IBM System/370 Model 155
| Industry | Computer hardware |
|---|---|
| Founded | June 16, 1911 in Endicott, New York, United States |
| Products | System/370 Model 155, System/370 Model 165 |
The IBM System/370 Model 155 and Model 165 were unveiled simultaneously on June 30, 1970, as part of IBM's System/370 mainframe series designed to meet the business computing demands of the 1970s. Alongside these, IBM also launched the System/370 Model 195 on the same date. These three systems represented the inaugural release within the System/370 lineup.
Significantly, these initial models lacked virtual memory—a feature that would later become a defining characteristic of the System/370 family—leading some analysts to consider the 155 and 165 as partial implementations rather than fully realized System/370 machines.
Three months following their debut, IBM broadened the product range by introducing the System/370 Model 145, which incorporated virtual memory capabilities. This enhancement made it more attractive to corporate clients requiring sophisticated computing functionality.
Virtual memory
The original System/370 Models 155 and 165 did not come equipped with virtual memory support.
In 1972, IBM offered an upgrade enabling virtual memory functionality. Unlike the IBM System/370 Model 145, which featured built-in virtual memory hardware from its June 1971 release and received a microcode update in 1972, Models 155 and 165 demanded significant hardware investments—$200,000 for the 155 and $400,000 for the 165—to activate virtual memory. The upgraded version of the 155 was referred to as IBM System/370 Model 155-II. These enhancements were not formally introduced as separate models; instead, they were part of the broader "System/370 Advanced Function" announcement that also included Models 158 and 168.
Physical memory
The joint announcement for Models 155 and 165 detailed specifications regarding physical memory, storage components (both main memory and disk), and a cache system identified as "buffer."
Model 155 provided seven main memory configurations ranging from 256 KB up to 2 MB; Model 165 offered five options spanning from 512 KB to 3 MB. Both systems incorporated high-speed buffer storage designed to support substantial main memory capacities.
It is worth noting that the 3155 utilized traditional ferrite core storage technology rather than more modern monolithic main storage. The associated 3360 disk units each offered either 256 or 512 KB capacity, with a maximum of four units attachable per system. Unlike subsequent models where microcode could be loaded via floppy disks facilitating easier updates, the microcode in the 3155 was fixed in hardware. Additionally, this model provided limited error logging capabilities and employed an IBM 3215 typewriter console for operator interaction.
Channels
Channel configuration included Channel 0 as a byte multiplexor channel, while Channels 1 through 5 functioned as block multiplexor channels. Channels 0 and 1 were standard features; Channels 2 through 5 were available as optional enhancements.
Operating systems
The System/370 Model 155 supported both DOS/360 and OS/360. Although part of the System/370 family, Models 155 and 165 initially lacked virtual memory support, which prevented them from running operating systems that required virtual memory.
Following its upgrade to Model 155-II with virtual memory enabled, supported operating systems expanded to include DOS/VS, OS/VS1, OS/VS2 Release 1 (SVS), Release 2 (MVS), as well as VM/370.