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Originally, the VAX allowed immediate operands (8F) to be used without restrictions in address mode instructions, either standalone or indexed. Starting with MicroVAX II, immediate indexed became reserved. This remained true for all subsequent chip implementations. The SRM was ECOed in March, 1985 to make immediate indexed unpredictable. In MicroVAX II, immediate g-floating operands didn't work correctly. The problem was found a couple of months after tape-out. While the index flows could be fixed, and were fixed according to the microcode revision history: ; 7-May-84 [RMS] Fixed FD problem in index flows (JLR) the problem in indexed immediate could only be fixed by a significant hardware change in an area that was already packed full. The VAX Architecture Team, which had always been very sympathetic to the VAX chip efforts, proposed a much simpler solution: make immediate indexed unpredictable. It was useless, in any case. I'm rather surprised that this wasn't flagged by the 780 diagnostics. Maybe it was never tested. It was tested in HCORE (the original MicroVAX I core diagnostic that is failing), but I removed it subsequently: ; 8-may-85 rms removed indexed immediate tests Bottom line - the simulator is right for the chip VAXes (including, I think, V11) and wrong for MicroVAX I and probably the 8600, 780, 750, and 730.