[simh] RSTS processor identification

Johnny Billquist bqt at softjar.se
Fri Mar 5 17:02:45 CST 2021

Nice writeup, Paul. And very interesting.

Just in case anyone wonder about RSX, here is how it's done in M+:

1. Test if SYSID register exists
    If SYSID register exists:
    2. Test if high bit of KISDR0 can be set and read back
       If high bit can be set and read back => 11/74 CPU
       If high bit cannot be set and read back => 11/70 CPU
3. Try MFPT instruction
    If that succeeds:
      If R0 == 1 => 11/44 CPU
      If R0 == 3:
        4. Try to read maintenance register
           If register exists => XT CPU (Pro)
           5. If register does not exist, try writing to SWR
              If fail to write => 11/23 CPU
              If succeed to write => 11/24 CPU
      If R0 is something else, it is a J11 CPU, see more below.
6. Execute OP-codes 076600,000400
    If that succeeds => 11/60 CPU
    If that fails:
    7. Execute OP-code 106700
       If that succeeds => 11/34 CPU
       If that fails:
       8. Try to read PIRQ register
          If that succeeds => 11/45 CPU
          If that fails:
            CPU is one of: 11-/04/05/10/15/20/40
            M+ will just assume 11/40, since that is the only possible
            model that could possibly be running this code. => 11/40 CPU

For J11 processors, after point 3, we get into a J11 probing.
9. If R0 <> 5, it is not a J11 processor after all. => Unknown CPU
10. Read maintenance register
     If fail => Unknown CPU
11. Check bits 4-7 of maintenance register:
     == 4: => 11/53 CPU
     == 3: => 11/73 CPU (not KDJ11)
     == 1: Write KISDR7+1 to KISDR7+1
             Check if W bit in KISDR7 was set.
               If set => M11 CPU
             Try opcodes 076660,156227
               If succeed => N11 CPU
     == 2: => 11/83 or 11/84 CPU (see step 12)
     == 5: => 11/93 or 11/94 CPU (see step 12)
12. Check if Unibus system based on maintenance register
     If Unibus system indicated, try read Unibus map register
       If Unibus map exist: => Unibus system. CPU 11/84 or 11/94 (see 11)
13. Qbus system. CPU 11/83 or 11/93 (see 11)

Note: M11 processor is called 11/95
Note: N11 processor is called 11/97

That concludes how RSX-11M-PLUS decides what CPU you have at boot.

There are then probes for TOY, clock and memory, but that's a different 

If anyone wants more information, the code is in LB:[12,10]SAVSIZ.MAC, 
routine $STCPU. But I'm happy to also answer any questions.

Also note that while doing these tests/probes, RSX is catching the 
illegal instruction trap, and just resumes execution but sets carry. So 
for some of these tests, the carry is cleared, and the instruction is 
attempted, and then there is a check if carry got set, as a way of 
seeing if it worked or not. The specific opcodes are for maintenance 
instructions that either are harmless on other models, or trap. And 
which do not affect the carry if executed on the assumed processor 
tested for.

Non-existant memory is also trapped, and execution resumed with carry 
set. Same kind of idea...


On 2021-03-05 19:38, Paul Koning wrote:
> I was just asked some questions about how RSTS identifies your processor type.  Since that topic might be of broader interest I figured I'd do some code reading and summarize the logic.
> In the RSTS initialization code (INIT.SYS), the first step is to identify what your hardware looks like.  That is a combination of CPU type, bus type, memory layout, and peripheral configuration lookup.  They aren't strictly separated into sequential blocks for those four activities, though naturally you'd want to know the bus type before you start looking for I/O devices on that bus.
> What I describe here is in RSTS/E V10.1.  The general idea of scanning the hardware was introduced in V6B, and I believe is basically the same from that time onward apart from the addition of support for more hardware types.  Prior to V6B, the assumption was that you had the hardware you specified during SYSGEN, neither more nor less.
> Here is an outline (not all the details) of the hardware scan flow:
> 1. If word 0 of the boot block contains a zero, this is a Pro (CT bus); otherwise it isn't.
> 2. Make sure the MMU exist; if not, halt.
> 3. Check the CPU type (MFPT instruction).  If it's an F-11, see if 177570 exist.  If yes, 11/24 (Unibus); if no, 11/23 (Qbus).  If it's a J-11, read the board type register at 177750 and use the bus type bit to distinguish Qbus from Unibus.
> 4. Check that there is a clock, and if possible determine the power line frequency.
> 5. Check if there is a CPU cache, and whether there is a cache error address register.
> 6. If Qbus, check whether there is memory above the 18 bit range.
> 7. Check that there is at least 96kW of memory (but the message says that 124kW is required -- the actual check value was apparently overlooked and not updated).
> 8. Check CPU features: EIS (required), FPP, FIS, switch register, display register, MED, two register sets, system ID register, CIS, Data space.
> 9. If Unibus, check for UMR.
> 10. Find where memory is.  This is done by looking at every 1kW address to see if it answers.  So unlike some other operating systems, RSTS will keep looking if it finds a hole in memory.  The kernel needs to be at 0 and contiguous, but holes above that are not a problem.
> 11. Scan the I/O bus for peripherals. This uses the fixed addresses and float rules for Unibus/Qbus (either, the code doesn't care) or the slot use bits and device type register codes for the Pro.
> 12. Find the vectors, which for almost every device is done by making it interrupt.
> 13. Identify specific device models if we care, like RL01 vs. RL02, Massbus disk type, DMC/DMR/DMP, etc.
> 14. Find which of these devices we were booted from.
> That's about it.  Once you get past that point the INIT prompt appears and you can ask what INIT found with "HARDWARE LIST".
> Incidentally, RSTS doesn't try to identify the exact CPU type you have.  Instead, it cares about features or distinctions that affect the code.  In a number of cases it does report the type -- if MFPT works then "hardware list" will report that information.  But for older CPUs, it doesn't say explicitly, though you can deduce it to some extent.  If no type is given but there is cache and more than 128 kW of memory, it's an 11/70.  If MED is available, it's an 11/60.  If it has FIS, it can only be an 11/40.  Etc...
> 	paul
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Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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