[Coco] 6309 microprocessor project - 10-24-2003

[KnudsenMJ at aol.com]

In a message dated 10/25/03 12:44:13 AM Eastern Daylight Time, 
gene.heskett at verizon.net writes:

> That 256 count of 8192 byte blocks limit uses all 8 available bits in 
>  the Tony D. method of expanding it to 2 megs.  That kit converts the 
>  5 bit DAT offset regular OS9 l2 used into a full 8 bit one, expanding 
>  the number of available blocks from 64 to 256.  This all fits within 
>  the regular OS9 l2 framework with a couple of very minor patches.  To 
>  go any higher would require fairly major surgery to OS9 which would 
>  IMO impact the speed of access noticeably.

OK, now I remember, that's the reason -- OS9L2 uses an 8-byte array to store 
each process' DAT image, one byte per 8K DAT block.  Each block uses the lower 
13 address bits as they come on the bus, and up to 8 bits can be appended on 
top from the DAT register, for a total of 21 bits or 2 Megs.

Tandy's GIME supplies the lower 6 bits of the 8, and Tony's hardware does the 
upper 1 or 2.

To go bigger would require doubling the size of OS9's DAT image arrays in its 
process control blocks, and would indeed require some serious hacking.  Not 
that the Nitro boys couldn't do it in half an hour (and probably have already, 
one rainy day when the cable was out), but now that each PCB is 8 bytes 
bigger, the limit on total simultaneous procs would have to be lowered, since I 
doubt there's ANY leftover system RAM.  Maybe not a bad trade-off.

>  There was a time when I 
>  contemplated it, but with 2 megs, I never came close to using it all 
>  except while testing myram when its descriptor was set for about 1.7 megs.


I must agree -- with a 192K RAM Disk, UME, Dynastar editor all running at 
once, and the entire C compiler loaded into RAM as well, I don't recall ever 
seeing a #207 Error in L2.  Well, maybe -- ISTR downsizing by RAM Disk from 256 to 
192K.  And probably L2 gives a different error number for "out of RAM 
blocks".  Whatever.


Anyway, 16M would make a RAM Disk as big as most of our hard drives, and 
really change the feel of Coco computing.  --Mike K.