Re: Commodore "Most Wanted" List

From: Gerrit Heitsch <>
Date: Thu, 03 May 2012 19:07:38 +0200
Message-ID: <>
On 05/03/2012 08:19 AM, Baltissen, GJPAA (Ruud) wrote:
> Hallo Didier,
>> Double	 PC/XT	 9	 80	 2	 720 Kio	 250
> kbit/s	
>> Haute	 AT	 18	 80	 2	 1 440 Kio	 500 kbit/s	
>> I dont remember the exact details but the speed of the data is
>> probably twice
> 9 sectors * 512 bytes/sector * 6 rev/sec * 8 bits = 221184 bits/sec
> IMHO the number is even a bit too low.

I thought a floppy rotates at 300 rpm and not at 360 rpm? That would 
make it 5 rev / sec.

And of course you must not forget the gaps between the sectors. The 
Amiga was able to squeeze 11 sectors per track onto the floppy since it 
was able to work without gaps.

> 221184 bits/sec = 27648 bytes/sec = 36 uS/byte which isn't a suprise.

And the reason they used the SO pin on the 6502 instead of an IRQ to 
signal the next byte in the 1541. Without it, it looks like you need 2 
MHz if the 1551 is any indication. Its 6510T runs on 2 MHz and doesn't 
use the SO pin since it doesn't have one. It also doesn't use an IRQ but 
polling. The only IRQ source in the 1551 is a 555 rigged to supply an 
IRQ every 10ms or so.

> Although a 720 KB is roughly four times a 1514 (after GCR), it also has
> twice the number of sides and tracks. These numbers tell us that even a
> 1 MHz 6502 can handle a 720 KB drive. More interesting: using the same
> argument, a 2 MHz 6502 should be able to handle a 1.44 MB drive.

Assuming you use the same data seperator hardware. How different is the 
WD1772 in that respect, meaning how many CPU cycles do you need to grab 
a byte from there? There must be a reason why the 1570/71 were able to 
switch to 2 MHz and the 1581 also runs on 2 MHz.

> So are there more bears on the road that could stop us to connect a 1.44
> MB FDD to a 1581 and use it?

Not every 1772 can be overclocked and you need to double the RAM.


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Received on 2012-05-03 18:00:10

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