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.
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.
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.
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.
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.
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.