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7

Here are a few further thoughts on the setting of flags and the SO (set overflow) pin mentioned in the other answers, in response to Joe's comment, The fact that there's a direct pin that can set the overflow flag is definitely a bit of a "i guess someone somewhere had a use case that needed it". The use of it in the 1540/1541(and I'd guess 1570/1571?)...


16

There is simply no need for setting Overflow. The same is as well true for Negative/Sign and Zero. No operation will be influenced by any of them, it's only used to signal an overflow during ADC and SBC (well, and BIT for testing bit#6). In fact, the question is rather, why there is a CLV present, as there is no reason, within the boundaries of the ...


37

Setting and clearing carry, the decimal or interrupt flags is useful: the carry flag because the 6502 offers only add and subtract with carry; the decimal flag because it changes the mode of the processor; and the interrupt flag because it masks or unmasks the maskable interrupt. Conversely, explicitly setting and clearing the other flags mostly isn't ...


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