PDP8 / ETOS

[Johnny Billquist]

On 2015-10-18 12:37, Rick Murphy wrote:
> At 01:22 PM 10/17/2015, David Gesswein wrote:
>> On Fri, Oct 16, 2015 at 11:03:51PM -0400, Rick Murphy wrote:
>>
>> System users guide pg 4-10. Fortran IV. Real time functions and floating
>> point processor functions are not currently supported. The internal
>> device handlers for high speed reader/punch and card reader are not
>> supported.
>> Device independent I/O must be used to access these devices.
>>
>> I don't know if that means the floating point instrutions won't work but
>> the software emulator fallback will or any attempt to use floating
>> point will
>> fail.
>
> The Fortran IV compiler emits FPP code, so what that has to mean is that
> you can't use a hardware floating point unit and must use the built-in
> emulator. That makes sense, as the FPP code has built-in 15-bit
> addresses that won't get swizzled by the ETOS time-sharing option.
>
> Guess I'll try to get a round tuit and bring up ETOS on my PiDP-8 to see
> if it can be made to work.

I decided to look into this a couple of days ago, since the basic 
workings of FRTS includes using interrupts, which is not possible if 
running under time sharing. That would also imply that it would not be 
possible to use F4 under RTS-8, which I had some memory of that it 
actually is possible.

To sum things up: The FPP-8 can usually not be used when in timesharing. 
First of all, you use IOTs to control the FPP, and all IOTs are trapped 
when in user mode. Second, as you note, the FPP-8 uses 15-bit addresses, 
which would make it impossible to use with virtual memory.

Also, the interrupt system is not available when in user mode.

However, FRTS actually have code to detect if it is running under RTS-8, 
and do not use the interrupt system in that case, but adopts. Also, 
since the FPP-8 IOTs are caught by RTS-8, and do not do anything, FRTS 
actually believes you are on a system without an FPP-8, even if you 
actually have one. So, FRTS will always use the FPP-8 emulator in this 
situation.

MULTOS-8 hook in to the same functionality, making FRTS believe it is 
running under RTS-8, which means that F4 programs will actually work 
under MULTOS-8, including ADVENT. I don't know if ETOS also implements 
the bits needed to make it look like OS/8 programs are actually under 
RTS-8. If it does, ADVENT should be possible to run under ETOS as well. 
Otherwise not.

Finally, as I noted, the FPP-8 is not that possible to use in user mode, 
both because of the IOT instructions being caught, and the 15-bit 
addressing. However, the FPP-8A have a mode where it only allows memory 
accesses within the same field, and will trap out if any memory 
reference goes to another field. With some work, and code, I think it 
could be possible to actually have access to the FPP-8 from user mode, 
if you have an FPP-8A, but I have never tried this, as I lack the hardware.

	Johnny

-- 
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