The IBM 305 RAMAC was the first commercial computer that used a moving-head hard disk drive (magnetic disk storage) for secondary storage. The system was publicly announced on September 14, 1956, with test units already installed at the U.S. Navy and at private corporations. RAMAC stood for "Random Access Method of Accounting and Control", as its design was motivated by the need for real-time accounting in business.
RAMAC was developed and manufactured at IBM's research facility in San Jose, California. In 1959, IBM's CEO Thomas J. Watson Jr. exhibited the RAMAC in Moscow. This led to a visit by Soviet leader Nikita Khrushchev to IBM's San Jose facility.
The first RAMAC to be used in the U.S. auto industry was installed at Chrysler's MOPAR Division in 1957. It replaced a huge tub file which was part of MOPAR's parts inventory control and order processing system.
During the 1960 Olympic Winter Games in Squaw Valley (USA), IBM provided the first electronic data processing systems for the games. The system featured an IBM RAMAC 305 computer, punched-card data collection, and a central printing facility.
More than 1,000 systems were built. Production ended in 1961; the RAMAC computer lost front-runner status in 1962 when the IBM 1405 Disk Storage Unit for the IBM 1401 was introduced, and the 305 was withdrawn in 1969.
The first hard disk unit was shipped September 13, 1956. The additional components of the computer were a card punch, a central processing unit, a power supply unit, an operator's console/card reader unit, and a printer. There was also a manual inquiry station that allowed direct access to stored records. IBM touted the system as being able to store the equivalent of 64,000 punched cards.
The 305 was one of the last vacuum tube computers that IBM built. It weighed over a ton.
The IBM 350 disk system stored 5 million alphanumeric characters recorded as six data bits, one parity bit and one space bit for eight bits recorded per character. It had fifty 24-inch-diameter (610 mm) disks. Two independent access arms moved up and down to select a disk, and in and out to select a recording track, all under servo control. Average time to locate a single record was 600 milliseconds. Several improved models were added in the 1950s. The IBM RAMAC 305 system with 350 disk storage leased for US$3,200 (equivalent to $36,700 in 2025) per month.
The original 305 RAMAC computer system could be housed in a room of about 9 m (30 ft) by 15 m (50 ft); the 350 disk storage unit measured around 1.5 square meters (16 sq ft). Currie Munce, research vice president for Hitachi Global Storage Technologies (which has acquired IBM's hard disk drive business), stated in a Wall Street Journal interview that the RAMAC unit weighed over a ton, had to be moved around with forklifts, and was delivered via large cargo airplanes. According to Munce, the storage capacity of the drive could have been increased beyond five megabytes, but IBM's marketing department at that time was against a larger capacity drive, because they did not know how to sell a product with more storage.
Programming the 305 involved not only writing machine language instructions to be stored on the drum memory, but also almost every unit in the system (including the computer itself) could be programmed by inserting wire jumpers into a plugboard control panel.
System architecture was documented in the 305 RAMAC Manual of Operation.
The 305 was a character-oriented variable "word" length decimal (BCD) computer with a drum memory rotating at 6000 RPM that held 3200 alphanumeric characters. A core memory buffer of 100 characters was used for temporary storage during data transfers.
Each character was six bits – plus one odd parity bit ("R") – composed of two zone bits ("X" and "O") and remaining four binary bits for the value of the digit in the following format:
Instructions could only be stored on 20 tracks of the drum memory and were fixed length (10 characters), in the following format:
Fixed-point data "words" could be any size from one decimal digit up to 100 decimal digits, with the X bit of the least significant digit storing the sign (signed magnitude).
Data records could be any size from one character up to 100 characters.
The drum memory was organized into 32 tracks of 100 characters each.
The color code of this table is: