76 lines
2.8 KiB
Text
76 lines
2.8 KiB
Text
## Metacompilation and Assembly
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RETRO 10 and 11 were written in themselves using a metacompiler.
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I had been fascinated by this idea for a long time and was able
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to explore it heavily. While I still find it to be a good idea,
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the way I ended up doing it was problematic.
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The biggest issue I faced was that I wanted to do this in one
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step, where loading the RETRO source would create a new image
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in place of the old one, switch to the new one, and then load
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the higher level parts of the language over this. In retrospect,
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this was a really bad idea.
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My earlier design for RETRO was very flexible. I allowed almost
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everything to be swapped out or extended at any time. This made
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it extremely easy to customize the language and environment, but
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made it crucial to keep track of what was in memory and what had
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been patched so that the metacompiler wouldn't refer to anything
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in the old image during the relocation and control change. It
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was far too easy to make a mistake, discover that elements of
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the new image were broken, and then have to go and revert many
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changes to try to figure out what went wrong.
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This was also complicated by the fact that I built new images
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as I worked, and, while a new image could be built from the last
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built one, it wasn't always possible to build a new image from
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the prior release version. (Actually, it was often worse - I
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failed to check in every change as I went, so often even the
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prior commits couldn't rebuild the latest images).
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For RETRO 12 I wanted to avoid this problem, so I decided to go
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back to writing the kernel ("Rx") in assembly. I actually wrote
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a Machine Forth dialect to generate the initial assembly, before
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eventually hand tuning the final results to its current state.
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I could (and likely will eventually) write the assembler in
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RETRO, but the current one is in C, and is built as part of the
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standard toolchain.
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My VM actually has two assemblers. The older one is Naje. This
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was intended to be fairly friendly to work with, and handles
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many of the details of packing instructions for the user. Here
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is an example of a small program in it:
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:square
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dup
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mul
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ret
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:main
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lit 35
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lit &square
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call
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end
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The other assembler is Muri. This is a far more minimalistic
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assembler, but I've actually grown to prefer it. The above
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example in Muri would become:
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i liju....
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r main
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: square
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i dumure..
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: main
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i lilica..
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d 35
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r square
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i en......
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In Muri, each instruction is reduced to two characters, and the
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bundlings are listed as part of an instruction bundle (lines
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starting with `i`). This is less readable if you aren't very
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familiar with Nga's assembly and packing rules, but allows a
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very quick, efficient way of writing assembly for those who are.
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I eventually rewrote the kernel in the Muri style as it's what
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I prefer, and since there's not much need to make changes in it.
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