[Simh] RQDX3 does not do odd byte transfers

[bsupnik at comcast.net]

There is no way to patch pdp11_rq.c, because the RQDX3 DOES NOT SUPPORT ODD BYTE TRANSFERS.

The real RQDX3 firmware checks explicitly for either an odd byte address or an odd byte count and generates an error i both cases.  The simulated RQ behaves exactly the same way.

The reason is simple: the RQDX3 uses a simple data mover to move data from its buffer to main memory over the Qbus.  This data mover always uses 16bit DATO transfers, which require even byte addresses.  Since the transfers are always multiples of 16bits, the byte count must be even as well.

The UDA50, on an 11/780, can support odd addresses (but not odd byte counts), because the 780's Unibus adapter has the ability to offset inbound addresses by one byte.  The Qbus adapter on the 3900 won't do that.

As far as I can tell, <no> DEC disk, except for the floppy disks, supports odd byte transfers.  The RK11, RK611, RL11, RH11 (Massbus), and UDA50 either have word count registers or expect byte counts to be even.

/Bob Supnik


> Today's Topics:
> 
>    1. Re: UDA-50 - want to use 'odd byte count' (Hoyer Falko)
> 
> 
> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Thu, 26 Jul 2007 14:51:37 +0200
> From: Hoyer Falko <Falko.Hoyer at austrocontrol.at>
> Subject: Re: [Simh] UDA-50 - want to use 'odd byte count'
> To: simh at trailing-edge.com
> Message-ID:
> 	<09344F86CC53BE4EA61557124E79A56F9A58B0 at EXCHSRV1.win.austrocontrol.at>
> Content-Type: text/plain; charset="iso-8859-1"
> 
> Hi Andreas,
> 
> thanks for the reply - 
> 
> taking your approach into account the following questions arise:
> ----------------------------------------------------------------
> - if using the massbus-adapter simulation then the maximum disk-size would be 
> 512MB (RP07) - correct?
> 
> - we are currently using VMS-7.3 and as far as I know 5.5-2 is the latest 
> running on a 780.
> 
> - we tried to compile simh for a VAX780 and we could not configure the 
> simvax.ini file correctly:
> do we need the "load ...ka655" stuff in that case ?
> has somebody a working .ini file for VAX-780 for reference ?
> 
> 
> apart from that - if we still keep on with vax-3900:
> ----------------------------------------------------
> as far as I understood the disk-controller used here (and using RA92) is a RQDX3 
> which is/was for the Q-bus.
> looking in the "VMS I/O User's Reference Manual" for the description of QIO 
> services to disk - we can see that the UDA50  (but NOT the RQDX3) is mentioned 
> for not supporting odd byte count:
> 
> "P2---The number of bytes that are to be read from the disk, or written from 
> memory to the disk. An even number must be specified if the controller is an 
> RK611, RL11, RX211, or UDA50."
> 
> now, I'm currently not sure if my assumption is true - do we really use the 
> UDA50 in the VAX-3900 emulation and not the RQDX3 ?
> If so, why is then odd byte count transfer impossible ?
> 
> any suggestions are very welcome (also if somebody has an idea how to patch the 
> PDP11_RQ.C file).
> 
> regards
> Falko