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#21 | |
Registered User
Join Date: Aug 2010
Location: Germany
Posts: 545
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Quote:
So, why 1660 bytes and not something in the low 1500's? The trackdisk.device MFM decoding does not rely upon the index sync interrupt to assist, neither does the Amiga 1000 Kickstart boot ROM. Once you turn on the DMA, the encoded bitstream data will start "pouring in" from whereever the read head is currently hovering over the track. Could the optional use of the index sync be a reason why the gap size is 1660 bytes? That is, could it have side effects which would be mitigated by a larger sector gap? The Amiga 1000 internal and external 3.5" disk drives (the latter being the A1010 model) were a different make from what was used in the Amiga 2000 and Amiga 500 models. I may be wrong on this, but I seem to recall that the Amiga 1000 3.5" disk drives were manufactured by Toshiba, but the disk drives used in the Amiga 2000 and Amiga 500 were manufactured by Chinon. Could the 1660 bytes be part of "future proofing" the design, for the inevitable mass-produced Amigas which Commodore engineering would have already started working on? It might be that cheaper 3.5" disk drives were expected to have different, worse tolerances than the more expensive models used in the Amiga 1000. |
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#22 | |
Defendit numerus
Join Date: Mar 2017
Location: Crossing the Rubicon
Age: 54
Posts: 4,533
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Quote:
- Portia was much simpler and didn't even have a mechanism for WORDSYNC (which in fact isn't present in KS1.x); - the index sync is a legacy of an old way of managing and synchronizing data on floppy, which in fact is totally ignored and ignorable on Amiga (except to be used by some protection); - the 'wide gap' is an inheritance for the support of old and no so good drives with bad or imprecise self-correcting motor rotation speed (the Amiga can theoretically support even drives with much more that 5% error on rotation speed.. this allows for very long tracks and to function without problems; or very short actually ![]() When in a previous message I wrote to you that I put the tolerance at 5% I really exaggerated, the vast majority of drives do not deviate from 1%... In the Chinon specs is defined a maximum variaton of "less than 1.5%", even long term. But probably when they wrote the first KSs it was not like that ![]() Or they simply wanted to be sure to support everything, also the first 3.5" drives ever built. |
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#23 |
Defendit numerus
Join Date: Mar 2017
Location: Crossing the Rubicon
Age: 54
Posts: 4,533
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Just out of curiosity I checked the sources of RNC diskio, which I would say can be considered a 'modern' professional standard for writing Amiga tracks without using trackdisk.device
It has probably been tested in thousands and thousands of drives. I thought I remembered it was $1980 words, instead it is even less: $1960+1 (the +1 is the safety word due to Paula's error writing the last bits of the stream). This means a gap of 12994-11968=1026 bytes This also seems like a synthetic safety margin, too similar to 1024+2.. So the reasoning could be: "I tried I don't know how many times and drives, the worst one I found gave me $345 as excess, let's round up to $400" ![]() |
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