Flags
Explore flags, the CPU’s way of recording the results of operations. Learn how flags like Zero, Carry, and Overflow are used to influence program control flow and implement logic like comparisons and conditional jumps.
When the CPU executes operations, it keeps information about the outcome of each instruction in something called flags. Each CPU has a different set of flags, but they usually follow similar patterns.
Flags are a way for the CPU to track the result of the most recent operation, and they are commonly used in conditional instructions (e.g., branch if zero, branch if negative, etc.) to decide what to do next in the program.
Flags are stored in a special register inside the CPU, often called the status register, condition code register (CCR), or flags register, depending on the architecture.
Here are some common flags you'll encounter in most CPU architectures:
-
Zero flag (Z): Set if the result of an operation is zero. Example: If you subtract two equal numbers, the result is zero, so the zero flag is set.
-
Negative flag (N): Set if the result of an operation is negative (usually if the highest bit is set in a signed value). Example: Subtracting a larger number from a smaller one.
-
Carry flag (C): Set if an operation results in a carry out of the most significant bit (used in unsigned arithmetic). Example: Adding two large numbers that overflow the size of the register.
-
Overflow flag (V or O): Set if the result of an operation caused a signed overflow (i.e., the result is too large or too small for the signed number range).
-
Sign flag (S): Sometimes used instead of or in addition to the Negative flag, depending on the architecture.
-
Parity flag (P): Set if the number of 1s in the result is even (used in some architectures like x86 for error checking).
These flags are typically updated automatically by the CPU after every arithmetic or logical operation.
For example, if you perform an add instruction, the CPU will update the zero, carry, and overflow flags based on the result.
Later instructions, like branch if zero (BEQ) or branch if negative (BMI), can then check these flags to determine whether or not to change the flow of execution.
Example:
cmp d0, d1 ; Compare d0 with d1 (sets flags based on result)
beq equal ; If result was zero (d0 == d1), jump to 'equal'
Here, the cmp instruction subtracts d1 from d0 and sets the flags based on the result, without storing the result anywhere.
Then beq checks the zero flag—if it's set, that means the values were equal, and the program jumps to the equal label.