![]() The same is true for unsigned overflow, but less so since the zero flag can be used as a substitute in these cases. ![]() ORA #%10000000 accumulator crossed from below $7F to above $80, but ORA doesn't affect the overflow flag.īVS ErrorHandler whether this branch is taken has NOTHING to do with the ORA instruction. INX although X went from $7F to $80, INX does not affect the overflow flag!īVS ErrorHandler whether this branch is taken has NOTHING to do with the INX instruction. Therefore, signed overflow can be "missed" by the CPU very easily if it occurs in other ways: The only arithmetic instructions that affect the overflow flag are ADC and SBC. Keep in mind that not all instructions affect the flags in the same way. LDA #$FFīCS ErrorHandler this branch will always be taken. These flags will automatically be set or cleared depending on the results of a calculation that can affect them.īVS ErrorHandler this branch will always be taken. BCC Branch if Carry Clear (unsigned overflow did not occur).BCS Branch if Carry Set (unsigned overflow has occurred).BVC Branch if Overflow Clear (signed overflow did not occur).BVS Branch if Overflow Set (signed overflow has occurred).The following instructions allow for branching based on the state of these flags: Unsigned overflow (crossing the FF-00 boundary) is detected by the CPU's carry flag C. Signed overflow (crossing the 7F-80 boundary) is detected by the CPU's overflow flag V. With the same program, if you unmask bit 20 by:īITFPO DC BL1'00001000' bit20=1 6502 Assembly 8-Bit Overflow You will have the S0C8 system abend code : fixed point overflow exception If you mask, you can test it in your program: Two non-privileged instructions (IPM,SPM) are available for retrieving and setting the program mask of the current PSW. ![]() You can choose to manage or not the binary integer overflow with the program mask bits of the PSW (Program Status Word).
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