Omnimaga
Calculator Community => TI Calculators => ASM => Topic started by: Matrefeytontias on March 23, 2013, 06:56:50 am
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Hi guys,
I'm working on a file system for cmdOS, and I want every files to be in archive to allow user programs to use a maximal amount of free RAM. I planned to handle it this way :- A 3-bytes pointer on a VAT at a static address in RAM consisting of a 2-bytes address (low byte then high byte) and a page number.
- At the location pointed to by this pointer and in the given flash page, the start of the VAT.
- After this location, each and every file whose structure will be discussed later.
The problem is : I don't know how to write to flash. I know how to swap pages using ports 6 and 7, but I also know that you can't just write to the corresponding areas like that.
How can I, say, write a byte to a flash page ?
Note that I don't have any built-in routine for that. I'm NOT using the TI-OS nor any library, and I don't want to. I just want to understand how to do that and then being able to implement it.
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I think there's some boot code stuff but IDK how to use it. :s
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How can I, say, write a byte to a flash page ?
That is not a good idea. The flash can't bear an infinite number of writes. That number is high (10000 or something) so in "normal conditions of use", you don't reach it, but if you plan on writing byte per byte, or even 10 bytes per 10 bytes, as long as you use it often, you have more chance of having a flash wear.
Also, sorry not to actually answer the question (:P), I think that you should ask thepenguin or DrDnar or Runer or other people like that about unlocking flash.
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What wears out Flash memories is erasing sectors / blocks / full chip, not writing to sectors / blocks :)
The OS and boot code contain BCALLs to those effects. They're certainly documented, and besides, the datasheet of the Flash memory chip could enable you finding them more easily.
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Well, if you can only erase sectors, how can you save datas into them ? O.O
And btw I won't be able to understand a code which does something I don't know anything about.
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In case anybody missed it, Matrefeytontias is working on building an OS, so he won't have some tools (but it should be easier). I think (http://wikiti.brandonw.net/index.php?title=83Plus:OS:Raw_Flash_Commands) this page might be useful.
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How could I have missed that, I read every page of Ti-83+:OS on WikiTi O.O (well, I thought)
Thanks a lot, that'll help me :D
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Hmm if you write to flash, be very careful to not accidentally write to the wrong page. If you accidentally erase the certificate, you're screwed.
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Yup, the raw commands are in the datasheet, which is why I suggested him to read the datasheet and then find the corresponding routines in TI's boot code :)
But if it's documented in WikiTI, even better.
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I'll go with the page Xeda112358 gave me, and then I'll see if I can do something with it :)
By the way, Hayleia said that writing a byte to flash wasn't a good practice, but the said page gives an entire writeFlashByte routine .. what are your thoughts on that ?
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Writing more than one byte is just a matter of embedding the code for writing a byte into a loop. As I wrote above, what wears out the Flash memory is erasing it, not writing to it.
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Erasing a sector is what the TI-OS does when Defragmenting after an app deletion right ?
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Yup, erasing a sector is necessary so that you can write again arbitrary data to it. When writing to Flash memory (both NOR and NAND), the write operation is effectively an AND operation: it can only turn bits from 1 (default state after erasing) to 0. Only erasing makes it possible to turn bits from 0 to 1, but erasing is a slow, bulk operation performed over large blocks (sectors).
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But then if I want to write a single 1 bit to a sector I need to erase it first !? *.*
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Only if you want to overwrite it. If you have erased the sector (a chunk of 4 flash pages), you can write to all of the bytes. That is why the OS needs to garbage collect. When you unarchive to edit, then rearchive, it gets written back to a different place instead of overwriting the whole sector just to replace the data.
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Oh I see. So instead of rearchiving it to the same place and then erase the entire sector, it archives it in a free sector. Then when there are no free sectors left, it Garbage Collect and then erase sectors with no variables inside.
But how do I know if a sector is OK to be erased or if there's something inside since you can't turn bits ON when they're OFF ?
EDIT : maybe I can turn to $FE the first byte of a sector when it's in use, and then turn it to 0 when it's ready to be erased ?
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The OS stores a byte (FE, maybe?) at the start of the file data. When it is marked for deletion, reset bit 1 to make it FC. Follow that by the size of the data so that you can figure out where the next variable is. Once you reach an FF byte, that is free to start writing data to. You have to be careful about how you handle crossing sectors. The OS usually does not let variable be split across sectors, but being split across flash pages is fine.
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Yeah I just edited :P
But the danger is that a data which was in flash actually had a $FF byte inside. And I can't put a size word since I can't easily write what I want.
EDIT : By the way, how do you read from flash ? The page only says how to write to it ...
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You can definitely put a size word O.o And if you put in a size word, you just jump over all of the data, so you don't need to worry about a program having an FF since you jump over it. The bytes might look like:
FE0900EF004048656C6C6FC9 FE040001020304 FFFFFFFF...
When you read the first byte, it is FE, so the next two bytes are 0900 (the size bytes for 9 bytes), so you skip the next 9 bytes and now the pointer is at FE, the next size bytes tell it to skip 4 bytes, and now you are pointing to an FF byte that you can overwrite.
Reading flash is just like reading RAM.
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If I were to write an OS, and this is just me, I'd have variable storage hinge entirely on my VAT, and update my VAT (or MFT) updated every time a variable is moved if I, say, need a new sector to be erased. I'd copy the contents of that sector to some area in RAM and then write that data to a new sector.
Please tell me I'm not a newb for suggesting this!
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I can't afford to copy the content of a whole sector to RAM, because let's say that the calc is running a 32K-large program (all of the available RAM of a 83+ BE), I can't copy 16K of datas without overwriting some parts of this program. I need everything to be done in archive.
That's why I'll keep everything related to a program bundled, including its content, which won't change since there are no ways to edit stuffs. As I said in the cmdOS thread, every feature to include will be up to 3rd-party programmers, which are way better than me anyway.
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Reading flash is just like reading RAM.
... for the NOR Flash found, for instance, in TI-Z80 and TI-68k calculators :)
The NAND Flash found in Nspires, and most large Flash chips in fact (NAND is denser, AFAIK), is another story.
The rule of thumb is that any random access / eXecute In Place Flash memory is NOR.
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The ASIC will only let code on certain flash pages unlock flash. See port 21 (http://wikiti.brandonw.net/index.php?title=83Plus:Ports:21) for a list of such pages and the page on protected ports (http://wikiti.brandonw.net/index.php?title=Category:83Plus:Ports:By_Address:Protected) for how to unlock flash. Prudence suggests that normally you keep flash locked, although frankly, it's actually hard to issue a write or erase command accidentally, so keeping flash unlocked all the time isn't that dangerous. Nevertheless, you should confine modifying protected ports and writing to flash to be an OS-only thing. But, it would be nice to have functions for overwriting bytes in existing files, so user programs can have the option of leaving blank space in files to be filled later for whatever purpose.
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It's even something hard to me to reset a bit of the archive, don't ask me to overwrite a byte <_<
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TIOS uses a swap sector, doesn't it?
Do that!
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If you really want to see how to write to flash, you should take a look at how the boot code does it. (Attached) (The random sub_48c5 runs the routine that IX is pointing to in ram).
The boot code does a pretty good job of flash writing, so, you don't have to copy the code line by line, but, in the end, yours should look pretty similar.
And like xeda said, reading from flash is no different than reading from ram. Just swap the page in and go. (Using ports 06 and 07 of course).
Before you get too far in your OS, I would definitely suggest you disassemble the boot code and at least disassemble the boot path through the OS. There are a few reasons I say this, and here they are:
1. You need to see what happens in the boot code before your OS gets control
2. Doing all of this will teach you a bit more about the hardware of the calculator
3. You can learn quite a bit from the different routines in the boot code (and you may actually want to use them)
4. Seeing how TI-OS handles the boot sequence could guide you in writing your own.
Now, you don't have to disassemble these two. But I would highly recommend you do the boot code. I learned so much when I did that and I think it would really help you.