Re:6502 “Illegal” Opcodes Demystified
by masswerk on Sunday June 06, 2021 @01:42PM
(#27414779)
The overflow flag and its intrinsics was quite a mystery for a while.
(Related fun fact: There is no such thing as a break flag, it just gets inserted, whenever the the status register is pushed to the stack by an instruction, i.e. by software, as opposed to a hardware interrupt. This is also, why it is commonly shown as 1, because this is the value it would show up with, if there was a PHP or BRK instruction at the given time, which is also: always. But this is really just a virtual state and there is no such flag in the CPU.)
(Related fun fact: There is no such thing as a break flag, it just gets inserted, whenever the the status register is pushed to the stack by an instruction, i.e. by software, as opposed to a hardware interrupt. This is also, why it is commonly shown as 1, because this is the value it would show up with, if there was a PHP or BRK instruction at the given time, which is also: always. But this is really just a virtual state and there is no such flag in the CPU.)