IBM System/360 - System/360 History - Backward Compatibility

Backward Compatibility

IBM's existing customers had a large investment in software that executed on second generation machines. Many models offered the option of emulation of the customer's previous computer (e.g. the IBM 1400 series on a Model 30 or the IBM 7094 on a Model 65) using a combination of special hardware, special microcode and an emulation program that used the emulation instructions to simulate the target system, so that old programs could run on the new machine. However, customers had to halt the computer and load the emulation program. The Model 85 and later System/370 retained the emulation options, but allowed them to be executed under operating system control alongside native programs.