Cache Result Register for Quick Cache Information Lookup

• US 20120173825A1
• Filed: 12/30/2010
• Published: 07/05/2012
• Est. Priority Date: 12/30/2010
• Status: Active Grant

• Current Assignee: NXP USA, Inc. (NXP Semiconductors NV)
• Original Assignee: Freescale Semiconductor, Inc. (NXP Semiconductors NV)
• Inventors: Ehrlich, Robert, Heuer, Kevin C., McGowan, Robert A.
• Granted Patent: US 8,572,323 B2
• Time in Patent Office: Days
• US Class Current: 711/145
• CPC Class Codes: G06F 11/362 Software debugging
• CPC Class Codes: G06F 12/0897 with two or more cache hierarchy levels with multilevel cache hierarchies G06F12/0811

Abstract

Each level of cache within a memory hierarchy of a device is configured with a cache results register (CRR). The caches are coupled to a debugger interface via a peripheral bus. The device is placed in debug mode, and a debugger forwards a transaction address (TA) of a dummy transaction to the device. On receipt of the TA, the device processor forwards the TA via the system bus to the memory hierarchy to initiate an address lookup operation within each level of cache. For each cache in which the TA hits, the cache controller (debug) logic updates the cache'"'"'s CRR with Hit, Way, and Index values, identifying the physical storage location within the particular cache at which the corresponding instruction/data is stored. The debugger retrieves information about the hit/miss status, the physical storage location and/or a copy of the data via direct requests over the peripheral bus.

Claims

1. A device comprising:

at least one processor core;

a memory hierarchy coupled to each of the at least one processor core and including a memory and one or more levels of cache, wherein each cache has a cache result register (CRR) which captures cache access information that can be utilized to access a location at which specific information is stored within the cache; and

transaction address processing logic within each cache of the memory hierarchy that responsive to receipt of a transaction address of a dummy transaction while the one or more levels of caches are in debug mode determines whether the transaction address of the dummy transaction hits within the cache;

wherein in response to a hit of the transaction address of the dummy transaction within the cache, the transaction address processing logic automatically updates the CRR of the cache with the cache access information and passes the transaction address down to a next level of the memory hierarchy; and

wherein in response to a miss of the transaction address of the dummy transaction within the cache, the transaction address processing logic passes the transaction address down to a next level of the memory hierarchy.

2. The device of claim 1, wherein the cache access information includes a hit/miss indicator of whether the transaction address hits within the particular cache and a way indicator value and an index value corresponding to a physical storage location within the cache at which the specific information corresponding to the transaction address of the dummy transaction resides within the particular cache.

3. The device of claim 2, wherein the CRR has a first pre-set number of bits assigned to store the way indicator value corresponding to the physical storage location of information corresponding to a transaction address that hits within the cache, a second pre-set number of bits assigned to store the index value corresponding to the physical storage location within the cache, and a third pre-set bit assigned to store the hit/miss indicator, wherein a first value of the third pre-set bit indicates a cache hit and a second value of the third pre-set bit indicates a cache miss.

4. The device of claim 3, wherein, when the cache is a data cache, the CRR also comprises a dirty bit indicator that is assigned to store an indication of whether data within the particular physical storage location of the data cache is dirty, wherein the data is dirty when the data has been recently updated, but has not yet been copied to memory.

5. The device of claim 2, further comprising:

a system bus that connects the at least one processor core to the associated memory hierarchy and on which the transaction address is transmitted to the associated memory hierarchy; and

a peripheral bus that connects a debug interface to the at least one processor core and to each cache within the associated memory hierarchy and which enables a debug probe of the debugger to obtain hit/miss information from the cache access information contained within the CRR of each cache and to perform an access to the physical storage location within a random access memory (RAM) of the cache utilizing the way indicator value and the index value from the cache access information contained within the CRR.

6. The device of claim 5, wherein said cache result register connects via said peripheral bus to a JTAG debug interface which provides access by the debug probe to said device.

7. The device of claim 1, wherein the one or more levels of cache comprises an L1 cache and a L2 cache and wherein the device is a data processing system that does not have hardware enforced memory coherency, such that each level of cache within the memory hierarchy is not necessarily coherent and does not necessarily contain a copy of an address and associated cached information corresponding to information from a highest level of the memory hierarchy that is being utilized by the processor core, wherein an update in a first cache occurs independent of and does not necessarily trigger a corresponding update in a second cache within the memory hierarchy.

8. The device of claim 1, further comprising:

a debug interface for connecting the device to a debugger located on an external device; and

cache debug logic that controls an exchange of information with the debugger based on received requests from the debugger;

wherein the cache debug logic provides access to the physical storage location within the cache indicated by the cache access information contained within each CRR, in response to receipt at the device from the debugger of a request for access to the data contained within the physical storage location;

wherein the cache debug logic forwards to the debugger a current value of the cache access information from each CRR in response to a request to populate a cache viewer of the debugger with current information of a physical storage location within the cache of the particular data; and

wherein the cache debug logic forwards to the debugger a copy of Hit/Miss indicator of the CRR for each cache at which the transaction address hits, to populate a memory window of the debugger; and

wherein the cache debug logic forwards to the debugger a copy of the particular data that is retrieved at the WAY and INDEX location from each cache at which the Hit/Miss indicator of the CRR indicates a hit of the transaction address and a dirty bit indicator indicates that the data is dirty.

9. The device of claim 8, wherein the peripheral bus connection to the cache debug logic and enables the debugger to perform an update of the information stored within the physical storage location utilizing the cache access information, wherein the update is performed within only those caches in which the transaction address of the dummy transaction hit, determined from reading the hit/miss indicator of the CRR, and without triggering a full cache address lookup operation.

10. The device of claim 1 further comprising a memory management unit (MMU) connected via the system bus to the processor core and to a first level of the one or more levels of cache and which receives a virtual address of the dummy transaction from the processor core and translates the received virtual address into a real address corresponding to a memory location during processing of the dummy transaction.

11. A computer implemented method for debugging a device using a debugger, the method comprising:

forwarding to the device, while the device is in the debug mode, a dummy transaction having a transaction address that triggers a cache access operation within each level of cache to determine whether the transaction address hits within a particular cache;

wherein the forwarding of the dummy transaction while the device is in debug mode further triggers each cache to update a cache result register (CRR) of the particular cache with cache access information that indicates whether the transaction address hits within the particular cache, and when the transaction address hits within the particular cache, further indicates a way and an index at which the information corresponding to the transaction address is located within the particular cache;

in response to completion of the dummy transaction within the device, triggering a debug probe coupled to the device to retrieve hit/miss information from the CRR and updating a memory window of the debugger with a copy of the data returned from the dummy transaction and hit/miss information corresponding to the dummy transaction for each cache within the memory hierarchy, wherein the debug probe of the debugger coupled to a debug interface of the device detects from each CRR of each cache whether the transaction address from the dummy transaction hit within the particular cache; and

in response to the transaction address having hit within the particular cache, retrieves a way indicator value and an index value from the CRR and performs an access to a specific physical location within the cache indicated by the way indicator value and the index value at which the information is stored within the particular cache;

wherein each CRR is accessible to the debug interface via a peripheral bus connecting the debug interface to the processor core and to each cache.

12. The method of claim 11, further comprising:

receiving one of a first request to update a memory window of the debugger with hit/miss information for a transaction address and a second request to update a cache viewer of the debugger with specific HIT/WAY/INDEX information from each cache at which a transaction address of a dummy read transaction hits;

responding to the first request by issuing the dummy transaction to the processor of the device, and triggering the debug probe to retrieve the hit/miss indicator value from each CRR for display of the caches in which the transaction address hits within the memory window of the debugger;

responding to the second request by issuing the dummy transaction to the processor of the device, and triggering the debug probe to access the CRR of each level of cache and return the hit/miss indicator value and the way and index values contained within each respective CRR for display within the cache viewer of the debugger;

responsive to receipt of the hit/miss values from the respective CRRs following the first request, updating the memory window with the information received, wherein said updating provides a copy of the data and information about which levels of the memory hierarchy contains a copy of the data; and

responsive to receipt of the hit/miss and way and index values from the respective CRRs, updating the cache viewer with the information received, wherein said updating provides information about a specific way and an index location at which the data resides within each cache contain a copy of the data.

13. The method of claim 11, wherein the device is coupled via a debug probe and a debug interface to a data processing system on which a debugger application executes.

14. The method of claim 13, wherein the debug probe is a JTAG probe.

15. A method for enabling debugging within a device having a processor core and a memory hierarchy having one or more levels of cache, the method comprising:

receiving at the processor core a transaction address associated with a dummy read, while the device is in a debug mode, wherein the processor core and each cache of the device are set to operate in the debug mode;

in response to receipt of the transaction address, forwarding the transaction address to a first level cache of a memory hierarchy that includes one or more levels of cache and a memory;

determining within each level of the one or more levels of cache whether the transaction address hits; and

in response to a hit of the transaction address within a cache, updating a cache result register (CRR) of that particular cache with cache access information including a hit/miss indicator value and a way indicator value and an index value corresponding to a physical location within the particular cache at which the information associated with the transaction address is located.

16. The method of claim 15, further comprising:

forwarding the transaction address to a next level of the memory hierarchy; and

in response to the dummy transaction being a read transaction and the transaction address reaching the memory, returning data associated with the transaction address from the memory;

forwarding a copy of the data to a debugger that is communicatively coupled to the device; and

responsive to a request for CRR hit information, forwarding to the debugger a value of the hit/miss indicator within each CRR, wherein the debugger utilizes the hit/miss indicator values to identify within a memory window of the debugger which caches within the memory hierarchy contains a copy of the data corresponding to the transaction address of the dummy transaction.

17. The method of claim 15, further comprising:

enabling a connected debug probe to access each CRR of each cache within the memory hierarchy to determine whether the transaction address hit within the particular cache; and

when the CRR provides an indication that the transaction address hit within the particular cache and a request is received from the debug probe for access to the information contained within the physical storage location, retrieving the way indicator value and the index value from the CRR and accessing the physical location within the particular cache to perform the requested access to the stored information;

wherein the access occurs only within those caches in which the transaction address of the dummy transaction hits and without triggering a full cache address lookup operation on the device.

18. The method of claim 17, wherein in response to the debugger issuing a request to modify the information found within the way and index within the CRR of a cache in which the transaction address hits, the method further comprises:

accessing the physical location indicated by the way indicator value and index value of the CRR of the particular cache; and

performing the modification of the information stored at the physical location within the particular cache.

19. The method of claim 15, further comprising:

in response to receipt of a request for CRR information from the caches within the memory hierarchy, forwarding to a debug probe a content of each CRR associated with each of the caches within the memory hierarchy; and

in response to receipt of a request for the actual data from the different levels of the memory hierarchy when a dirty bit is indicated within the CRR, accessing the way and index location within each cache in which the transaction address hit and for which the dirty bit indicator of the CRR indicates that the stored data is dirty, and retrieving and forwarding to the debugger a copy of the data stored at that particular way and index location for each cache containing the data.

20. The method of claim 15, wherein the CRR comprises a first pre-set number of bits assigned to store the way indicator value corresponding to a physical location of information corresponding to a transaction address that hits within the cache, a second pre-set number of bits assigned to store the index value corresponding to the physical location within the cache, and a third pre-set bit assigned to store a hit/miss indicator, wherein a first value of the third pre-set bit indicates a cache hit and a second value of the third pre-set bit indicates a cache miss and wherein, when the cache is a data cache, the CRR further comprises a dirty bit indicator that indicates whether the data stored at the way and index location within that cache is dirty.