39 lines
1.8 KiB
Plaintext
39 lines
1.8 KiB
Plaintext
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GOAL:
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live-program an emulator in assembly
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- a program is a graph that is flattened into a set of bytestreams.
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- first step: make the damn graph.
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APPROACH:
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- program is split into "ROM" and "RAM" - ROM is code that generally doesn't change while the program is running, RAM is program data
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- a program is assembled from snippets that refer to other snippets by name
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- a code snippet can be marked as a "loop point", a safe spot to pause and replace a running program's code
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- this is also be a good spot to take snapshots of "RAM" which we could roll back to
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- this assumes an empty stack + no code references in RAM - perhaps a special typed variable is needed for snippet pointers?
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- for now, we will depend on gsplus' socket-based remote debugging capabilities to poke the program into emulated memory
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WHAT IS A SNIPPET
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- label + block
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- block = bytestring, labels (offset into instruction list), fixups (label references)
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- can concatenate blocks; snippets are stored "loose" - accessed by absolute JMP or JSR
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- other compositional ideas:
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- "scopes" - keep labels "alive" only for a short period, for autogenerated loops, if statements, etc
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- could also just gensym
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(lda 13) ; immediate
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(lda (addr #x2000)) ; absolute
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(lda (addr #x20)) ; zero-page
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(lda (off-x (addr #x20))) ; zp,x
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(lda (off-y (addr #x20))) ; zp,y
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(lda (off-x@ (addr #x20))) ; (zp,x)
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(lda (@off-y (addr #x20))) ; (zp),y
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(lda (off-x (addr #x2000))) ; abs,x
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(lda (off-y (addr #x2000))) ; abs,y
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label = lazy address
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maybe treat zero-page and absolute as different types? we will always plan what goes into zero-page. zp has addressing modes abs doesn't.
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each opcode is a function that returns a block, usually 0-3 bytes with optionally associated labels + fixups
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(fixup is a tuple: local offset, label reference, and whether we're expecting an absolute, zero-page, or relative address)
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