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.
# Conflicts:
# VAX/vax_cpu.c
