Rico Pajarola <rp(a)servium.ch> wrote: The 4000/300 works like any other uVAX (VAX processor implemented in a single IC) in this respect: a halt is a special exception that works like regular VAX macrocode exceptions except that it saves the PC and the PSL in special IPRs rather than on the stack (so that it doesn't depend on a stack), vectors to the hardwired ROM address 20040000 rather than through SCB (so that it doesn't depend on SCB), and MAPEN is cleared (so that it goes to the physical address 20040000 in the ROM rather than whatever that address may be virtually). The CPU board firmware than displays the halt message and the >>> prompt as prescribed by the VAX Architecture (DEC STD 032-0). KA650/655 works exactly the same way. While I haven't played with simh and don't intend to in the near future (due to my general lack of interest in emulators), thus I don't claim to be right here, but from your session logs it appears that simh is not doing the above uVAX halt exception but instead it handles halts like an 11/780, by "physically" stopping all instruction execution. That is fine and dandy, the 11/780 way is certainly cooler, but the problem is it ain't a KA655 any more, and one should not expect the KA655 boot ROM to work right then (and IMAO it has no right to report the KA655 SID code either). When I took a cursory look at SIMH/VAX, which consisted only of glancing at its brief documentation, I read that the KA655 ROM image it comes with is not the real one but hacked to work with the poor emulator. This suggests to me that you are able to boot some OSes because Bob has gutted the (quite complicated) KA655 console and boot logic to the bare minimum he had emulated. I wouldn't be surprised if he hacked out the KA655 diagnostic executive almost entirely, so that on "power-up" one goes through an absolutely minimal init sequence without a single halt/unhalt cycle occuring (of which on a real KA655 several occur on every power-up during diagnostics), gets to the >>> prompt, and then when booting, runs VMB, unhalts once (the SIMH probably doesn't even know that it unhalts, as the uVAX unhalt is simply an RFI: on real hardware it knows that this RFI is an unhalt and lights the RUN LED because the SSC chip compares instruction fetch addresses against the halt range and detects the unhalt, but I doubt that SIMH emulates this), and then it all goes well (cross your fingers three times!) never goes through a proper KA655 halt/unhalt again. But if something goes wrong in the boot and VMB's error handler gets invoked, SIMH's very poor man's emulation is not prepared for it. But the rub is that because of SIMH talking the KA655 talk but not walking the KA655 walk, you can't see what exactly is the problem with your bootblocks: VMB's error messages aren't displayed. MS