OS/8 FORTRAN IV

[Rick Murphy]

On 12/29/2016 2:42 AM, Vincent Slyngstad wrote:
> From: Charles Dickman: Wednesday, December 28, 2016 7:43 PM
>> What happens when a PDP8/e executes an IOT to a non-existent device?
>> My PDP8/e is skipping when it executes a printer flag test for a
>> device that is not present.
>
> IOT instructions for non-existent devices should be a no-op.
> Skipping for a non-existent device is a bug. 
Agreed.
I'll bet that one of your devices is improperly decoding IOTs and is 
causing the skip.

You could verify by writing a simple toggle-in that does the skip to test:
0200/ 6661  - skip on the flag
0201/ 7402 - halt if it doesn't skip
0202/ 6662 - clear the flag
0203/ 6661 - did that clear it?
0204/ 7402 - yes
0205/ 7402 - no

The FRTS does the following:
LPTEST, LSF
         IFNZRO LPTEST-415 <****ERROR.....Change Loader****>
         JMP     NOTLPT
LPTLCF, LCF             /CLEAR FLAG
         TAD I   LPGET
         SNA CLA         /CHECK FOR SPURIOUS INTERRUPT
JMPDIS, JMP     DISMIS  /GO AWAY IF SO

So, it's skipping, then clearing the flag. So, if there was real 
hardware, this would only trigger once.
Try the above on a minimal system, then add boards until the problem 
comes back.
     -Rick