A packet parser has a set of marker elements each comprising a one bit latch and connected to store flag values from the results of the application of parser rules. Some marker elements are connected to provide the stored marker values as input to the parser rule logic to be taken into account in the processing of subsequent parser rules and some are connected to control external hardware. Some markers are reset at the end of each packet. A special toggle marker element toggles its value when its address is selected and other marker elements are connected to store, when its own address is selected, the value of the toggle element. Other markers toggle their own value when selected.