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recipes/rc2014: stage2 WIP

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Virgil Dupras 2020-04-07 22:36:59 -04:00
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recipes/rc2014/README.md
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@ -108,11 +108,135 @@ Once bootstrapping is done you should see the Collapse OS prompt. That's a full
Forth interpreter. You can have fun right now. Forth interpreter. You can have fun right now.
However, that long boot time is kinda annoying. Moreover, that bootstrap code However, that long boot time is kinda annoying. Moreover, that bootstrap code
being in source form takes precious space from our 8K ROM. We already have our being in source form takes precious space from our 8K ROM. That brings us to
compiled dictionary in memory. All we need to have a instant-booting Forth is building stage 2.
to combine our stage1 with our compiled dict in memory, after some relinking.
TODO: write this, do this. ### Building stage 2
You're about to learn a lot about this platform and its self-bootstrapping
nature, but its a bumpy ride. Grab something. Why not a beer?
Our stage 1 prompt is the result of Forth's inner core interpreting the source
code of the Full Forth, which was appended to the binary inner core in ROM.
This results in a compiled dictionary, in RAM, at address 0x8000+system RAM.
Unfortunately, this compiled dictionary isn't usable as-is. Offsets compiled in
there are compiled based on a 0x8000-or-so base offset. What we need is a
0xa00-or-so base offset, that is, something suitable to be appended to the boot
binary, in ROM, in binary form.
We can't simply adjust offsets. For complicated reasons, that can't be reliably
done. We have to re-interpret that same source code, but from a ROM offset. But
how are we going to do that? After all, ROM is called ROM for a reason.
Memory maps.
What we're going to do is to set up a memory map targeting our ROM and point it
to our RAM. Then we can recompile the source as if we were in ROM, right after
our boot binary. Forth won't ever notice it's actually in RAM.
Alright, let's do this. First, let's have a look around. Where is the end of
our boot binary? To know, find the word ";", which is the last word of icore:
> ' ; .X
097d>
> 64 0x0970 DUMP
:70 0035 0958 00da ff43 .5.X...C
:78 003b 3500 810e 0020 .;5....
:80 0043 0093 07f4 03ef .C......
:88 0143 005f 0f00 0131 .C._...1
:90 3132 2052 414d 2b20 12 RAM+
:98 4845 5245 2021 0a20 HERE !.
:a0 3a20 4840 2048 4552 : H@ HER
:a8 4520 4020 3b0a 203a E @ ;. :
See that `_` at 0x98b? That's the name of our hook word. 4 bytes later is its
wordref. That's the end of our boot binary. 0x98f, that's an address to write
down.
Right after that is our appended source code. The first part is `pre.fs` and
can be ignored. What we want starts at the definition of the `H@` word, which
is at 0x9a0. Another address to write down.
So our memory map will target 0x98f. Where will we place it? It doesn't matter
much, we have plenty of RAM. Where's `HERE`?
> H@ .X
8c3f>
Alright, let's go wide and use 0xa000 as our map destination. But before we do,
let's copy the content of our ROM into RAM because there's our source code
there and if we don't copy it before setting up the memory map, we'll shadow it.
Let's be lazy and don't even check where the source stop. Let's assume it stops
at 0x1fff, the end of the ROM.
> 0x98f 0xa000 0x2000 0x98f - MOVE
> 64 0xa000 DUMP
:00 3131 3220 5241 4d2b 112 RAM+
:08 2048 4552 4520 210a HERE !.
:10 203a 2048 4020 4845 : H@ HE
:18 5245 2040 203b 0a20 RE @ ;.
:20 3a20 2d5e 2053 5741 : -^ SWA
:28 5020 2d20 3b0a 203a P - ;. :
:30 205b 2049 4e54 4552 [ INTER
:38 5052 4554 2031 2046 PRET 1 F
Looks fine. Now, let's create a memory map. A memory map word is rather simple.
It is called before each `@/C@/!/C!` operation and is given the opportunity to
tweak the address on PSP's TOS. Let's go with our map:
> : MMAP
DUP 0x98f < IF EXIT THEN
DUP 0x1fff > IF EXIT THEN
[ 0xa000 0x98f - LITN ] +
;
> 0x98e MMAP .X
098e> 0x98f MMAP .X
a000> 0xabc MMAP .X
a12b> 0x1fff MMAP .X
b66e> 0x2000 MMAP .X
2000>
This looks good. Let's apply it for real:
> ' MMAP (mmap*) !
> 64 0x980 DUMP
:80 0043 0093 07f4 03ef .C......
:88 0143 005f 0f00 0131 .C._...1
:90 3132 2052 414d 2b20 12 RAM+
:98 4845 5245 2021 0a20 HERE !.
:a0 3a20 4840 2048 4552 : H@ HER
:a8 4520 4020 3b0a 203a E @ ;. :
:b0 202d 5e20 5357 4150 -^ SWAP
:b8 202d 203b 0a20 3a20 - ;. :
But how do we know that it really works? Because we can write in ROM!
> 'X' 0x98f !
> 64 0x980 DUMP
:80 0043 0093 07f4 03ef .C......
:88 0143 005f 0f00 0131 .C._...X
:90 0032 2052 414d 2b20 .2 RAM+
:98 4845 5245 2021 0a20 HERE !.
:a0 3a20 4840 2048 4552 : H@ HER
:a8 4520 4020 3b0a 203a E @ ;. :
:b0 202d 5e20 5357 4150 -^ SWAP
:b8 202d 203b 0a20 3a20 - ;. :
> 64 0xa000 DUMP
:00 5800 3220 5241 4d2b X.2 RAM+
:08 2048 4552 4520 210a HERE !.
:10 203a 2048 4020 4845 : H@ HE
:18 5245 2040 203b 0a20 RE @ ;.
:20 3a20 2d5e 2053 5741 : -^ SWA
:28 5020 2d20 3b0a 203a P - ;. :
:30 205b 2049 4e54 4552 [ INTER
:38 5052 4554 2031 2046 PRET 1 F
TODO: continue
[rc2014]: https://rc2014.co.uk [rc2014]: https://rc2014.co.uk
[romwrite]: https://github.com/hsoft/romwrite [romwrite]: https://github.com/hsoft/romwrite