[Simh] Configuring DMC11 Devices

[Paul Koning]

> On Jan 4, 2016, at 2:38 PM, Paul Koning <paulkoning at comcast.net> wrote:
> 
> It looks like the problems RSTS is having with DMC/DMR is due to unexpected behavior of the master clear bit.  It's documented as self-clearing, so the driver sets it and then subsequently assumes it will read as cleared.  That isn't actually happening correctly, at least not if two accesses are close together:
> 
> DBG(4466384761)> DMC REG: dmc_wr(DMC0), addr=0x003fe038, SEL0, newdata=0x4000, olddata=0x8200
> DBG(4466384761)> DMC REG: DMC0: Setting SEL0 (0x4000) MCLR 
> DBG(4466384764)> DMC REG: dmc_wr(DMC0), addr=0x003fe038, BSEL0, newdata=23, olddata=0x00
> DBG(4466384764)> DMC REG: DMC0: Setting SEL0 (0x4023) MCLR RQI 
> 
> This is the driver doing a master clear, quickly followed by a Request In asking to set the Base address.  That second operation is a byte write to BSEL0, and it relies on MCLR being either write-only, or at least self-clearing fast enough.  In this case, clearly that is not working; MCLR is still set so the byte write sets not just RQI and BASEI, but also MCLR -- so RDYI is never set, the driver times out, and the cycle repeats.
> 
> This patch (hack?) fixes the issue, and now my two cross-connected DMCs successfully start:
> 
> diff --git a/PDP11/pdp11_dmc.c b/PDP11/pdp11_dmc.c
> index e0e0111..6034683 100644
> --- a/PDP11/pdp11_dmc.c
> +++ b/PDP11/pdp11_dmc.c
> @@ -3592,7 +3592,7 @@ else {
> 
> if ((dmc_getsel(reg) == 0) || (dmc_getsel(reg) == 1)) {/* writes to SEL0 and SEL2 are actionable */
>     if (0 == controller->dmc_wr_delay) {    /* Not waiting? */
> -        controller->dmc_wr_delay = 10;      /* Wait a bit before acting on the changed register */
> +        controller->dmc_wr_delay = 1;      /* Wait a bit before acting on the changed register */
>         sim_activate_abs (controller->unit, controller->dmc_wr_delay);
>         }
>     }

It would be nice to get this change into the official code, because right now that's the only thing keeping the DMC from working with RSTS.  (I just checked, after being prompted by R. Voorhorst).

	paul