Magnetic disk storage was a critical component of the computer revolution. IBM was a pioneer in this area. This article surveys the major IBM computer disk drives introduced in the 1950s, 1960s and early 1970s.
The basic mechanical arrangement of hard disk drives hasn't changed since the IBM 1301. Disk drive performance characteristics are specified the same way today as they were in the 1950s. A modern (2004) PC hard drive is included for comparison. The reader may wish to calculate the ratio of cost-per-megabyte between the IBM 1301 and the 2004 PC hard drive. Few products in history have enjoyed such a meteoric decline in cost and size with such a stellar improvement in capacity.
The IBM 350 was part of the IBM RAMAC 305, the computer that introduced disk storage technology to the world on September 4, 1956. RAMAC stood for "Random Access Method of Accounting and Control." The 350 stored 5 million characters. It had fifty 24-inch diameter disks with 100 recording surfaces. Each surface had 100 tracks. The disks spun at 1200 RPM. Data transfer rate was 8,800 characters per second. 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. A third arm was added as an option. Several improved models were added in the 1950s. The IBM RAMAC 305 system with 350 disk storage leased for $3,200 per month. The 350 was officially withdrawn in 1969.
The 350's cabinet was 60 inches long, 68 inches high and 29 inches deep. IBM had a strict rule that all its products must pass through a standard 29.5 inch doorway. This rule presumably dictated the maximum diameter of the disks.
The IBM 355 was announced on September 14, 1956 as an addition to the popular IBM 650. It used the same mechanism as the IBM 350 and stored 6 million decimal digits. Data was transferred to and from the IBM 653 magnetic core memory, an IBM 650 option that stored just sixty 10-digit words, enough for a single sector of disk or tape data.
The IBM 1405 Disk Storage Unit was announced by 1961 and was designed for use with the IBM 1401 series medium scale business computers. The 1405 stored 10 million characters on a single module. Each module had 25 large disks, yielding 50 recording surfaces. The disks spun at 1200 RPM. The Model 1 had one module, the Model 2 had two modules, stacked vertically. Each recording surface had 200 tracks and 5 sectors per track. Data was read or recorded at 22,500 characters per second. A single arm moved in and out and up and down. Access time ranged from 100 to 800 milliseconds (Model 2).
The IBM 1301 Disk Storage Unit was announced on June 2, 1961. It was designed for use with the IBM 7000 series mainframe computers and the IBM 1410. The 1301 stored 28 million characters on a single module (25 million characters with the 1410). Each module had 20 large disks and 40 recording surfaces, with 250 tracks per surface. The 1301 Model 1 had one module, the Model 2 had two modules, stacked vertically. The disks spun at 1800 RPM. Data was transferred at 90,000 characters per second.
A major advance over the IBM 350 and IBM 1405 was the use of a separate arm and head for each recording surface, with all the arms moving in and out together like a big comb. This eliminated the time needed for the arm to pull the head out of one disk and move up or down to a new disk. Seeking the desired track was also faster since, with the new design, the head would usually be somewhere in the middle of the disk, not starting on the outer edge. Maximum access time was reduced to 180 milliseconds.
The 1301 also featured heads that were aerodynamically designed to fly over the surface of the disk on a thin layer of air. This allowed them to be much closer to the recording surface, which greatly improved performance.
The 1301 was connected to the computer via the IBM 7631 File Control. Different models of the 7631 allowed the 1301 to be used with a 1410 or 7000 series computer or shared between a 7000 and a 1410 or between two 7000's.
The IBM 1301 Model 1 leased for $2,100 per month or could be purchased for $115,500. Prices for the Model 2 were $3,500 per month or $185,000 to purchase. The IBM 7631 controller cost an additional $1,185 per month or $56,000 to purchase. All models were withdrawn in 1970.
The IBM 1302 Disk Storage Unit was introduced in September 1963. Improved recording quadrupled its capacity over that of the 1301, to 117 million 6-bit characters per module. Average access time was 165 ms and data could be transferred at 180 K characters/second, more than double the speed of the 1301. A second arm accessed a separate group of 250 tracks. As with the 1301, there was a Model 2 with twice the capacity. The IBM 1302 Model 1 leased for $5,600 per month or could be purchased for $252,000. Prices for the Model 2 were $7,900 per month or $355,500 to purchase. The IBM 7631 controller cost an additional $1,185 per month or $56,000 to purchase. The 1302 was withdrawn in February 1965.
The IBM 1311 Disk Storage Drive was announced on October 11, 1962 and was designed for use with the IBM 1401, IBM 1440, IBM 1620 and IBM 1710 medium-scale business and scientific computers. The 1311 was about the size and shape of a top-loading washing machine and stored 2 million characters on a removable IBM 1316 disk pack. Each disk pack was 4 inches high, weighed 10 pounds and contained six 14-inch diameter disks, yielding 10 recording surfaces (the outer surfaces were not used). The disks spun at 1500 RPM. Each recording surface had 100 tracks with 20 sectors per track. Each sector stored 100 characters. Several models of the 1311 were introduced during the 1960s. They were withdrawn during the early 1970s.
The IBM 1316 Disk Packs were covered with a clear plastic shell and a bottom cover when not in use. A lifting handle in the top center of the cover was rotated to release the bottom cover. Then the top of the 1311 drive was opened and the plastic shell was lowered into the disk drive opening (assuming it was empty). The handle was turned again to lock the disks in place and release the plastic shell, which was then removed and the drive cover closed. The process was reversed to remove a disk pack.
The IBM 2311 Direct Access Storage Facility was introduced in 1964 for use throughout the System/360 series. It was also available on the IBM 1130. The 2311 mechanism was largely identical to the 1311, but recording improvements allowed higher data density. The 2311 stored 7.25 million bytes on a single removable IBM 1316 disk pack (the same type used on the IBM 1311). Each recording surface had 200 tracks. Average seek time was 85 ms. Data transfer rate was 156 K bytes/sec.
Because the 2311 was to be used with a wide variety of computers within the 360 product line, its electrical interconnection was standardized. This created an opportunity for other manufacturers to sell plug compatible disk drives for use with IBM computers and an entire industry was born.
The IBM 2314 Disk Storage Drive was introduced on April 22, 1965, one year after the System/360 introduction. It was used with the System/360 and the System/370 lines. The 2314 mechanism was similar to the 2311, but further recording improvements allowed higher data density. The 2314 stored 29.2 million characters on a single removable IBM 2316 disk pack. Each disk pack was contained eleven 14-inch diameter disks, yielding 20 recording surfaces. Each recording surface had 200 tracks. Access time was initially the same as the 2311, but later models were faster. Data transfer rate was doubled to 310 K bytes/sec.
The IBM 2316 disk pack was similar in design to the 1316, but higher and heavier.
The IBM 2310 Removable Cartridge Drive was introduced with the IBM 1130 in 1965. It could store 512,000 words (1,024,000 bytes) on a IBM 2315 cartridge. A single disk spun in a plastic shell with openings for the read/write arm and head.
The IBM 3330 Direct Access Storage Facility, code named Merlin, was introduced in June 1970 for use with the IBM System/370 and the IBM System 360/195. Its removable disk packs held 100 megabytes (the 1973 Model 11 featured IBM 3336 Disk Packs that held 200 megabytes). Access time was 30 millisecond and data transferred at 806 K bytes/sec. A major advance introduced with the 3330 was the use of error correction, which made the drives more reliable and reduced costs because small imperfections in the disk surface could be tolerated. The circuitry could correct error bursts up to 11 bits long. The 3330 was withdrawn in 1983.
The IBM 3340 Direct Access Storage Facility, code named Winchester, was introduced in March 1973 for use with IBM System/370. It was named after the famous 30-30 Winchester rifle. Its removable disk packs were sealed and included the head and arm assembly. There was no cover to remove during the insertion process. Access time was 25 millisecond and data transferred at 885 K bytes/sec. Three versions of the removable IBM 3348 Data Module were sold, one with 35 megabyte capacity, another with 70 megabytes, the third also had 70 megabytes, but with 500 kilobytes under separate fixed heads for faster access. The 3340 also used error correction. It was withdrawn in 1984.
The floppy disk
Another important IBM innovation was little noticed when it was introduced with the System/370 in 1971. IBM needed a way to load new microcode into the 370 and developed the floppy disk for this purpose. These first floppies were 8 inches in diameter and held 80 K bytes of data. Floppies were not used for regular program or data storage on the 370, but they became key to the development of the personal computer in the late 1970s.
Disk storage in 2004
IBM sold its disk drive operation to Hitachi in 2002. For comparison purposes, an Hitachi Deskstar 7K250 PC hard drive stores 250,000,000,000 bytes (250 Gigabytes) on three 3.5-inch diameter platters spinning at 7200 RPM. It has a sustained average transfer rate of 61,400,000 bytes per second over a serial ATA bus. The average seek time is 8.5 milliseconds. It weighs 640 grams (1.4 lb). Like all 3.5 inch hard drives, it's about as long as the carrying handle on an IBM 1316 disk pack. You could buy one retail in July 2004 for about $200.
- A Quarter Century of Disk File Innovation (http://www.research.ibm.com/journal/rd/255/ibmrd2505ZC.pdf) – IBM Journal of Research and Development, 1981 (PDF)