CPU flags are NOT set. JS uses the (S) flag.
The (S) flag is set if a calculation gives a negative result.
Assembler
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Machine Code
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Explanation
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JS Forward
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C3 12
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Jump if the (S) flag is set.
If the (S) flag is set, jump forward 12 places.
If the (S) flag is set, add 12 to (IP).
If the (S) flag is NOT set, add 2 to (IP).
The assembler calculates the jump distance.
The biggest possible forward jump is +127.
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JS Back
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C3 FE
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Jump if the (S) flag is set.
If the (S) flag is set, jump back 2 places.
If the (S) flag is set, add -2 to (IP).
If the (S) flag is NOT set, add 2 to (IP).
The assembler calculates the jump distance.
The biggest possible backward jump is -128.
FE is -2. This is explained here.
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