IBM Mainframe Magnetic Storage Media

Magnetic Media
Photo: Frank da Cruz, Jul 2010; Click to enlarge

Left is the well-known 10.5-inch diameter half-inch wide reel of 9-track magnetic tape, with autoloading tape seal (this one happens to be a Kermit software distribution tape; we used to send out dozens of these a week before there was an Internet). The Computer Center had a library of thousands of these tapes, complete with tape librarian; users could "mount" them from the IBM mainframes or DEC-20s (a mount request caused a message to appear on the operator console; the operator would fetch the desired tape and mount it on a free drive and then reply to the request, which granted access to the user). CLICK HERE to see a typical 9-track tape drive.

9-track tapes were used on almost every kind of minicomputer and mainframe throughout the 1970s and 80s and well into the 1990s for backup, data archiving, and data interchange. Physically, at least, they were well standardized for interchange. Of course the logical formats differed (ASCII, EBCDIC; OS, ANSI, BACKUP, DUMPER, TAR, CPIO; fixed records, variable records, etc), so we found or wrote conversion programs. If the tape was mounted with a write-ring, the user could write on the tape; otherwise it was read-only. The operator would insert the ring according to the mount request unless the tape had a NO RING sticker.

9-track tapes could be recorded in different densities: 800 bpi (bits per inch), 1600 bpi, and 6250 bpi; the higher the density, the newer the technology. 1600 and 6250 bpi tape drives could recognize lower densities (e.g. a 1600 bpi drive could read an 800 bpi tape). "9-track" refers to recording 8 data bits plus one parity bit across the tape (edge to edge). Thus 1600 bpi is the same as 1600 cpi (characters to per inch). At 1600 bpi, a 2400-foot 9-track tape could hold about 50MB (depending on blocking, etc). In earlier times there were 4-track and 7-track drives at lower densities such as 200 and 556 bpi.

In the 1990s, 9-track tapes gave way to a panoply of more-compact higher-density cartridge media: IBM (3850?) "square tapes", DEC TK50 and TK70, QIC, 8mm and 4mm DAT, and so on, most of which rapidly vanished from the scene.

The remaining items are peculiar to IBM. On top is an IBM MSS cartridge (1982), which holds 50MB. There were 2000 such cartridges in our MSS (plus some spares), staged to four IBM 3350 317.5MB disk drives (which ran in 3330 Mod I (100MB) emulation mode since the MSS did not support 3350s).

On the right is a tape strip from the IBM Data Cell, the precursor to the MSS cartridge, mid-1960s, capacity: 200,000 bytes. This is the last remaining relic of Columbia's Data Cell.


Frank da Cruz / fdc@columbia.edu / Columbia University Computing History / May 2001 / Updated July 2010