how to set properly the flags v and c

for ADC:

Carry is set if the addition resulted in an unsigned overflow (sum > 255), and is cleared otherwise (<= 255).

Overflow is set if the addition resulted in a signed overflow (sum > 127, or < -128), and is cleared otherwise (sum >= -128 and sum <= 127).

In an emu you probably have all your numbers as unsigned, so checking for Carry is pretty simple:



Overflow is a bit weirder if you're dealing with unsigned logic. The common way to do it is check to see if:

Positive + Positive = Negative
or
Negative + Negative = Positive

If either of those are true, you set V, otherwise you clear it. In this case "negative" just means that the high bit is set. So:



where 'val' is the value being added to A, 'A' is the accumulator before addition, and 'sum' is the resulting sum of A+val+C.

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for SBC:

SBC logic is just ADC inverted. You can implement SBC with the following, and it will work perfectly (provided your ADC is implemented correctly:



Or if you want to implement SBC on its own:

C is set if the unsigned result was >= 0, and is cleared if the unsigned result was < 0.

V is set under same conditions as for ADC (result caused signed overflow). Logic to implement this with unsigned types:

Positive - Negative = Negative
Negative - Positive = Positive

Ways to implement this:



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for CMP/CPX/CPY

Do not change V at all.

C is set the same as it is for SBC -- however note that while SBC will subtract an extra 1 if C is clear, CMP/CPX/CPY never subtract that extra 1.

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for ASL/LSR/ROL/ROR

C is set or cleared as a result of the bit shifted out.
V is unchanged.

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the N flag (sign / negative flag)

is just set if the high bit (bit 7: 0x80) of the result is set, and is cleared otherwise: