The 2301 drum was direct access storage, the same as a disk drive, but had one fixed head per track.
Whereas with a disk drive the head moved to the track, (which relatively speaking took a long time)
then there was the rotational delay until the required data block arrived at the head,
with the drum there was no move just rotational delay. This made the drum access time a tenth of a disk drive.
In addition the data was read/written four bits simultaneously from four tracks. So the data transfer rate
was four times that of a disk drive. This is a very fast device.
The drum was not used to store ordinary user programs or data but as an extension of main memory.
The 1MByte of main memory and the 4MByte of drum storage was formatted as 2KByte pages and the whole,
along with special hardware and software created a large virtual memory to support interactive terminals.
We do not have a 2301 drum but do have one of the read/write head assemblies.
| Mus.Cat. NEWUC:2004.30 | Mnfctr: IBM | Date: 1967 | No: 629 16 | - |
| Comp: Drum head assmy | LENGTH: 148 mm | WIDTH: 66 mm | DEPTH: 47 mm | WEIGHT: 358 g |
Each assembly had 11 pads with a pair of read/write gaps.
There were 10 bays around the drum with 4 assemblies in each bay making 880 tracks per 2301.