DEC PDP-8/E wanted - still looking

dave.g4ugm at dave.g4ugm at
Fri Nov 27 04:29:12 CST 2020


This is the e-mail I got from Bob Armstrong..





From: sparetimegizmos at <mailto:sparetimegizmos at>  [mailto:sparetimegizmos at] 
Sent: 15 July 2015 15:53
To: sparetimegizmos at <mailto:sparetimegizmos at> 
Subject: RE: [sparetimegizmos] Any builders of the SBC6120 in the UK


  The FORTRAN RTS uses interrupts, whereas most OS/8 software does not, and so it reveals bugs that are not otherwise apparent.


  As I recall the problem was with the VGA/PS2 console implementation in the FPGA.   Jim didn’t handle clearing the flags and the interrupt request quite correctly, and it made FORTRAN hang up in an interrupt loop.   It should be easy enough to fix by revising the VHDL, but I’m not sure Jim ever did.  Make sure you have the latest IOB6120 firmware.


  FWIW,  FORTRAN does work with the SBC6120 onboard serial console enabled so you can use the IOB so long as you’re not using the VGA console.


  This was an issue for many people because it broke the ADVENTure game, which is also written in FORTRAN :-)





From: Tom Hunter <ccth6600 at> 
Sent: 27 November 2020 01:08
To: Dave Wade <dave.g4ugm at>
Cc: General Discussion: On-Topic and Off-Topic Posts <cctalk at>
Subject: Re: DEC PDP-8/E wanted - still looking


Hi Dave,

Could you please explain how exactly the VT emulation "kills" the Fortran run time system in OS/8?

What is the fault? How do you trigger it? Have you got a Fortran code snippet which reproduces the problem?

Was there some previous discussion about it answering my questions above?

I have a reasonable level of experience with VHDL and Xilinx ISE so maybe I can fix it if I can reproduce it.

Thanks and best regards

Tom Hunter


On Thu, Nov 26, 2020 at 6:28 PM <dave.g4ugm at <mailto:dave.g4ugm at> > wrote:


If it has an IOB6120 then it the VT emulation kills the Fortran run time system in OS8.


> -----Original Message-----
> From: cctalk <cctalk-bounces at <mailto:cctalk-bounces at> > On Behalf Of Tom Hunter via
> cctalk
> Sent: 26 November 2020 06:02
> To: General Discussion: On-Topic and Off-Topic Posts
> <cctalk at <mailto:cctalk at> >
> Subject: DEC PDP-8/E wanted - still looking
>  Tom Uban fully built "Spare Time Gizmos" SBC6120 based computer
> complete
> with front-panel and IO board is on its way to me.   :-)
> I am still looking to buy one of the following PDP-8 models:
> PDP-8/F
> PDP-8/E
> PDP-8/L
> PDP-8/I
> PDP-8/M
> Dave, Chris, Paul and Robert I am looking forward to getting more details
> from you.
> I am happy to pay a reasonable price for the right PDP-8, so a seller won't be
> disappointed.
> Thanks and best regards
> Tom Hunter

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