Fix order of operands in word-sized `cmp`.
Chris Pressey
6 years ago
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Subtracts the contents of src from dest (without considering carry) but
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does not store the result anywhere, only sets the resulting flags.
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This means that `z` is set if src and dest are equal,
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and `c` is set if dest is greater than or equal to src
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(`c` is unset if dest is less than src.)
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* It is illegal if src OR dest is uninitialized.
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In addition, if dest is of `word` type, then src must also be of `word`
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type, and in this case this instruction trashes the `a` register.
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Note that, like `cmp` is not suitable for making a
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Note that `cmp` is not suitable for making a
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signed comparison; this article, which mentions
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techniques that a SixtyPical compiler could use to
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implement `cmp`, also explains why that is:
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if isinstance(src, LocationRef) and src.type == TYPE_WORD:
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src_label = self.get_label(src.name)
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dest_label = self.get_label(dest.name)
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self.emitter.emit(LDA(Absolute(src_label)))
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self.emitter.emit(CMP(Absolute(dest_label)))
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self.emitter.emit(LDA(Absolute(dest_label)))
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self.emitter.emit(CMP(Absolute(src_label)))
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end_label = Label('end_label')
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self.emitter.emit(BNE(Relative(end_label)))
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self.emitter.emit(LDA(Absolute(Offset(src_label, 1))))
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self.emitter.emit(CMP(Absolute(Offset(dest_label, 1))))
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self.emitter.emit(LDA(Absolute(Offset(dest_label, 1))))
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self.emitter.emit(CMP(Absolute(Offset(src_label, 1))))
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self.emitter.resolve_label(end_label)
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return
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cls = {
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