[Simh] Subtle H316 TTY interrupt bug? (Bob Supnik)

[Adrian Wise]

Interesting question. This spurred me into action to get the ASR 
interface from netlisted schematics to work with the H316 CPU that I've 
had simulating for a while. In the process finding a few more bugs in 
the schematics - both of the ASR interface and the CPU itself.

For the record you can see (an image of) simulation results illustrating 
the point in question here:

http://www.series16.adrianwise.co.uk/hardware/images/ASR_simulation.png

running the code pasted below. (I've wound the baud rate up to 9600, 
since the simulations take a *really* long time at 110 baud.)

(Or download the schematics and run it yourself, if you'd like to dig 
around in a waveform viewer.)

So, I can confirm Bob Supnik's analysis, both in Verilog simulation, and 
also ran it on real hardware (a DDP516).

Also I agree that my simulator, like simh, doesn't properly model the 
Dummy cycle.

 > From: "Bob Armstrong" <bob at jfcl.com>
 > ...
 >    FWIW, I'm trying to resurrect some real H316 software that uses 
exactly
 > the OCP 104/ OTA 4 sequence described, so it's not entirely an academic
 > concern.

It's probably worth pointing out that any program that actually relies 
on this behaviour is itself faulty - it has a race.

The issue, to recap, is if you (with interrupts turned on and masked 
correctly) put the ASR interface into output mode, and then immediately 
write a character to the ASR interface. This is a race, because you're 
gambling on reaching the OTA that outputs a character in the main thread 
before the one in the interrupt service routine.

Most of the time you'll win - but we're working with interrupts turned 
on, and if some other, unrelated, interrupt is serviced (or perhaps even 
a DMA/DMC request is serviced) which places a significant delay between 
the OCP to select output mode and the OTA, you'll lose - and the first 
character output by the ISR (and probably several others too) will be 
output before the one from the main thread.

It's also a really weird thing to want to do. If you've decided to put 
the message you want printing in a buffer and use interrupts to print 
it, why would you want to special-case the first character by sending it 
from the main program?

Test program for code run in simulation:
ABS                                                             PAGE    1



     0001                         ABS
     0002                         ORG   '1000
     0003 01000   34 0104         SKS   '104              READY?
     0004 01001   0 01 01000      JMP   *-1
     0005 01002   14 0004         OCP   '4                START IN INPUT 
MODE
     0006 01003   0 02 01027      LDA   PINT
     0007 01004   0 04 00063      STA   '63               SET UP INTERRUPT
     0008 01005   0 02 01053      LDA   ='000040          BIT 11 SET
     0009 01006   74 0020         SMK   '20               SET MASK
     0010 01007   140040          CRA
     0011 01010   0 04 01030      STA   CNT               CLEAR COUNTER
     0012 01011   000401          ENB                     ENABLE INTERRUPTS
     0013 01012   0 35 01052      LDX   =-200
     0014 01013   0 12 00000      IRS   0                 WAIT A BIT
     0015 01014   0 01 01013      JMP   *-1
     0016 01015   34 0104         SKS   '104              READY?
     0017 01016   0 01 01015      JMP   *-1
     0018 01017   0 35 01052      LDX   =-200
     0019 01020   0 12 00000      IRS   0                 WAIT A BIT
     0020 01021   0 01 01020      JMP   *-1
     0021 01022   0 02 01051      LDA   ='360             'P' - 4 HI 
BITS, 4 LO
     0022 01023   14 0104         OCP   '104              OUTPUT MODE
     0023 01024   74 0004         OTA   '4
     0024 01025   0 01 01024      JMP   *-1
     0025 01026   0 01 01026      JMP   *                 WAIT FOR EVER
     0026                    *
     0027 01027   0 001033   PINT DAC   INTR
     0028 01030   000000     CNT  OCT   0
     0029 01031              SA   BSS   1
     0030 01032              SK   BSS   1
     0031                    *
     0032 01033   0 001033   INTR DAC   *
     0033 01034   0 04 01031      STA   SA
     0034 01035   000043          INK
     0035 01036   0 04 01032      STA   SK
     0036 01037   0 12 01030      IRS   CNT
     0037 01040   0 02 01050      LDA   ='325             'U' - 
ALTERNATE HI/LO BITS
     0038 01041   74 0004         OTA   '4
     0039 01042   0 01 01041      JMP   *-1
     0040 01043   0 02 01032      LDA   SK
     0041 01044   171020          OTK
     0042 01045   0 02 01031      LDA   SA
     0043 01046   000401          ENB
     0044 01047  -0 01 01033      JMP*  INTR
     0045 01050   000325          END
          01051   000360
          01052   177470
          01053   000040



                                 ABS 
                           PAGE    2



     CNT    001030A   INTR   001033A   PINT   001027A   SA     001031A
     SK     001032A
     0000 WARNING OR ERROR FLAGS
     DAP-16 MOD 2    REV. C    01-26-71