on a band with a gap between sectors equivalent to 32 digits and there are eight bands per drum. Provision is also made for multiple block transfers using consecutive sectors on one band of a drum. The drum diameter is 14 inches and the useful length 12.75 inches.

1  sector                      =     512 forty-eight bit words.
1  band    = 6 sectors         =   3,072 words.
1  drum    = 8 bands           =  24,576 words.
1  cabinet = 4 drums           =  98,304 words.
Maximum capacity = 8 cabinets  = 786,432 words.

Speed

The drums revolve at a nominal 4735 r.p.m. but are not synchronised. This gives a revolution time of 12.67 ms and a transfer rate of

   2 µs per bit per track (24 tracks in parallel) 
   4 µs per forty-eight bit word 
2048 µs per sector.

After the selection of a band or drum different from that selected previously, approximately 1.5 ms is allowed for the selection to settle before a transfer is allowed to start.

Parity checking

With each 24 digits of information on the drum a parity digit is stored (c.f. the parity check on the core store). The parity of each half word is checked on transfers to and from the drum.

Extracodes for Drum Transfers

It is envisaged that most programmers will use the core and drum as a one-level store. However, in certain circumstances it may be useful to be able to specify that a given block of information should be either in the core store or that it can be written to the drum store. For this purpose the following drum transfer extracode orders are provided.

1050 Read block P
If there is no suitable empty page a block of information at present in the core store is written to the drum. If the block is already in the core store nothing is done. The original (drum) copy is lost i.e. the sector originally containing it is indicated as empty in the sector directory.
1051 Write block P
If P is already on the drum nothing is done. The page in the core store originally containing the information is now regarded as free.
1052 Duplicate
Duplicate block P1 calling the copy P2; P2 in core store/drum - depending on whether P1 was on the drum/core store originally.
1053 Rename
Rename block P1 and call it block P2. The original block called P2 (if any) is lost.

Before using extracodes 1054, 1055, 1056, which refer to sector numbers, drums are reserved by a statement in the title of the program. The programmer always numbers the drums allocated to him as drum 0,1 etc.

1054 Read n blocks
Read n blocks beginning at sector s and label the new copies in the core store blocks P onwards.
The transfer continues round the specified band of the drum so that n ≤ 6. The contents of the sectors are retained but any original blocks P to (P + n - 1) are lost.
1055 Write n blocks
Write n blocks from P onwards to n sectors beginning at sector s. The transfer continues round the specified band of the drum so that n ≤ 6.
The blocks are retained in the core store and the original information on the sectors is overwritten. If waiting time between successive transfers in a multiple length transfer to (or from) the drum is to be avoided it is necessary that the sectors concerned are all on the same band, This leads to two limitations in this and the previous instructions
  1. n ≤ 6 and
  2. the sectors written to or read from are all on the same band.
For example, if n=4, P=10, s=3 
block 10 in the core store is written to sector 3
block 11 in the core store is written to sector 4
block 12 in the core store is written to sector 5
block 13 in the core store is written to sector 0 (and not on band 1).
If, therefore, a multiple length transfer involving two (or more) bands is required, it must be specified in two (or more) instructions.
1056 Lose Sector n
The sector n is made empty and becomes available to the one-level store.
1057 Lose block P
The sector or page occupied by block P is made empty.
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