Abstract:
A chip multiprocessor die supports optional stacking of additional dies. The chip multiprocessor includes a plurality of processor cores, a memory controller, and stacked cache interface circuitry. The stacked cache interface circuitry is configured to attempt to retrieve data from a stacked cache die if the stacked cache die is present but not if the stacked cache die is absent. In one implementation, the chip multiprocessor die includes a first set of connection pads for electrically connecting to a die package and a second set of connection pads for communicatively connecting to the stacked cache die if the stacked cache die is present. Other embodiments, aspects and features are also disclosed.

Description:
BACKGROUND 
     1. Technical Field 
     The present application relates generally to processors and memory for computer systems. 
     2. Description of the Background Art 
     Conventional two-dimensional (2-D) microprocessors, including conventional chip multiprocessors, are formed on a single silicon die. In order to increase performance of these microprocessors, further components, such as more processor cores, caches and memory controllers, are generally being integrated into the single silicon die. 
     Recently, however, technologies for stacking of silicon die have been developed. In order to apply the stacking technologies to chip multiprocessors, various proposals have been made. Each of these proposals provide an architecture or design for implementing the chip multiprocessor on a stack of silicon dies. For example, one set of proposals splits each core of the chip multiprocessor between multiple stacked die. 
     Applicants have observed that each of the proposals for applying stacking to chip multiprocessors makes the natural assumption that stacking will be required. In other words, the designs are optimized assuming stacking of silicon dies. 
     SUMMARY 
     One embodiment relates to a chip multiprocessor die supporting optional stacking of additional dies. The chip multiprocessor includes a plurality of processor cores, a memory controller, and stacked cache interface circuitry. The stacked cache interface circuitry is configured to attempt to retrieve data from a stacked cache die if the stacked cache die is present but not if the stacked cache die is absent. In one implementation, the chip multiprocessor die includes a first set of connection pads for electrically connecting to a die package and a second set of connection pads which can be configured for communicatively connecting to the stacked cache die if the stacked cache die is present. 
     Other embodiments, aspects, and features are also disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic cross-sectional diagram depicting a modular (stackable) chip multiprocessor with a variable number of stacked die for high-performance system applications in accordance with an embodiment of the invention. 
         FIG. 1B  is a schematic planar-view diagram depicting two sets of contact pads for a modular chip multiprocessor having at least one stacked die in accordance with an embodiment of the invention. 
         FIG. 2A  is a schematic cross-sectional diagram depicting a modular chip multiprocessor without any stacked die for low-cost system applications in accordance with an embodiment of the invention. 
         FIG. 2B  is a schematic planar-view diagram depicting two sets of contact pads for a modular chip multiprocessor having no stacked die in accordance with an embodiment of the invention. 
         FIG. 3  is a schematic diagram showing an example logic design for a modular 3-D chip multiprocessor in accordance with an embodiment of the invention. 
         FIG. 4  is a flow chart of a method performed by a memory controller of a modular 3-D chip multiprocessor in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present application discloses an architectural design for a chip multiprocessor die in embodiments of the present invention, where the chip microprocessor die is configured to be modular in that the 3-D stacking of additional levels of cache memory is optional, i.e. possible but not required. In this architecture, all the cores are contained on a single processor die. Additional cache levels may be optionally stacked using additional die. 
       FIG. 1A  is a schematic cross-sectional diagram depicting a modular (stackable) chip multiprocessor with a variable number of stacked die for high-performance system applications in accordance with an embodiment of the invention. In this embodiment, the chip multiprocessor (CMP) die  102  includes the multiple processor cores and one or more of near cache levels. 
     The heat sink  104  may be advantageously attached to the chip multiprocessor die  102 , and the package  106  (including connectors  108  for power and input/output) is preferably attached to at least one stacked cache die (for example, the topmost stacked die  110 - 2  in the illustrated example), if any. Of course, while the CMP  102 , stacked cache die  110 , and package  106  are shown spaced apart in  FIG. 1  for purposes of depicting the stacking order, an actual implementation would not typically have the spacing between the components. Instead the stacked cache die  110  would be stacked directly on top of the CMP  102 , and package  106  would be configured on top of the stacked cache die  110 . 
       FIG. 1B  is a schematic planar-view diagram depicting two sets of contact pads for a modular chip multiprocessor having at least one stacked die in accordance with an embodiment of the invention. Note that the particular arrangement, shape and scaling of the contact pads shown in  FIG. 1B  are arbitrary for purposes of explanation. 
     As shown in  FIG. 1B , the contact layer of the base die  102  for the chip multiprocessor is preferably configured to have two sets of connection pads. A first set of pads  120  preferably includes pads with larger surface areas and may either be connected directly to a package  106  or routed through one or more stacked die  110  to a package  106 . The interconnections from this first set of connection pads are shown by the thicker lines  112  in  FIG. 1A . A second set of pads  130  preferably include pads having smaller surface areas and would preferably only be used for communicating with stacked die  110  if they were present in the system. The interconnections for these connection pads are shown by the thinner lines  114  in  FIG. 1A . 
       FIG. 2A  is a schematic cross-sectional diagram depicting a modular chip multiprocessor without any stacked die for low-cost system applications in accordance with an embodiment of the invention. In this embodiment, the base die of the chip multiprocessor  102  is present, but the stacked cache die  110  are absent. As such, the thicker interconnections  112  to the package  106  are used, but there are no thinner connections  114  to the absent stacked cache die  110 . 
       FIG. 2B  is a schematic planar-view diagram depicting two sets of contact pads for a modular chip multiprocessor having no stacked die in accordance with an embodiment of the invention. Note again that the particular arrangement and scaling of the contact pads shown in  FIG. 2B  are arbitrary for purposes of explanation. 
     As shown in  FIG. 2B , the contact layer of the base die for the chip multiprocessor  102  is again configured to have two sets of connection pads. The first set of pads  120  preferably includes pads with larger surface areas and may be connected directly to a package  106 . The interconnections from this first set of connection pads are shown by the thicker lines  112  in  FIG. 2A . The second set of pads  130  preferably include pads having smaller surface areas and would preferably only be used for communicating with stacked die  110  if they were present in the system. In this case, however, there are no (optional) stacked die  110  present. Hence, the second set of pads  130  remain unconnected and un-used. 
     Advantageously, using this architectural design, the number of stacked stacked cache die  110  is variable. In the particular implementation shown in  FIG. 1A , two stacked cache die  110 - 1  and  110 - 2  are shown. This implementation may correspond to a high-performance high-cost multiprocessor system for applications with large memory needs. On the other hand, in the particular implementation shown in  FIG. 2A , no stacked cache die  110  are shown. This implementation may correspond to a lower-performance lower-cost multiprocessor system for applications with smaller memory needs. 
     Furthermore, this architectural design advantageously positions the cores, which typically dissipate the vast majority of power and hence generate the most heat, nearest to the heat sink  104  and the optional stacked cache die, which typically generate much less heat, further from the heat sink  104 . 
     An example logical design for a chip multiprocessor  102  in accordance with an embodiment of the present invention is illustrated in  FIG. 3 . While a particular CMP design (i.e. one with private L1 caches, semi-private L2 caches, and a shared L3 cache) is shown in  FIG. 3 , other specific CMP designs may be utilized in accordance with other embodiments of the invention. 
     As shown in  FIG. 3 , the chip multiprocessor  102  includes multiple processor cores  302 . Level one instruction (L1I) and level one data (L1D) caches may be provided for each core  302 . In this particular implementation, semi-private level two (L2) caches  304  are each shared by two cores  302 . Further in this particular implementation, inter-core interconnect circuitry  306  interconnects the L2 caches with a shared level 3 (L3) cache  308 . The shared L3 cache  308  is shown divided into banks. 
     As further shown in  FIG. 3 , one or more memory controllers  310  on the chip multiprocessor die  102  may be configured to communicate by way of the relatively thicker conductive connections  112  which interconnect those contact pads  120  with input/output connections (see  108 ) of the package  106 . The one or more memory controllers  310  also connect to stacked cache interface circuitry  312  which is on the CMP die  102 . While one block of circuitry is depicted in  FIG. 3  for the stacked cache interface circuitry  312 , the stacked cache interface circuitry  312  may comprise one block or multiple blocks of circuitry. The stacked cache interface circuitry  312  may be configured to communicate by way of the relatively thinner conductive connections  114  which interconnect those contact pads  130  with the optional stacked cache die  110 . 
     The stacked cache interface circuitry  312  may be small and so may be implemented without adding much cost to the CMP die  102  in the case where the CMP die  102  is not stacked (i.e. where no stacked cache die  110  are used and there are no stack die connections  114 ). Also, power to the stacked cache interface circuitry  312  may be configured so as to be unconnected in the case where the CMP die  102  is not stacked. 
     The memory controllers  310  may be configured to signal the stacked cache interface circuitry  312  so as to find out if one or more stacked cache die are present or if there are no stacked cache die. The stacked cache interface circuitry  312  may be configured to detect the presence of the optional stacked cache (e.g., a level 4 cache) die  110  by several mechanisms. One such mechanism comprises receiving a reply (acknowledgement signal) to signals transmitted to the stacked cache die  110  to indicate presence of the stacked cache die  110  or not receiving a reply to such signals which would indicate an absence of the stacked cache die  110 . Another mechanism comprises an absence of a signal path due to a lack of stacking (i.e. the signal path is open circuit when there is no stacked cache die  110 ). 
       FIG. 4  shows a logical method  400  performed by a memory controller  310  of a chip multiprocessor  102  in accordance with an embodiment of the invention. As shown by the branch point  401 , processing of a memory request is different depending on whether or not at least one stacked cache die is present. In accordance with an embodiment of the invention, the determination  401  as to whether one or more stacked cache die is present or absent may be performed at power-up by the memory controllers  310 . For example, in accordance with one embodiment of the invention, power to the stacked cache interface circuitry  312  may be disconnected during manufacture of the die  102  if there are no stacked cache die to be included in the system. In that case, the presence or absence of power to the stacked cache interface circuitry  312  may be used by the memory controllers  310  as an indication of the presence or absence of stacked cache die. 
     The memory controller  310  receives  402  a memory request. If no stacked cache die is present, then the memory controller  310  fetches  406  the requested data from the main memory (for example, from the memory DIMMs). In other words, in the lower-performance lower-cost configuration shown in  FIG. 2A , the memory controller  310  on the chip multiprocessor  102  accesses main memory for the missing data. 
     On the other hand, if there is a stacked cache (i.e. if there is one or more cache die)  110 , then the memory controller  310  attempts to find the data in the stacked cache  110  by sending  408  a memory request signal to the stacked cache interface circuitry  312 . If  410  the data is found in the stacked cache (i.e. a stacked cache hit), then the memory controller  310  receives  412  the requested data from the stacked cache interface  312 . If the data cannot be found in the stacked cache (i.e., a stacked cache miss), then the memory controller  310  resorts to fetching  406  the data from memory. 
     In accordance with one embodiment, the chip multiprocessor die  102  includes one or more near cache levels, and the stacked cache die(s)  110  include optional cache levels which are higher (farther) than those levels on the chip multiprocessor die  102 . In that case, memory requests would be processed by first checking the near cache levels on the CMP die  102 . Upon near cache misses such that the data requested is not found on the CMP die  102 , the memory controller  310  would then send a memory request signal to the stacked cache interface circuitry  312 . If the data is found in the stacked cache (i.e. a stacked cache hit), then the memory controller  310  receives  412  the requested data from the stacked cache interface  312 . If  410  the data cannot be found in the stacked cache (i.e., a stacked cache miss), then the memory controller  310  resorts to fetching  406  the data from memory. 
     The architecture disclosed in the present application has several advantages or potential advantages. First, processor manufacturers are generally interested in providing a range of products to cover different markets. However, designing different products for different markets is typically rather expensive. Instead, with optional stacking, manufacturers may sell a stacked chip microprocessor in markets that had higher performance demands and required a more powerful memory system (e.g., enterprise servers and high-performance computing applications), while the base die may be used in lower-performance cost-sensitive applications (e.g., home consumer and laptop applications). The lower-performance lower-cost version of the product may be built simply by omitting some or all of the stacked die. 
     Second, by placing all the cores on a single die, this architecture produces a low thermal resistance between the cores and the heat sink. Since the cores dissipate the vast majority of the power, this yields the lowest operating temperature (i.e. most efficient heat sinking) for the stack as a whole. 
     In the above description, numerous specific details are given to provide a thorough understanding of embodiments of the invention. However, the above description of illustrated embodiments of the invention is not intended to be exhaustive or to limit the invention to the precise forms disclosed. One skilled in the relevant art will recognize that the invention can be practiced without one or more of the specific details, or with other methods, components, etc. In other instances, well-known structures or operations are not shown or described in detail to avoid obscuring aspects of the invention. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
     These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.