VAX420: Added ROM patch to correct boot problem on MicroVAX 3100 M10/M20

VAX/ka41a_patch.com

@@ -0,0 +1,24 @@
+$!
+$! This procedure patches KA41A.BIN (V1.6) Boot ROM image to work under
+$! the SIMH simulator
+$!
+$ PATCH /ABSOLUTE /NEW_VERSION /OUTPUT=KA41A.BIN KA41A_ORIG.BIN
+!
+! Test D - NVR
+!
+!   - This appears to be a bug in the ROM code, which
+!     causes an endless loop if the NVR is not initialised.
+!     The subroutine loops until a particular value is found
+!     in the SIE register however the SIE is a constant and
+!     never matches the expected value. There are no other
+!     exit conditions from this loop. The KA41-D V1.0 ROM
+!     has the same subroutine however the loop is exited
+!     when the expected value is not found in the SIE.
+!
+REPLACE/INSTRUCTION 0284C = 'BNEQ    0000280E'
+'BNEQ    0000285B'
+EXIT
+!
+UPDATE
+EXIT
+$

VAX/vax420_sysdev.c

@@ -1022,9 +1022,6 @@ if (*rom == 0) {                                        /* no boot? */
 #if defined (VAX_41A) || defined (VAX_41D)
 rom_wr_B (ROMBASE+4, sys_model ? 2 : 1);                /* Set Magic Byte to determine system type */
 #endif
-#if defined (VAX_41A)
-rom_wr_B (ROMBASE+7, 8);                                /* ROM goes into endless loop without this? */
-#endif
 for (i = 0; i < OR_COUNT; i++)                          /* unmap all option ROMs */
     or_unmap (i);
 for (i = 0; (cdptr = sim_devices[i]) != NULL; i++) {    /* loop over all devices */