BACKGROUND OF THE INVENTION 
     Integrated circuits combining at least one high performance processor with on-chip cache memory have become common in the art. Integrated circuits combining more than one high performance processor with on-chip cache memory are also available. 
     Memory references from a processor that are found in a cache are known as cache “hits”. Memory references that do not “hit” in cache are cache “misses”. 
     Many high performance processor integrated circuits have two power domains, a first power domain operating at a relatively low, core, voltage; and a second power domain operating at a higher, I/O (Input/Output) voltage. This typically permits design of processor and cache circuitry with transistors of smaller dimensions, including oxide thickness, than those used in the I/O pad ring. Typical processors, including the Pentium-3 (trademark of Intel Corporation) processors, require that both the core and I/O power domains receive power for correct operation. 
     Standard real time clock circuits also typically have two power domains. A first power domain operates I/O pad ring and most other logic of the circuit. A second power domain provides power to the clock circuitry, and often to memory circuitry, on the same integrated circuit. These circuits are typically powered on both I/O and clock power domains during normal operation of a system, and powered by a battery on the clock power domain when the system is in a low-power state. It has been found that interface logic between the power domains must be carefully designed to prevent excessive load on the attached battery, and to prevent corruption of clock domain memory and logic during power transitions of the I/O power domain. 
     It is well known in the art of integrated circuits that manufacturing processes are imperfect—on each wafer some, but not all, integrated circuits function fully. It is also known that defects tend to occur in clusters. Therefore, for circuits having multiple, large, functional units, such as processors, there will be a substantial population of manufactured integrated circuits where one functional unit is defective, or even a small portion of a function unit is defective, but other functional units on the same integrated circuit function properly. 
     The probability that an integrated circuit will have one or more defects increases as the size of the integrated circuit increases. Further, the cost of fabrication increases as integrated circuit size increases. A high performance single or multiple-processor integrated circuit can be quite large. It is therefore desirable to find ways of selling at least some of those large, multiple processor, integrated circuits that contain one or a few defects. 
     High performance processor integrated circuits are known to consume copious amounts of power, power requirements may exceed one hundred twenty-five watts. Multiple processor integrated circuits can have even higher power requirements, potentially as much as one hundred and fifty watts. 
     High power consumption can result in undesirably short battery life in portable systems. High power consumption is environmentally undesirable, requires that cooling apparatus be provided, and can result in critical circuitry overheating. In addition, high power processors require a larger and more expensive infrastructure in terms of power delivery and heat removal. This infrastructure decreases the density of processors that can be attained within a given volume. This density of computation is an increasingly important metric. 
     It is known that load requirements on computer systems vary with time. Computer system performance can be degraded to conserve power during times of low load, and returned to full performance during times of high load. 
     Processor power typically has two components, static power that is constant with operating frequency, and dynamic power that is a function of operating frequency. Many notebook computers are equipped with adjustable clock circuits whereby processor operating frequency may be reduced, thereby reducing power consumption, during times of low load. 
     It is desirable to have additional ways of adjusting power consumption and performance to system load. 
     SUMMARY OF THE INVENTION 
     A multiple processor integrated circuit has three separate power supply connections. A first power connection provides power to the first processor and cache of the circuit, which form a first power domain. The integrated circuit has a second power connection providing power to the second processor and cache of the circuit, and forming a second power domain. The third power connection provides power to common logic of the integrated circuit, forming a third power domain. There may be additional power connections to the integrated circuit, forming additional power domains. Each power connection may have multiple pins. 
     In particular embodiments, the integrated circuit has interface circuitry between the first and third, and the second and third, power domains. This interface circuitry controls signals crossing domain boundaries to prevent improper operation of the circuit as power connections change voltage. 
     In an alternative embodiment, the third power connection is common with one, but not both, of the first and second power connections. 
     A system embodying the multiple processor integrated circuit has three independent power supply subsystems, each connected to one of the three power connections of the multiple processor integrated circuit. Each power supply subsystem is independently controllable by Input/Output (I/O) circuitry of the system. The system is capable of adjusting system power consumption and performance to system load by independently controlling the power supply subsystems. The system is capable of shutting off power to the first processor, while providing power to, and operating, the second processor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a system embodying a multiple-processor integrated circuit according to the invention; and 
         FIG. 2 , a portion of a schmoo plot of a typical processor. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     An multiple processor integrated circuit  100  ( FIG. 1 ) has a first processor  104  with cache  106 , a second processor  108  and cache  110 . Cache misses from cache  106  and I/O references from first processor  104  are passed through an interface  112  to common circuit elements, which includes a memory bus interface  114 . Cache misses from cache  110  and I/O references from second processor  108  are passed through an interface  116  to common circuit elements including the memory bus interface  114 . 
     In a particular embodiment, the common circuit elements include clock circuits  118  and an additional level of cache  120 . The multiple processor integrated circuit  100  has three separate power supply connections. A first power connection  122  provides power to the first processor  104  and cache  106  of the circuit, which together form a first power domain. The integrated circuit  100  has a second power connection  124  providing power to the second processor  108  and cache  110  of the circuit, and forming a second power domain. Each cache  106  and  110  may be a single level cache, or in a particular embodiment, each cache  106  and  110  is a multilevel cache system of at least two levels. The third power connection  126  provides power to common logic, including the memory bus interface, optional additional cache  120 , and clock circuits  118  of the integrated circuit  100 , forming a third power domain. Each power connection may have multiple pins, in particular it is anticipated that the third power domain, having the common circuitry, may have many pins to permit control of switching transients that may occur as circuit pins change 
     In an alternative embodiment, (not shown) there are four processors, each having its own first level cache, each having independent power connections forming four power domains. In this embodiment, the common logic circuitry has two power connections, one at a low, core, voltage and another at a higher, I/O interface, voltage, forming another two power domains. It is anticipated that other embodiments may have additional power connections to the integrated circuit, forming additional power domains. 
     In particular embodiments, the integrated circuit has interface circuitry between the first and third, and the second and third, power domains. This interface circuitry controls signals crossing domain boundaries to prevent improper operation, or destruction, of the circuit as power connections change voltage. In particular, for multiple processor integrated circuits fabricated in P-well or twin-well CMOS processes, this interface circuitry is designed according to the following rules:
         1. To prevent forward-biasing of junctions, signals driven by gates powered by the third power domain are never connected to any P-diffusion located in a well that is electrically connected to either the first and second power domain;   2. To prevent corrupt operation, circuitry in the third power domain is designed to ignore all transitions in the first power domain when power provided to the first power domain is less than that required for normal operation;   3. To prevent corrupt operation, circuitry in the third power domain is designed to ignore all transitions in the second power domain when power provided to the first power domain is less than that required for normal operation;       

     In an alternative embodiment, the third power connection is common with one, but not both, of the first and second power connections. In this embodiment, the processor powered by the common supply can not be turned off in operation, while the processor powered by the separate supply can be. 
     A system  130  embodying the multiple processor integrated circuit  100  has three independent power supply subsystems  132 ,  134 , and  136 , each connected to one of the three power connections of the multiple processor integrated circuit  100 . Power supply  132  connects to the first power connection  122 , power supply  134  connects to the second power connection  124 , and third power supply  136  connects to the third power connection  126 . 
     The memory bus interface  114  of the multiple processor integrated circuit  100  is configured to pass memory references from first processor  104  and second processor  108  that miss in cache  106 ,  110 , and  120 , through a system controller  140  to main memory  142 . Similarly, I/O references are passed through system controller  140  to a display adapter  144  and I/O devices  146 . The system controller  140  is additionally configured to permit access to main memory  142  by the display adapter  144  and I/O devices  146 . 
     The system controller  140 , memory  142 , display adapter  144 , and I/O devices  146 , are provided with power from a source other than the first and second power supplies  132  and  134 . 
     Processor power supply subsystems  132  and  134  are independently controllable by I/O devices  146  of the system. The system is capable of adjusting system power consumption and performance to system load by independently controlling power supply subsystems  132  and  134 . The system is capable of shutting off power to the first processor, by turning off power supply  132 , while providing power to, and operating, the second processor through power supply  134 . 
     In an alternative embodiment of the system, for use when the first processor  104  or first cache  106  is known to be defective but second processor  108  and cache  110  is known functional; the first power supply subsystem is either permanently disabled or deleted from the system. 
     It is known that processor integrated circuits have maximum frequencies of operation that vary with power supply voltage. A plot of operating frequency versus voltage is known as a “schmoo plot”. In particular, it is known that maximum operating frequency  300  ( FIG. 2 ) of a processor typically increases as processor voltage is increased. It is also known that power consumption of a processor typically increases proportional to the square of the processor voltage, and that excessive processor voltage can cause destruction of a processor. 
     It is therefore possible to choose a first, high performance, operating point  302  of voltage and frequency for a processor, such as processor  104 . High performance operating point  302  has a higher frequency and higher operating voltage than a low performance operating point  304 . Operation at high performance operating point  302  will cause processor  104  to dissipate more power than operation at the low performance operating point  304 . 
     In a particular embodiment, power supplies  132  and  134  are adjustable under control of I/O devices  146 . 
     With reference to  FIGS. 1 and 2 , When it is desired to operate the system  130  at maximum performance, both the first  104  and second  108  processors are operated at the high performance operating point  302  by adjusting the clock circuit  118  and power supplies  132  and  134  appropriately. When it is desired to conserve power by degrading performance to a first level of degradation, power supply  132  is turned off and the first processor  104  and cache  106  are disabled. The system continues to operate using the second processor  108  and cache  110 . When it is desired to conserve additional power by degrading performance further, the clock circuit  118  is set to the frequency of the low performance operating point  304 , and after the frequency is set the power supply  134  attached to the second processor  108  and cache  110  is adjusted to the voltage of the low performance operating point. 
     While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various other changes in the form and details may be made without departing from the spirit and scope of the invention. It is to be understood that various changes may be made in adapting the invention to different embodiments without departing from the broader inventive concepts disclosed herein and comprehended by the claims that follow.
A multiple processor integrated circuit has a first processor-first level cache combination powered by a first power terminal, and a second processor-first level cache combination powered by a second power terminal. There is common circuitry coupled to each processor-cache combination. In a particular embodiment, the processor-cache combinations are capable of receiving independently controlled power over the power terminals.