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doc: add reverse engineering notes for trs80-4p
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@ -210,3 +210,34 @@ always successful.
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66 BANK B=func C=bank BZ Memory bank use
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67 BREAK HL=vector HL Set Break vector
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68 SOUND B=func - Sound generation
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## Personal reverse engineering
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This section below contains notes about my personal reverse engineering efforts.
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I'm not an expert in this, and also, I might not be aware of existing, better
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documentation making this information useless.
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### Bootable disk
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I'm wondering what makes a disk bootable to the TRS-80 and how it boots it.
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When I read the raw contents of the first sector of the first cylinder of the
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TRS-DOS disk, I see that, except for the 3 first bytes (`00fe14`), the rest of
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the contents is exactly the same as what is at memory offset `0x0203`, which
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seems to indicates that the bootloader simply loads that contents to memory,
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leaving the first 3 bytes of RAM to either random contents or some predefined
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value (I have `f8f800`).
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A non-bootable disk starts with `00fe14`, but we can see the message "Cannot
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boot, DA TA DISK!" at offset `0x2a`.
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I'm not sure what `00fe14` can mean. Disassembled, it's
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`nop \ rst 0x28 \ ld b, c`. It makes sense that booting would start with a
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service call with parameters set by the bootloader (so we don't know what that
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service call actually is), but I'm not sure it's what happens.
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I don't see any reference to the `0x2a` offset in the data from the first
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sector, but anyways, booting with the non-bootable disk doesn't actually prints
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the aformentioned message, so it might be a wild goose chase.
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In any case, making a disk bootable isn't a concern as long as Collapse OS uses
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the TRS-DOS drivers.
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