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94 lines
3.9 KiB
Markdown
94 lines
3.9 KiB
Markdown
# Writing to a AT28 from Collapse OS
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## Goal
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Write in an AT28 EEPROM from within Collapse OS so that you can have it update
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itself.
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## Gathering parts
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* A RC2014 Classic
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* `stage2.bin` from the base recipe
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* An extra AT28C64B
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* 1x 40106 inverter gates
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* Proto board, RC2014 header pins, wires, IC sockets, etc.
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## Building the EEPROM holder
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The AT28 is SRAM compatible so you could use a RAM module for it. However,
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there is only one RAM module with the Classic version of the RC2014 and we
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need it to run Collapse OS.
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You could probably use the 64K RAM module for this purpose, but I don't have one
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and I haven't tried it. For this recipe, I built my own module which is the same
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as the regular ROM module but with `WR` wired and geared for address range
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`0x2000-0x3fff`.
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If you're tempted by the idea of hacking your existing RC2014 ROM module by
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wiring `WR` and write directly to the range `0x0000-0x1fff` while running it,
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be aware that it's not that easy. I was also tempted by this idea, tried it,
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but on bootup, it seems that some random `WR` triggers happen and it corrupts
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the EEPROM contents. Theoretically, we could go around that by putting the AT28
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in write protection mode, but I preferred building my own module.
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I don't think you need a schematic. It's really simple.
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### Assembling stage 3
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Stage 2 gives you a full interpreter, but it's missing the "Addressed devices"
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module and the AT28 driver. We'll need to assemble a stage 3.
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When you'll have a system with function disk block system, you'll be able to
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directly `LOAD` them, but for this recipe, we can't assume you have, so what
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you'll have to do is to manually paste the code from the appropriate blocks.
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Addressed devices are at B140. To know what you have to paste, open the loader
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block (B142) and see what blocks it loads. For each of the blocks, copy/paste
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the code in your interpreter.
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Do the same thing with the AT28 driver (B150)
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If you're doing the real thing and not using the emulator, pasting so much code
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at once might freeze up the RC2014, so it is recommended that you use
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`/tools/exec` that let the other side enough time to breathe.
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After your pasting, you'll have a compiled dict of that code in memory. You'll
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need to relocate it in the same way you did for stage 2, but instead of using
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`RLCORE`, which is a convenience word hardcoded for stage 1, we'll parametrize
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`RLDICT`, the word doing the real work.
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`RLDICT` takes 2 arguments, `target` and `offset`. `target` is the first word
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of your relocated dict. In our case, it's going to be `' ADEVMEM+`. `offset` is
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the offset we'll apply to every eligible word references in our dict. In our
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case, that offset is the offset of the *beginning* of the `ADEVMEM+` entry (that
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is, `' ADEVMEM+ WORD(` minus the offset of the last word (which should be a hook
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word) in the ROM binary.
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That offset can be conveniently fetched from code because it is the value of
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the `LATEST` constant in stable ABI, which is at offset `0x08`. Therefore, our
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offset value is:
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' ADEVMEM+ WORD( 0x08 @ -
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You can now run `RLDICT` and proceed with concatenation (and manual adjustments
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of course) as you did with stage 2. Don't forget to adjust `run.fs` so that it
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runs `ADEV$`.
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## Writing contents to the AT28
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The driver provides `AT28!` which can be plugged in adev's `A!*`.
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First, upload your binary to some place in memory, for example `a000`. To do so,
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run this from your modern computer:
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./upload <tty device> a000 <filename>
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Then, activate `AT28!` with `' AT28! A!* !` and then run
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`0xa000 0x2000 <size-of-bin> AMOVE`. `AT28!` checks every myte for integrity,
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so it there's no error, you should be fine. Your content is now on the EEPROM!
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Why not upload content directly to `0x2000` after having activated `AT28!`?
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Technically, you could. It was my first idea too. However, at the time of this
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writing, I always get weird mismatch errors about halfway through. Maybe that
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the ACIA interrupt does something wrong...
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