CP/M survey

[Allison]

>
>Subject: Re: CP/M survey
>   From: "Chuck Guzis" <cclist at sydex.com>
>   Date: Thu, 19 Apr 2007 15:30:37 -0700
>     To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org>
>
>On 19 Apr 2007 at 13:07, Allison wrote:
>
>>> While the OS didn't do that it was easy to have your own FCB(s) 
>and 
>> the OS would not limit you. 
>
>....unless you were porting to MP/M, in which case too-ambitious 
>manipulation of FCBs could come back and bite you, since MP/M did 
>track file opens

True but many applications ran well under it anyway.

>> CP/M was a big step up from OSs like NSdos that only did sequential 
>> allocation and even more limited user interface.
>
>Sequential or consecutive?   Consecutive allocation was not a bad 
>thing, provided that it allowed for expansion of a file by adding 
>additional extents.  Indeed, it could be much faster than simple 
>granular allocation when seek time is an issue.  I've worked on a 
>couple of mainframe allocation systems that used consecutive-with-
>extension allocation with no particular problems.    I routinely run 
>into them in conversion (e.g. IBM DIsplaywriter).  A Smith-Corona 
>typewriter uses sequential allocation  in that each allocation unit 
>is placed physically later on the disk than the previous one, but not 
>necessarily adjacent to the preceding one. 

Sequential and consecutive.  However NSdos (like RT11) does not have 
a way to allocate addional space. For example, in File A,B,C are in 
place and A needs to be enlarged.  Under NSdos you have to copy A to A1,
append the data to it and delete A and rename A1 to A. Now if you need 
the space on the disk that A occupied you must compact the disk.  This 
was particulary nasty if the file was larger than half the disk size 
as you run out of space.  NSdos was a bag and tag file system.


>> CPM3 and MPM allowed for 512byte sectors and 32mb max logical drive size.
>
>You must be looking at MP/M I.  The maximum drive size for MP/M II is 
>512MB using 16K allocation units.  The maximum file size, however is 
>still 8 MB. 

I was.  


>> CP/M2 is non multitasking, V3 and MPM which are related (same filesystem 
>> and bdos calls) it can be an issue. However, the non-multitask status 
>> of CP/MV2 didn't prevent things like background printing or interrupt 
>> driven IO though it meant the BIOS implmentor had to do the work.
>
>Didn't the CP/M SPOOL program simply hook the printer BIOS vector and 
>install itself below CCP like the XSUB program?  It's been a long 
>time, so it might have been above the CBIOS also.

That was one of the few that did that.  There was nothing to prevent many
apps from doing that.


>> Other oddities is there was no MBR or on disk partition tables for 
>> large drives. The partition info was kept in the DPH/DPB inside the BIOS.
>
>I suspect that DRI considered that area to be an issue left to the 
>implementor.  We certainly allowed two OS-es to reside on the same 
>drive by simply implementing our own partition table scheme and 
>making the hard disk access routines aware of it.  MS-DOS scarcely 
>does anything much more elegant.

Back then two OSs on a disk would have been truly extravagant.


>> Only required for floppy or the uncommon removable harddisk 
>> (CDC hawk anyone).
>
>....or Syquest removables (SQ100) which were around early enough, 
>albeit after the PC, to find their way onto some Z80 CP/M systems.

My point of reference was pre PC.  By post PC thre weree enough things 
changing like the availability of inexpensive (under $1000) hard disks
and controllers to be significant.  Prior to that (especially pre1980)
it was 8" and 14" fixed drives and a few 14" removeables.

I still ahve a Syquest270 (with parallelport adaptor) that both works 
and I have about 15 disks for it.

>I seem to recall seeing an OS being advertised in one of the mags in 
>the late 70's that offered CP/M functional compatibility, but also 
>featured a hierarchical directory structure.  I don't recall the 
>name, but a friend was all fired up about it.   

There may have been one but I never saw one in action. The idea of 
hierarchical directory pre 1980 was pretty radical for a micro system.


Allison