Editing IBM System/360 (section)

===Backward compatibility===
IBM's existing customers had a large investment in software that ran on [[Transistor computer|second-generation machines]]. Several System/360 models had the option of [[Emulator|emulating]] the customer's existing computer using special hardware<ref name=SY77-6835/> and [[microcode]], and an emulation program that enabled existing programs to run on the new machine.

{| class="wikitable"
|-
! rowspan="2" | System/360 model !! colspan="6" | Emulated systems
|-
|[[IBM 1401|1401]]
|[[IBM 1440|1440]], [[IBM 1460|1460]]
|[[IBM 1410|1410]], [[IBM 7010|7010]]
|[[IBM 7070|7070, 7072, 7074]]
|[[IBM 7080|7080]]
|[[IBM 709|709]],<br />[[IBM 7090|7090, 7094, 7094 II]],<br />[[IBM 7040|7040, 7044]]
|-
| [[IBM System/360 Model 20|Model 20]]|| {{yes}}
|
|
|
|
|
|-
| [[IBM System/360 Model 30|Model 30]]|| {{yes}}
|{{yes}}
|
|
|
|
|-
| [[IBM System/360 Model 40|Model 40]]|| {{yes}}
|{{yes}}
|{{yes}}
|
|
|
|-
| [[IBM System/360 Model 50|Model 50]]|| {{yes}}
|{{yes}}
|{{yes}}
|{{yes}}
|
|
|-
| [[IBM System/360 Model 65|Model 65]]||
|
|
|{{yes}}
|{{yes}}
|{{yes}}
|-
| [[IBM System/360 Model 85|Model 85]] ||
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|{{partial|Under OS control}}
|}

Customers initially had to halt the computer and load the emulation program.<ref>{{cite book
 | publisher = [[IBM]]
 | title     = System/360, Model 30 1401 Compatibility Feature
 | id        = A24-3255-1
 | date      = April 1964
 | url       = http://www.bitsavers.org/pdf/ibm/360/1401_emulator/A24-3255-1_Model30_1401_Compatibility_Apr64.pdf
 | quote     = Mode status (System/360, Model 30, mode or 1401 compatibility mode) is set during the read-in of the compatibility initialization deck.}}</ref>
IBM later added features and modified emulator programs to allow emulation of the 1401, 1440, 1460, 1410 and 7010 under the control of an operating system.
The Model 85 and later System/370 maintained the precedent, retaining emulation options and allowing emulators to run under OS control alongside native programs.<ref>{{cite book
 | publisher = [[IBM]]
 | title     = Emulating the IBM 7094 on the IBM Models 85 and 165 using OS/360 - Program Number for M/85: 360C-EU-734 - Program Number for M/165: 360C-EU-740 - OS Release 20
 | id        = GC27-6951-2
 | edition   = Third
 | date      = November 1971
 }}
</ref><ref>{{cite book
 | publisher = [[IBM]]
 | title     = 7094 OS Emulator on Models 165/168 Reference - Program Number for OS/MFT and OS/MVT - 360C-EU-740 and Program Number for OS/VS1 and OS/VS2 5744-AM1
 | id        = GC27-6983-0
 | edition   = First
 }}
</ref>