Block programmable priority encoder in a CAM
View Patent ↗A priority encoder (PE) for a CAM, comprising a plurality of PE blocks, each receiving a plurality of match results corresponding to data entries in a corresponding data array block and, for determining an address of a highest priority data entry based on a physical location in the data array block during a CAM search-and-compare operation a register for storing a user defined priority value assigned to each PE block and means for evaluating priority values and the address determined by the plurality of PE blocks to select a PE block having the highest priority data entry.
1. A priority encoder for providing a low physical address match data or a high physical address match data as a highest priority match data, the low physical address match data having higher priority than the high physical address match data, comprising:
a multiplexor for passing the high physical address match data as the highest priority match data in response to a first state of a control signal, the multiplexor passing the low physical address match data in response to a second state of the control signal;
logic circuitry for receiving a first priority value corresponding to the low physical address match data and a second priority value corresponding to the high physical address match data, the logic circuitry comparing the first priority value to the second priority value and generating the first state of the control signal if the second priority value is greater than the first priority value.
2. The priority encoder of claim 1 , wherein the low physical address match data and the high physical address match data each include a match address and a match flag.
3. The priority encoder of claim 2 , wherein the low physical address match data includes the first priority value, and the high physical address match data includes the second priority value.
4. The priority encoder of claim 1 , wherein the logic circuitry includes a subtractor for subtracting the first priority value from the second priority value, the subtractor generating the control signal if a difference between the second priority value and the first priority value is greater than 0.
5. The priority encoder of claim 4 , further including a logical AND circuit having a first input for receiving the control signal and a second input for receiving a match flag corresponding to the high physical address match data, the logical AND circuit passing the control signal if the match flag corresponds to a match condition for the high physical address match data.
6. A method for providing a low physical address match data or a high physical address match data as a highest priority match data, the low physical address match data having higher priority than the high physical address match data, comprising:
a) receiving a first priority value corresponding to the low physical address match data and a second priority value corresponding to the high physical address match data;
b) comparing the first priority value to the second priority value; and,
c) passing the high physical address match data as the highest priority match data if the second priority value is greater than the first priority value.
7. The method of claim 6 , wherein the step of comparing includes subtracting the first priority value from the second priority value.
8. The method of claim 7 , wherein the step of comparing further includes generating a control signal when the second priority value is greater than the first priority value.
9. The method of claim 8 , wherein the step of passing includes passing the high physical address match data in response to the control signal.
10. The method of claim 9 , wherein the step of generating a control signal includes passing the control signal in response to a logical state of a match flag corresponding to the high physical address match data.
11. The method of claim 8 , wherein the control signal is passed with the high physical address match data.
12. The method of claim 6 , wherein the low physical address match data and the high physical address match data each include a match address and a match flag.
13. The method of claim 6 , wherein the low physical address match data includes the first priority value, and the high physical address match data further includes the second priority value.