Re:IBM System/360 Model 67
by YZF on Saturday August 22, 2015 @12:55AM
(#10101551)
Not sure when that came historically but I remember the diag instruction that was used to talk to the "hypervisor". E.g. diag 8 was used to issue CP commands from your program. That's at a lower level than CMS which ran in the user's space and interacted with the virtual hardware (and would handle something like file i/o)...
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That's exactly right. CP trapped the "diagnose" instruction in the virtual machine to allow the VM to talk to the hypervisor. The "diagnose" instruction was convenient, since it was an opcode that was never used in normal programs - on a real (non-virtual) machine, it was used by IBM engineers to diagnose hardware problems and did different weird things on each different model of machine.
The normal SVC (supervisor call) instruction was the API for programs to talk to CMS, the operating system that was most commonly run on top of the virtual machines. It was actually possible to run several different operating systems on the virtual machines, including CP (the hypervisor) itself. Systems programmers would do that for testing new versions of CP on a live system.