[simh] RSTS processor identification

Johnny Billquist bqt at softjar.se
Fri Mar 5 20:02:12 CST 2021

On 2021-03-06 02:33, Paul Koning wrote:
>> On Mar 5, 2021, at 7:22 PM, Johnny Billquist via cctalk <cctalk at classiccmp.org> wrote:
>> ...
>>> Maybe this weekend I'll hack that SSD floppy thingie and load up the P/OS 3.2 disks to see how that works.
>> Can't run split I/D space on any version of P/OS. Neither does it support supervisor mode. Also, the J11 on the Pro-380 is running a bit on the slow side. Rather sad, but I guess they didn't want to improve the support chips on the Pro, which limited speed, and they didn't want to start having Pro software that didn't run on all models, which prevented the I/D space and supervisor mode.
>> In the end I would probably just put it down to additional ways DEC themselves crippled the Pro, which otherwise could have been a much better machine.
> The most embarassing blunder with the Pro is that the bus supports DMA, but no I/O cards use it.  Even though a bunch of them should have -- hard disk controller obviously, network adapter possibly as well.

That's another good point about weird things about the Pro.

> I/D and supervisor mode work fine on RSTS.  :-)

Really? That's nice. I would have loved to try and build a version of 
the latest RSX for the Pro, but unfortunately the code for the disk 
controllers are missing for me.
The handling of the screen and keyboard are still in the RSX sources.

> The explanation I heard for the slow J-11 clock is that the original J-11 spec called for it to operate at 20 MHz.  When Harris failed to deliver and the max useable clock speed ended up to be 18 MHz, most designs had no trouble.  But the Pro support chips were designed to run synchronous with the CPU clock and for various other reasons needed a clock frequency that's a multiple of 10 MHz, so when 20 MHz was ruled out that left 10 MHz as the only alternative.

I do think it sounds weird that the support chips would require a clock 
that is a multiple of 10 MHz. But I wouldn't know for sure.
Somewhere else I read/heard that they didn't work reliable above 10 MHz, 
but for the F11 that was ok. When the -380 came, they just reused those 
support chips.
Also, no caches. The -380 could have been so much better...

> I would have liked better comms.  The USART has such a tiny FIFO that you can't run it at higher than 9600 bps even with the J-11 CPU.  At least not with RSTS; perhaps a lighter weight OS can do better.  The printer port is worse, that one can't run DDCMP reliably at more than 4800 bps.  I normally run DDCMP on the PC3XC, which is a 4-line serial card that uses two dual UART chips (2681?) with reasonable FIFO.

Hmm. I'm pretty sure I was running my -380 with the printer port for 
DDCMP on HECnet for a while, and at 9600 bps.

But with P/OS, you are not using the console port as such. That's all on 
the graphics side.
But unless I'm confused, that's the same port. The printer port just can 
also be the console port, if you short pins 8-9, right? Except it won't 
fully work the same as the DL11, since interrupts work differently. But 
polled I/O will work the same.
But I would expect the speed characteristics to be the same for the 
console as for the printer port.


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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