First Disk Storage Unit – 1956 AD

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First Disk Storage Unit

Reynold B. Johnson (1906–1998)

“Faster than tape but slower than main memory, magnetic disk drives have been an important part of computing since they were invented by IBM and publicly demonstrated on September 14, 1956.

The IBM 305 RAMAC (Random Access Method of Accounting and Control) was designed to store accounting and inventory files that had previously been stored as boxes of IBM punch cards or on tape. To do this, the RAMAC shipped with the IBM 350 disk storage unit, a new device that stored data on 50 spinning disks, each 24 inches (61 centimeters) in diameter and revolving at 1,200 revolutions per minute (RPM). Arranged in 100-character blocks that could be randomly accessed, read, and rewritten, the RAMAC made it possible for a computer with only a few kilobytes of main memory to rapidly access 5 million characters—the equivalent of 64,000 punch cards.

Unlike modern drives, which have a head for every disk, the RAMAC had a single head that moved up and down to select the disk, and then in and out to select the specific block where data would be read or written. The average access time was six-tenths of a second.

The RAMAC also came equipped with a rotating drum memory that spun at 6,000 RPM and stored 3,200 characters on 32 tracks of 100 characters each.

Over the 60 years that followed, the capacity of disk-drive systems increased from 3 megabytes to 10 terabytes—a factor of 3 million—thanks to improvements in electronics, magnetic coatings, drive heads, and mechanical head-positioning systems. But the time it takes for the disk to reposition its head to read the data, something called the seek time, only dropped from an average of 600 milliseconds to 4.16 milliseconds, a factor of just 144. That’s because reducing seek times depended on improving mechanical systems, which, unlike electronics, are subject to constraints resulting from friction and momentum: in all the years since the RAMAC was introduced, rotation rates have only increased from 1,200 RPM to 10,000 RPM for even the most expensive hard drives.”

SEE ALSO Magnetic Tape Used for Computers (1951), Floppy Disk (1970), Flash Memory (1980)

“The RAMAC actuator and disk stack, with fifty 24-inch (61-centimeter) disks spinning at 1,200 RPM, held 5 million characters of information.”

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