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System Flags Tutorial

System Flags

The 6502 CPU contains a status register and each individual bit is a FLAG. A flag indicates the result of the previous operation.

N result is negative.
V result too big, causing an overflow.
B  set by software BRK command.
D  in decimal mode.
I   an interrupt has occurred.
Z  result zero.
C carry.

The flags are tested by the CPU and, depending on their state, determine which part of the program the CPU will jump to.

This is how computers are able to make decisions.

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