Re: Commodore "Most Wanted" List

From: didier derny <>
Date: Wed, 02 May 2012 10:55:08 +0200
Message-ID: <>
Le 02/05/2012 07:50, Baltissen a crit :
> - ROM from a 1591 disk drive - the high density 1581 - heard prototype
> was thrown away :(
> To be honest, I haven't an idea what hardware is needed to run a 1.44 MB
> drive at all. FYI, I have never seen the insides (or schematics) of an
> CMD FD2000. For example, is DMA a must?  If it can be run using the 1581
> hardware, IMHO two things have to be done:
> - the 1581 stores a complete track in memory. If we want to do that here
> as well, at least 8 KB have to be added.
> - change the sofware. OK, I know this is obvious but it can be done in
> two ways:
>   - with the RAM expansion: just double the number of sectors/track
>   - without the expansion: read/write every track like it is done on a
> 1541
> FYI, in some spare times I'm still busy with 1581IDE project so I dare
> to say that I know where I am talking about.

it may be a problem of speed, on a standard floppy 3 1/2 floppy as far 
as I can remember, we have 13 or 14 microseconds
to transfer the data. with a HD it's probably faster and probably why a 
dma is needed.

on the 6502 the commodore floppy disk controller they are using the SO 
line (set overflow)
and the assembly code:

NEXT:       CLV                                    ;-  2 cycles
LABEL:     BVC    LABEL                      ;-  3 cyles
                  LDA    PORT                       ;-  4 cycles    here 
overflow is set and the data are ready
                  STA     $BUFFER, Y           ;-  5 cycles
                  INY                                    ;-  2 cycles
                  BNE    NEXT                     ;-  3 cycles
                                                            ;- 19 cycles

(I used something similar to read a 3 1/2 floppy disk with a 6809...)

on a 3 1/2 HD disk we probably need either DMA or a faster speed for the 
I dont remember very well the details but it was the origin of the 
problem and the use of SO line


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Received on 2012-05-02 10:00:32

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