[Coco] MPI and NitrOS-9

[Bill]

Was there some information lost in this section? Seems disjointed to me:

"Much of this is better explained by a detailed read of the superdesc.asm file in the nitros9 build directory. Because bits in the descriptor are hard to find spares for adding these new functions, 1 or more bits may control the meaning of 3 or 4 other bits, so read, and re-read until the interactions become clear.

dmode, which can adjust a descriptor in memory on the fly, or a descriptor on the disk to build the next boot disk, will be your best friend while learning all this."

-----Original Message-----
From: Coco [mailto:coco-bounces at maltedmedia.com] On Behalf Of Gene Heskett
Sent: Thursday, October 20, 2016 11:07 PM
To: coco at maltedmedia.com
Subject: Re: [Coco] MPI and NitrOS-9

On Thursday 20 October 2016 22:14:12 Bill wrote:

> So, if I were able to build a ribbon cable to connect all four drives, 
> I could connect it to one controller? I might try that tomorrow. I 
> have the connectors and PLENTY of ribbon cable, but what about the 
> twists on the cable? Am I building a cable basically identical to the 
> two existing, but with four connectors?

Bill; There are 4 drive select wires in the cable, but one is commonly used for side select, so that you can connect only 3 physical drives at any one time without useing another controller. Two would allow 6 drives for instance. IIRC that would need some tricky stuff not usually played with in the descriptors to isolate the two controllers shareing common access addresses in the $FF40-$FF48 range.

This means also that there are no missing teeth in any of the connectors, nor is a cable twist required.  What is required is that you go thru the drives and by solder bridges, flea clips or dip switches, each drive has its own unique address from 0 to 2. But the 3.5" drives have only 2 possible addresses, and as shipped they are all set to address 1, and the twisted cable makes the drive on the end of the cable into drive 0 by swapping those 2 select wires.  Works fine on a doggoned pc, but not for os9.

So common practice here at the coyote.den is to have a pair of 3.5" 
drives, with one having its addressing solder blobs reset to make it drive 0. The next, is already drive 1, and a 5.25" of your drive choice at the drive 2 address. Since this drive can be a 35 track ss, a 40 track ds, or an 80 track ds, Robert and I worked out a patch for the
nitros9 rb1773.dr floppy driver such that if a 40 track disk is inserted in the 80 track drive, that drive will be double stepped so the heads stay in the center of the wider 40 track pattern for reads, and if an attempt is made to write to that disk, it will be found to be write protected since the narrower 80 track heads will not properly overwrite the wider track.  So without the write protection, the disk, which may be the only one you have with that data on it, will refuse the write, which if allowed, normally destroys the disk and it must be reformatted to use it again.

Much of this is better explained by a detailed read of the superdesc.asm file in the nitros9 build directory. Because bits in the descriptor are hard to find spares for adding these new functions, 1 or more bits may control the meaning of 3 or 4 other bits, so read, and re-read until the interactions become clear.

dmode, which can adjust a descriptor in memory on the fly, or a descriptor on the disk to build the next boot disk, will be your best friend while learning all this.