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Binary 8-bit subtraction of X from Y with carry in. Pops the operands off the stack, leaves the result in X. If the subtraction produces a borrow, sets the carry flag.
The R register does double duty as result flags for binary arithmetic instructions.
From dialect 23 bit 0 of R is the carry flag. A carry out of an ADD, ADC, SUB or SBC instruction will set bit 0 of R.
The same bit 0 the acts as the carry input to ADC and SBC.
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