[Simh] Problem with MT_ASTLVL on the VAX-11/780

[Matt Burke]

Whilst experimenting with a new device on VAX/VMS V4.5 I got a fatal
bugcheck (exception whilst above ASTDEL). The problem turned out the be
a reserved operand fault and I traced it back to the following code
(shown here from the VMS 3.0 source listings):

MOVL    PCB$L_PHD(R0),R0        ; Get PHD address
MOVB    PHD$B_ASTLVL(R0),R0     ; And fetch ASTLVL
MTPR    R0,#PR$_ASTLVL          ; Update current value

As you can see R0 will contain an S0 address after the first
instruction, then only the lower byte is overwritten with the new AST
level. The whole longword is then written to the ASTLVL IPR. Currently
the test for this IPR in vax_cpu1.c is:

if (val > AST_MAX)
    RSVD_OPND_FAULT;

Due to S0 addresses having bit 31 set this triggers the reserved operand
fault. The VMS code is clearly expecting a mask to be applied to the
value. I've checked various manuals to try and find the answer but the
results were not totally conclusive.

EB-17835-18 - VAX-11/780 Hardware Handbook (1979)
    http://manx-docs.org/details.php/1,8111
    Page 71 - Bits <31:3> ignored, returns 0

EK-VAXV2-HB-002 - VAX Maintenance Handbook (VAX-11/780) (Mar 1983)
    http://manx-docs.org/details.php/1,2826
   
http://www.bitsavers.org/pdf/dec/vax/780/EK-VAXV2-HB-002_vaxMaintHbk_Mar83.pdf
    Page 119 - Reserved operand fault if not valid i.e. MBZ /= 0

EP-ES0AA-DL-124 - VAX-11/780 Microcode (Jan 1982)
   
http://www.bitsavers.org/pdf/dec/vax/780/fiche/EP-ES0AA-DL-124_4of6_780uCode_Jan82.pdf
    Page 678 - A mask of 0x7 is applied at line 25908

I've also looked at the VAX-11/750 and came up with similar results:

EK-VAXV3-HB-001 - VAX Maintenance Handbook (VAX-11/750) (Mar 1983)
    http://manx-docs.org/details.php/1,2827
    Page 20 - Reserved operand fault if not valid i.e. MBZ /= 0

CMT098.MCX - VAX-11/750 Microcode (Nov 1983)
   
http://www.bitsavers.org/pdf/dec/pdp11/xxdp/fiche_200dpi/0187_CMT098.MCX_750uCod.pdf
    Page 855 - A mask of 0x7 is applied at line 35030

I guess the microcode listings are the ones to trust and the two VAX
Maintenance Handbooks are probably incorrect. I've prepared a fix to add
a machine specific test for the AST level IPR but I just wanted to open
this up for comments before sending a pull request in case I might have
overlooked something.

Matt