Amiga floppy checksums

catphish · 01 October 2019, 01:41

I'm working on a tool to read and write Amiga disks with a PC. All is going well, but I one thing about the disk format is confusing me - the sector checksums.

Checksums appear to be calculated by xor'ing the MFM encoded data, ignoring the clock bits. The result appears to be a checksum where all odd bits are zero.

This checksum is then encoded into 2 longs (one for odd bits and one for even bits). The result of this therefore seems to be that the "checksum odd" fields of the header are *always* zero. Is this actually correct? It seems like a bafflingly odd design if so.

This does appear to be the case with the disk I'm testing with, but I wanted some input on whether this is correct!

Thanks! I will likely have more questions and weird and wonderful disk formats soon.

lesta_smsc · 01 October 2019, 01:50

Just to clarify. Are you using a hardware interface to help as floppy disk controller? As far as I was aware, the PC cannot natively support Amiga disk format?

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catphish · 01 October 2019, 01:58

Yes. I have designed an STM32 based USB floppy controller and I'm now working on the software.

https://i.imgur.com/TEDFmct.jpg

lesta_smsc · 01 October 2019, 02:21

Fantastic! I'm not sure if it will help, but take a look at this project which uses Arduino for the same purpose. It is open source so you may be able to get some ideas for your situation in the code / concept?

http://amiga.robsmithdev.co.uk/

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ross · 01 October 2019, 08:26

Quote:

Originally Posted by catphish

This checksum is then encoded into 2 longs (one for odd bits and one for even bits). The result of this therefore seems to be that the "checksum odd" fields of the header are *always* zero. Is this actually correct? It seems like a bafflingly odd design if so.

Yes, and this is the reason why I deliberately decided to ignore it in my loader.
This increases the decoding speed and does not affect the correctness of the data.
(well, actually you could check if that value is actually zero

).
Could it be a legacy of encoding and decoding done with blitter in 1.x systems? (to align data)

catphish · 01 October 2019, 20:15

Thanks for confirming. I will ignore the odd checksum field. It doesn't even matter if the field is corrupt

Wiking67 · 02 January 2020, 16:02

here another: http://nickslabor.niteto.de/projekte/adf-copy-english/

lesta_smsc · 02 January 2020, 16:10

All these projects are great! It would be nice if there was a way to modify and ready made USB FDD but not sure what chipset is in these and whether it would allow any modification.

01 October 2019, 01:41	#1
catphish Registered User Join Date: May 2019 Location: Dorset, UK Posts: 8	Amiga floppy checksums I'm working on a tool to read and write Amiga disks with a PC. All is going well, but I one thing about the disk format is confusing me - the sector checksums. Checksums appear to be calculated by xor'ing the MFM encoded data, ignoring the clock bits. The result appears to be a checksum where all odd bits are zero. This checksum is then encoded into 2 longs (one for odd bits and one for even bits). The result of this therefore seems to be that the "checksum odd" fields of the header are always zero. Is this actually correct? It seems like a bafflingly odd design if so. This does appear to be the case with the disk I'm testing with, but I wanted some input on whether this is correct! Thanks! I will likely have more questions and weird and wonderful disk formats soon.

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01 October 2019, 01:50	#2
lesta_smsc Registered User Join Date: Feb 2012 Location: United Kingdom Posts: 3,173	Just to clarify. Are you using a hardware interface to help as floppy disk controller? As far as I was aware, the PC cannot natively support Amiga disk format? Sent from my SM-G925F using Tapatalk

01 October 2019, 01:58	#3
catphish Registered User Join Date: May 2019 Location: Dorset, UK Posts: 8	Yes. I have designed an STM32 based USB floppy controller and I'm now working on the software. https://i.imgur.com/TEDFmct.jpg

01 October 2019, 02:21	#4
lesta_smsc Registered User Join Date: Feb 2012 Location: United Kingdom Posts: 3,173	Fantastic! I'm not sure if it will help, but take a look at this project which uses Arduino for the same purpose. It is open source so you may be able to get some ideas for your situation in the code / concept? http://amiga.robsmithdev.co.uk/ Sent from my SM-G925F using Tapatalk

01 October 2019, 20:15	#6
catphish Registered User Join Date: May 2019 Location: Dorset, UK Posts: 8	Thanks for confirming. I will ignore the odd checksum field. It doesn't even matter if the field is corrupt

02 January 2020, 16:02	#7
Wiking67 Blowing Out The Flame Join Date: Apr 2009 Location: Lulea Sweden Age: 57 Posts: 214	here another: http://nickslabor.niteto.de/projekte/adf-copy-english/

02 January 2020, 16:10	#8
lesta_smsc Registered User Join Date: Feb 2012 Location: United Kingdom Posts: 3,173	All these projects are great! It would be nice if there was a way to modify and ready made USB FDD but not sure what chipset is in these and whether it would allow any modification.

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