I don't know what twos-complement means or what they meant by it but these 5 instructions IMHO are ADC,SBC,CLV,BIT as you said and the last one PLP.
Plus, the original 6502 has a "Set Overflow" Pin. Through it, external hardware can set the flag. Useful for extremely fast polling loops like
LOOP: BVC LOOP
This loop takes only 3 cycles. For example, the Commodore disk drives use it to find out when a byte has been read completely from the disk, since a normal
LOOP: BIT $XXXX
takes 7 cycles, which would be too slow, so the byte could be lost since one bit of the next byte has already come in by the time you get to read the byte from the hardware register.
I'm not sure it the S.O. pin is connected to anything in the Ataris, but most computers leave it unconnected.
Edited by tearex, Thu Jun 10, 2010 7:08 AM.