ASME-Landmark:IBM 350 RAMAC Disk File

The IBM 350 RAMAC (Random Access Method of Accounting and Control) disk drive storage development led to the breakthrough of online computer systems by providing the first storage device with random access to large volumes of data. Making information directly available for computer processing on demand meant that no longer would processors stand idle while searches were made through reels of magnetic tape or data was punched into cards and sorted for processing. After its introduction on September 4, 1956, the IBM 350 became the primary computer bulk-storage medium.

Fifty disks stacked on a cantilevered, rotating spindle recorded in one hundred concentric tracks on each side of each disk, providing 5 million characters of data storage. Developments, such as the pressurized-air head bearing, made disk storage feasible. Access to any of the 20,000 tracks could be obtained mechanically through an access arm under servo control that would move two read/write heads to the desired disk.

Revolutionary at the time, the 350's 24-inch disks revolved at a speed of 1,200 revolutions per minute; the resulting data rate was 100,000 bits per second. Its fifty 24-inch disks contained a total capacity of 5 million binary decimal encoded characters, stored 2,000 bits of information per square inch and had an average access time of 600 milliseconds; essentially, it stored one megabyte of information on an area the size of a pool table.

Development of the disk drive was pioneered by IBM engineers in San Jose and led to IBM's first computer manufacturing plant in California. See ASME website for more information