[simh] RSTS processor identification

Paul Koning paulkoning at comcast.net
Fri Mar 5 19:33:04 CST 2021

> 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.

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

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 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.


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