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doc: add reverse engineering notes for trs80-4p

This commit is contained in:
Virgil Dupras 2020-02-21 10:39:51 -05:00
parent bd38d46892
commit 08328b86a9

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