The GE 645 also had an execute double instruction, which would execute pairs of instructions. I think there were severe constraints on the even/odd-ness of the source, and on the overlaps (none allowed) between segments. Truly a nightmare for hardware. Not sure it ever really worked.
On Mar 15, 2018, at 8:13 PM, Henry Baker <hbaker1@pipeline.com> wrote:
Problems with XCT (execute one instruction):
* no clean/obvious semantics; semantics have to be worked out for each different instruction being "executed"
* only one level of execute (at least for IBM)
* originally used for *tables* of instructions, but what about variable-length instructions? (Similar problem to "skip" instructions)
* what are the semantics in the case of interrupts?
We now have architectures which support "virtual machines"; analogous problems at much larger scale.
At 04:47 PM 3/15/2018, rcs@xmission.com wrote:
PS: Does anyone else miss the XCT instruction? The 7094 & PDP6/10 had it, but it seems to have vanished. I guess it's an architectural nightmare for the hardware folks. --Rich
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