Fields with `flag' behavior behave like most edge/event-sensitive interrupt flags in commercial peripherals work: occurance of the event sets the flag bit, and writing a one to the bit through MMIO clears it again.

Usually many of these flags are combined into a single register. Canonical usage by software is then to read the register to determine which events have occurred, write the read value back to the register, and then handle the events. If a new event occurs between the read and write, its flag will not be cleared, because a zero will be written to it by the write action. This event will then be handled the next time the software reads the flag register.

It normally isn't possible to detect how many events have occurred for a single flag, just that there was at least one occurrance since the last read of the flag. If this information is necessary, the counter behavior can be used instead. If only the knowledge that an overflow occurred is needed, bit-overflow-internal can be used to drive an internal-flag field and/or an internal interrupt.

This structure supports the following configuration keys.

Configure the existence and behavior of the hardware read port.

The following values are supported:

• disabled (default): no read port is generated.

• simple: only the data output is generated.

This key is optional unless required by context. If not specified, the default value (disabled) is used.

Configures the reset value.

The following values are supported:

• no (default): the internal data register resets to 0, with the valid flag set.

• yes: the internal data register resets to 1, with the valid flag set.

• an integer: the internal data register resets to the given value, with the valid flag set.

• generic: the reset value is controlled through a VHDL generic.

This key is optional unless required by context. If not specified, the default value (no) is used.

Configures strobing an internal signal when a bit-set operation to a bit that was already set occurs. This essentially serves as an overflow signal for flag fields.

The following values are supported:

• null (default): the feature is disabled.

• a string matching [a-zA-Z][a-zA-Z0-9_]*: an internal signal with the given name is created (if necessary) and strobed when a bit-set operation occurs to an already-set bit.

This key is optional unless required by context. If not specified, the default value (null) is used.

Configures strobing an internal signal when a bit-clear operation to a bit that was already cleared occurs. This essentially serves as an underflow signal for flag fields.

The following values are supported:

• null (default): the feature is disabled.

• a string matching [a-zA-Z][a-zA-Z0-9_]*: an internal signal with the given name is created (if necessary) and strobed when a bit-clear operation occurs to an already-cleared bit.

This key is optional unless required by context. If not specified, the default value (null) is used.