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Krill
Registered: Apr 2002
Posts: 2839 |
Keeping a few bits of information in a hostile environment
If not reserving space in RAM, where would read-writable data be most likely to survive throughout the run-time of any random demo or game?
$D800-$DC00 is often overwritten entirely, $00/$01 in RAM are too cumbersome to access.
$DD03 (parallel port data direction register) might be good, or maybe the 2x4 CIA TOD registers at $DC08 and $DD08.
Or are they? What else could be usable for that purpose? =) |
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Krill
Registered: Apr 2002
Posts: 2839 |
Quoting JackAsserSo let that one bit determine if it's 1541 or not. The other types can have more drive code for further queries. Good idea, but... would like to somehow ignore that bit without adding extra code when loading, and pretend-loading an existing file (with just discarding the incoming blocks) just to determine the drive-type doesn't sound very elegant either. =) |
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Copyfault
Registered: Dec 2001
Posts: 466 |
Quoting JackAsserSo let that one bit determine if it's 1541 or not. The other types can have more drive code for further queries. Already suggested it via pm, but since I did not fully understand what makes it fail I'll ask again (a little bit adjusted to what JA wrote): couldn't you always send the 1541-save routine and have some watch-dog-alike routines on 1571- and 1581-side that will cancel the send-procedure and request "their" routine when the watch-dog detects the (first bytes of the) 1541-save-routine incoming? |
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chatGPZ
Registered: Dec 2001
Posts: 11107 |
All in 4 Bytes :=) |
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JackAsser
Registered: Jun 2002
Posts: 1989 |
I think we’re getting to the point that you should let the user store the returned config value during loader install then ask them to pass that value to the save-installer. Anyone can keep track of that byte.. |
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Krill
Registered: Apr 2002
Posts: 2839 |
Quoting JackAsserI think we’re getting to the point that you should let the user store the returned config value during loader install then ask them to pass that value to the save-installer. Anyone can keep track of that byte.. We have passed that point already some way up in this thread. :)
I am defaulting to $dd03 at the moment, planning to add a variable to the custom drive code upload parameter structure to override/provide drive type. |
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Rastah Bar
Registered: Oct 2012
Posts: 336 |
Quote: Quoting Rastah BarF.e., let the C64 try to load a nonexisting file and let the loader return a "file not found" error code that is different for each drive type. Not a bad idea, but not really feasible in practice either.
The byte in question sent by the drive encodes a status/error code ($00: end of file, $ff: file not found) or $01-$fe for current highest consecutive bytestream position (for decrunching while loading with blocks coming in out of order) or $01-$fe for the block size of the file's last block (which of the two $01-$fe options is determined elsewhere).
So other no other values left. If there were, however, reducing the 3 values for a file not found error would take some extra space in the resident C-64-side loader code, plus a definitely non-existing filename would have to be chosen somehow, and then things like no disk inserted can happen, too (although malicious user error is not very relevant in practice).
Switching codes $00 and $ff for one of the drives might provide one bit of information. |
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Copyfault
Registered: Dec 2001
Posts: 466 |
Quoting GroepazAll in 4 Bytes :=) Oh noes, that's not much /o\ Got the impression that there's still more free space, at least for drivecode on the non-1541-drives.
So I take it if you look at it spot on the details, especially the code on c64-side would've to be changed too much in order to make a watch-dog-driven handshake for uploading the save-routine work.
On an abstract level, I still think this would be most elegant ;) |
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Flavioweb
Registered: Nov 2011
Posts: 447 |
I guess i haven't really understood the problem here... but i try: why don't reserve a file name/file number which return a byte containing drive model value? |
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Rastah Bar
Registered: Oct 2012
Posts: 336 |
Can't you just always upload the highscore saver right away, that is, together with the loader code?
Or is there a backwards compatibility issue? Or do you want to save RAM? Or disk space?
How does a random user know in the first place that the loader has been installed on the drive? |
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Krill
Registered: Apr 2002
Posts: 2839 |
Quoting Copyfaultcouldn't you always send the 1541-save routine and have some watch-dog-alike routines on 1571- and 1581-side that will cancel the send-procedure and request "their" routine when the watch-dog detects the (first bytes of the) 1541-save-routine incoming? Quoting GroepazAll in 4 Bytes :=) Quoting CopyfaultOh noes, that's not much /o\ Got the impression that there's still more free space, at least for drivecode on the non-1541-drives. The main problem with such an approach (it's nothing to do with watchdogs, though) is that it would most likely add bytes either to the 1541's regular loader code or the C-64-side resident loader code. Bytes i don't have or don't want to afford. =)
The custom code upload part re-uses the C-64->drive routines normally used to send the filename.
Some kind of receive-rejection could be implemented by the receiver permanently asserting the data line and the sender detecting that the bit it put on the bus isn't the same it reads back.
Works only with one of the two bit values, though, but should work with a canary byte that is neither $00 nor $ff.
I am still not quite certain if this can or cannot work without the mentioned drawbacks...
But then i think the simple and safe solution of having the user shuffle over the drive type from installer return values struct to save/code upload parameter struct is acceptable.
Not as elegant, of course. =) |
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