Re: Disk II Controler problems

[jdb8042@blkbox.COM (John D. Baker)]

First of all, Digital Research, Inc. produced CP/M, not Micro$loth.  MS
simply wrote a BIOS module to allow CP/M to run on their Z80 card (SoftCard)
and integrated it with the Intel MDS-800 'standard' CP/M system.

The MicroSoft SoftCard CP/M system requires that at the first pair of
Disk ][ drives be in Slot 6.  In such a limited system as the SoftCard,
keeping code to a minimum is paramount and the code to locate all the
drives attached to the system and keep track of them would steal valuable
TPA in a machine that's hurting for RAM to begin with.

Additional Disk ][ drives were expected to be located in lower-numbered
slots.  Thus A: and B: were S6,D1&D2.  C: and D: were S5,D1&D2.  Part of
the reason for doing it this way is that CP/M must often be reloaded from
the boot tracks of the disk (a "warm boot") when a user program ends so
that the CCP can be restored.  The A: drive is always searched for
valid boot tracks for the "warm boot" so having fixed disk controller
assignments speeds up the process.  BEsides, does one really rearrange
the disks while the machine is running?  S6,D1 had already been established
as the customary "boot drive" anyway.

Of course the MS SoftCard system knew nothing of high-capacity 3.5" disks
or hard disks, so there was no call to make the system flexible. It wasn't
until v4.x and up of AE' CP/AM system that such things were supported on
a SoftCard-type system.

In the PCPI AppliCard and SLS CardZ180 systems, any type of disk drive
may be located anywhere, except that additional pairs of drives of any
one type were expected to be in the next-lower-numbered slot.  By
default, the first pair of Disk ][-style drives were expected to be in
S6,D1&D2 simply because that was conventional.  They could, in fact be
in any of slots 6, 5, or 4.

The AppliCard/CardZ180 systems could also use (and boot from) the
UniDisk 3.5 (and later the IIgs smartport drives).  The flexible
device driver arrangement of these systems BIOSes allowed any
drive to be assigned anywhere as long as multiple drives of any
one type took on consecutive drive letters.

Further, the "boot drive" was not necessarily drive A: in the
running system.  It was perfectly possible to boot from a Disk ][,
but have the first UniDisk 3.5 be drive A:.

In fact, I have a ][+ with an Applicard which must boot from a Disk ][,
but drives A:, B:, and C: are a Corvus flat-cable hard disk (ProDOS) and
the Disk ]['s show up as D: and E:.

Hope this helps explain the CP/M side of disk controller slot locations.


John D. Baker ->A TransWarp'802'd Apple //e CardZ180 Z-System nut //
Internet:  jdb8042@tamsun.tamu.edu, @blkbox.com, jdbaker@taronga.com
BBS:  JOHN BAKER on PIC of the Mid-Town [(713) 961-5817] 1:106/31,
Z-Node #45 [(713) 937-8886], The Vector Board [(716) 544-1863]