Hallo Anders,


> how much the DOS logic differs between a 8050 and a 1541.

The IEEE drives have two CPU's onboard, the 1541 only one. The one 
in the the 1541 does in fact the same job but now on its own. The 
DOS of the 2031 is derived from the 4040 most probably. The DOS of 
the 1541 is derived from the 2031 on its turn. That's why you still 
find references to two drives.

The two main reasons not paying much attention to, let's call it 
8250IDE, is that 1) I have CBM-HD that forfills all my needs here 
and 2) 1541IDE addresses a larger user group IMO. 

But because of this 90x0 discussion I paid more attention to it. And 
I started wondering: why do the dual IEEE drives need two CPU's? Do 
they REALLY need it? The 1541 prooves it can be done with one CPU. 

So I really started to wonder if things could not be archieved by 
just by one CPU? AFAIK both DOS systems work the same: they put a 
command in the job queue and then start checking a memory address to 
see if the job is done. In the dual IEEE drives this address is 
updated by the second CPU, in the 1541 it is done by the same CPU 
during an interrupt. 
The harddisk has does not depend on timers, interrupts or, quite 
important, this SO-input of the 6502. So why cannot the CPU execute 
the job by just calling a subroutine? I know it is possible because 
I replaced the completely interrupt driven format routine by a small 
routine that just fills all sectors with zeros in one go. 

In this way I solve a problem I have with the dual IEEE drives: what 
to do with the 6530? It has ROM, I/O and RAM on board. The RAM is 
used as zeropage and stack and cannot be ommited. So it has to be 
replaced by something that has at least RAM and ROM on board. Doable 
but it takes some work. Only using one CPU means that lazy me can 
drop the whole subject :) (hmmm, what controls the LEDs ???)

OK, enough for now. The garage called, I have to pick my car from 
its annual check.


--
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  / /  |_/     Groetjes, Ruud Baltissen
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   \___|       http://Ruud.C64.org







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