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54 lines
2.0 KiB
Markdown
54 lines
2.0 KiB
Markdown
Disk: Apple II
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==============
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Apple II disks are nominally fairly sensible 40-track, single-sided, 256
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bytes-per-sector jobs. However, they come in two varieties: DOS 3.3 and
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above, and pre-DOS 3.3. They use different GCR encoding systems, dubbed
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6-and-2 and 5-and-3, and are mutually incompatible (although in some rare
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cases you can mix 6-and-2 and 5-and-3 sectors on the same disk).
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The difference is in the drive controller; the 6-and-2 controller is capable
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of a more efficient encoding, and can fit 16 sectors on a track, storing
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140kB on a disk. The 5-and-3 controller can only fit 13, with a mere 114kB.
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Both formats use GCR (in different varieties) in a nice, simple grid of
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sectors, unlike the Macintosh. Like the Macintosh, there's a crazy encoding
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scheme applied to the data before it goes down on disk to speed up
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checksumming.
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In addition, a lot of the behaviour of the drive was handled in software.
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This means that Apple II disks can do all kinds of weird things, including
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having spiral tracks! Copy protection for the Apple II was even madder than
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on other systems.
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FluxEngine can only read well-behaved, DOS 3.3 6-and-2 disks. It doesn't even
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try to handle the weird stuff.
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Sadly, DOS 3.3 also applies logical sector remapping on top of the physical
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sector numbering on the disk, and this _varies_ depending on what the disk is
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for. FluxEngine doesn't attempt to remap sectors, instead giving you an exact
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copy of what's on the disk, so you may need to do some work before the images
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are usable in emulators.
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Reading discs
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-------------
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Just do:
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```
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fluxengine read apple2
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```
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You should end up with an `apple2.img` which is 143360 bytes long.
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**Big warning!** The image may not work in an emulator, due to the
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logical sector mapping issue described above.
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Useful references
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-----------------
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- [Beneath Apple DOS](https://fabiensanglard.net/fd_proxy/prince_of_persia/Beneath%20Apple%20DOS.pdf)
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- [MAME's ap2_dsk.cpp file](https://github.com/mamedev/mame/blob/4263a71e64377db11392c458b580c5ae83556bc7/src/lib/formats/ap2_dsk.cpp)
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