Editing IBM System/360 (section)

==="Family" concept===
Producing a single system design with support for all of these features, at a price acceptable to low-end customers and with a performance level acceptable to high-end customers, would border on impossible. Instead, the SPREAD concept was based on the separation of the defined feature set from its internal operation, with a family of machines with different performance and different internal designs. 

Specifically, depending on the machine, some components might not be directly implemented in hardware, and would instead be completed using small programs referred to as [[microcode]] or microprograms. These small programs, or subprograms, would be stored in [[read only memory]] (ROM)<ref group=NB>The original models used [[transformer read-only storage]] (TROS) and capacitor read-only storage (CROS)</ref> inside the machine. Some models<ref group=NB>[[360/20]], [[360/22]], [[360/25]], [[360/30]], [[360/40]], [[360/50]], [[360/65]], [[360/67]], [[360/85]]</ref> use microcode in the [[central processing unit]] (CPU) to implement instructions while others<ref group=NB>[[360/44]], [[360/75]], [[360/91]], [[360/95]], [[360/195]]</ref> use only hardware. Some models<ref group=NB>[[360/20]], [[360/22]], [[360/25]], [[360/30]], [[360/40]], [[360/50]]</ref> use [[cycle stealing|cycle-stealing]] microcode in the CPU to implement I/O channels while others<ref group=NB>[[360/44]], [[360/65]], [[360/75]], [[360/85]], [[360/91]], [[360/95]], [[360/195]]</ref> use only hardware in separate<ref group=NB>Originally 2860 and 2870, but IBM later announced the 2880 to support the [[History of IBM magnetic disk drives#IBM 2305|2305]] on the [[360/85]] and [[360/195]].</ref> units. Today this approach is known as [[microcode]].<ref>{{cite journal|last1=Tucker|first1=S. G.|title=Microprogram control for SYSTEM/360|doi=10.1147/sj.64.0222|journal=IBM Systems Journal|volume=6|issue=4|pages=222–241|year=1967}}</ref>

This meant that a single lineup could have machines tailored to match the price and performance niches that formerly demanded entirely separate computer systems, where software was specific to each system. This flexibility greatly lowered barriers to entry. With most other vendors customers had to choose between machines they might outgrow or machines that were potentially too powerful and thus too costly. In practice, this meant that many companies simply did not buy computers. Now, a customer could purchase a machine that solved a particular requirement, knowing they could switch models as their needs changed, without losing support for the programs they were already running.<ref name= gamble/>

For instance, in the case of a firm that purchased an accounting system and was now looking to expand their computer support into engineering, this meant they could develop and test their engineering program on the machine they already used. If they ever needed more performance, they could purchase a machine with floating-point hardware, knowing that nothing else would change, it would simply get faster. Even the same peripherals could be used, allowing, for instance, data from the engineering system to be written to tape and then printed using a high-speed [[line printer]] already connected to their accounting system. Or they might replace the accounting system outright with a system with the performance to run both tasks.<ref name= gamble/>

The idea that a single design could address all the myriad ways that the machines could be used gave rise to the name, "360" is a reference to 360 degrees in a circle, and circles of machines and components featured prominently in IBM's advertising.<ref name= gamble/>