Address conversion unit for multiprocessor system
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to the field of data processing systems employing multiple memories and more specifically to multiprocessing systems employing a common memory for all of the processors and a private memory for one of the processors. The common memory and private memory have distinct memory spaces, and the invention allows the processor having access to the private memory to also be able to access memory locations in both the memory space for the common memory and the memory space for the private memory. 
     2. Description of the Prior Art 
     The reduction in the cost and size of processors used in electronic data processing systems over the last few years has given rise to a dramatic increase in the number of processors that are used in data processing systems. A number of data processing systems have been developed in which several processors are used to process user programs. In some of these systems, the different processors may be specially designed to execute certain classes of instructions, such as fixed or floating point instructions, matrix instructions, or instructions that operate on character string. In other systems, different processors may process the same classes of instructions; in such a system, the operation is enhanced by the plural processors processing a number of user programs more quickly. Furthermore, several other elements of data processing systems, such as control units that control many of the peripheral elements such as disk and tape storage systems and input/output control units, employ one or more processors. 
     Multiprocessor systems can be constructed with one common memory that is accessable by all of the processors. However, if the system requires a high rate of memory accesses, it can be slowed down by contentions for the memory. If there is substantial duplication among the processors of programs and data, which may be the case, for example, in some multiprocessor systems in which several processors process user programs, a single common memory may be desirable. Such a system may also include several interleaved memories to reduce delays due to contentions for memory. 
     Alternatively, it often is desirable to provide a private memory that is accessible by only one of the processors. This may be done if the processor has specialized functions other than, or in addition to, the functions of the other processors. For example, if the multiprocessor system employs a master processor to schedule and coordinate processing by a number of slave processors, only the master processor need have access to the programs and data that allow it to perform this function. Similarly, for processors that control disk or tape drives in a peripheral controller, a private memory may store data and programs that allow them to perform these operations. A common memory may also be provided to store data and programs that are used by a number of processors. This arrangement would reduce the number of accesses of the common memory, and can enhance system performance. 
     Problems arise, however, in multiprocessor systems having one memory common to the processors and one or more private memories for the various processors. In such a systems, each of the memories may have a distinct address &#34;space&#34;, or set of addressable locations, and the system must be configured to be able to distinguish between addresses that are intended for the common memory and addresses that are intended for the private memories to ensure that the correct locations are accessed. 
     Furthermore, the processors that have access to a common memory as well as to a private memory must be provided with addresses which are usable by them in processing their programs. These addresses normally must be addresses in their private memory spaces, even when processing programs that require references to programs or data that are stored in the common memory. Such processors typically use the addresses of the private memory space when processing their programs, and, to be able to refer to information stored in the common memory, must be provided with addresses in the private memory space that they may use to refer to the actual locations in the common memory in order to process such programs. 
     SUMMARY 
     It is therefore an object of the invention to provide a new addressing arrangement for a multiprocessing system. 
     It is a further object of the invention to provide a new and improved multiprocessing system which includes both a common memory for all of the processors and a private memory for one processor, and in which the processor having the private memory may access memory locations in both memory spaces. 
     In brief, the invention provides a multiprocessing system having a common memory for all of the processors, and in which one or more of the processors have a private memory. The common memory and a private memory have separate and distinct memory spaces. A processor having a private memory has an address conversion unit that, in response to addresses from the processor in a portion of its memory space that is within the private memory space of the processor, but distinct from the range of addresses for the private memory, generates addresses that are in the address space for the common memory. 
     The address conversion unit includes a set of registers which are loaded with pointers to blocks of addresses in the common memory address space. When the processor transmits an address within the range for the address conversion unit, the address conversion unit retrieves the most significant bits of the contents of a register selected by the address from the processor as the most significant portion of the addressed location in common memory. The least significant bits of the address transmitted by the processor are concatenated onto the end of the bits retrieved from the register as the least significant portion of the addressed location in common memory. 
     In addition, the address conversion unit, in response to an address in the address space of the common memory, generates addresses that are in the private memory space that its processor may use when referring to the common memory address while processing its programs. The address in the common memory may be provided by any of the processors in the system. The address conversion unit, on receipt of the common memory address, stores the address in a register. Its processor may read the register to determine the common memory address. When the processor reads the register, the address conversion unit converts the common memory address that is stored in the register to an address in the processor&#39;s private memory space by forming an address comprising, as the most significant portion, the private memory space identification of the address conversion unit and the register in which the common memory address is stored, and, as the least significant portion, the least significant bits of the common memory address. 
     The flexibility of the invention is enhanced by providing multiple sets of the registers and a index register loaded by the processor to point to a specific set of registers that is active at any one time. The processor can change the active set of window address registers merely by changing the contents of the index register. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is pointed out with particularity in the appended claims. The above and further objects and advantages of the invention may be better understood by referring to the following description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a multiprocessor system constructed in accordance with this invention; 
     FIG. 2 is a diagram depicting memory maps for the private memory and the common memory which is useful in understanding the invention; and 
     FIG. 3 is a diagram useful in understanding the operation of the address conversion unit depicted in FIG. 1. 
    
    
     DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT 
     The invention will be described in terms of a multiprocessor system in which a master processor schedules and coordinates the activities of a plurality of slave processors. The invention could also be advantageously used in multiprocessor systems that do not use the master/slave arrangements for scheduling and coordination, as well as in systems, as described above, in which the processors are special purpose processors used in controlling peripheral or input/output units in the system. 
     Referring to FIG. 1, the basic elements of a multiprocessor system that embody the invention include a master processor 10 connected to several slave processors 12 and 14, and a common memory 16 over a common bus 18. The master processor 10 includes a processor unit 20 that connects to a private memory 22, an address conversion unit 24, and a common bus interface 26 over a private bus 28. The processor unit 20 transmits address and data with the private memory 22 and address conversion unit 24 over private bus 28, and with the common memory 16 through common bus interface 26 over common bus 18. Furthermore, processors 10, 12 and 14 may interrupt each other and transmit interrupt information over common bus 18. All of this communication may be by means well known in the art, and will not be discussed further herein. 
     The internal organization of the slave processors 12 or 14 may be similar to the organization of master processor 10 as depicted in FIG. 1. That is, each of slave processors 12 and 14 may also include a private memory for storing data and control programs executed only by that slave processor. Alternatively, all of the control programs and data for the slave processor may be stored in common memory 16. Since it will be apparent to those skilled in the art that the invention may be used in connection with any of the processors 10, 12 and 14 forming the multiprocessor system depicted in FIG. 1, attention here will be directed only to the master processor 10. 
     Accordingly, FIG. 2 is a memory map 30 depicting the organization of a private address space used by master processor 10 for its internal processing. This address space, in one specific embodiment, has the range (177777, octal, through 000000, octal). In particular, the private address space includes addresses that are allocated to private memory 22, whose storage locations may be directly addressed by processor unit 20 using addresses within the range of the private memory addresses 32 depicted in FIG. 2. Similarly, in its internal processing, processor unit 20 may use addresses in the range depicted as common memory window addresses 34 in memory map 30. Furthermore, input/output units that may be connected to master processor 10 may be addressed using addresses in the range indicated as input/output addresses 36 in memory map 30. 
     FIG. 2 also depicts a memory map of the address space of the common memory 16, which also includes a plurality of addressable storage locations. This address space, in one specific embodiment, has an address range of (377777, octal, through 000000, octal). As can be seen, the address space of addressable locations in common memory 16 and the private address space used by master processor 10 in its internal processing are separate and distinct. Accordingly, to permit master processor 10 to communicate with common memory 16, it must produce addresses within the address space for the common memory. This is done by address conversion unit 24. 
     In brief, with reference to FIG. 2, when the processor unit 20 of master processor 10 transmits addresses in certain address ranges in the processor&#39;s internal address space, particularly in the input/output range 36 in memory map 30, as indicated by arrow (A), address conversion unit 24 intercepts the address and generates an address in the address space of the common memory 16, as indicated by arrow (B). With reference to FIG. 3, address conversion unit 24 includes a window index register 50 which when loaded by processor unit 20 points to one of the plurality of sets of window address registers 52A through 52N. In one particular embodiment, window index register 50 can identify one of up to 128 (177 octal) sets of window address registers. The window index register 50 enables one set of the window address registers to be active, as described below. Since all of the sets of window address registers are identical, the discussion below will be restricted to set 52C, hereinafter generally referred to as set 52. The address conversion unit 24 also includes a window bus register 54, whose purpose will be made clear below. 
     Window address register set 52 includes eight registers, identified as window address registers WADR0 60 through WADR7 67. Prior to using address conversion unit 24, either processor unit 20, or slave processors 12 or 14, loads the registers with a value that point to, or identifies, an address in common memory 16. An example of such a value is depicted in FIG. 3 in window address register WADR1 61, which has the value 410 (octal). How this value is obtained will be made clear below. 
     After the window address registers 52 are loaded, processor unit 10 uses address conversion unit 24 to generate an address identifying a location in common memory 16. Specifically, processor unit 20 transmits an address in the private memory space onto private bus 28. The address may identify an addressable location in private memory 22 or in an input/output unit (not shown) if it is in the range indicated as private memory addresses 32 or input/output addresses 36 (FIG. 2), or, if the address is in the range of common memory window region 34 (FIG. 2) it will be intercepted by address conversion unit 24. 
     Assuming for purposes of this description that the private address from processor 20 is intercepted by address conversion unit 24, the address conversion unit decodes the private address and selects one of the registers 52 in the active set, as identified by the contents of window index register 50, to take part in the address conversion process. As an example in one specific embodiment of the invention using sixteen bit private addresses and in which the address space for the common memory, particularly depicted in FIG. 3, address 70 is an address in the processor&#39;s 20 private address space. If the four high order bits (15:12) of address 70 have a value 16 (octal), the internal address is in the range for common memory window region 34 (FIG. 2) for conversion to the address space for common memory 16, and address conversion unit 24 intercepts the internal address. The address conversion unit 24 uses private address bits (11:9) to identify the window address register whose contents are to be used in the conversion process. In the example depicted in FIG. 3, private address bits (11:9) contain the value (001) which identifies window address register WADR1 61. 
     In the example depicted in FIG. 3, the value 410 (octal) has previously been stored in window address register WADR1 61. The address conversion unit 24 then produces the address for the location in common memory 16. The nine high order bits of the contents of the window address register WADR1, which are identified generally by reference numeral 68 in FIG. 3, form the nine high order bits of the address for common memory 16. Bits (8:1) of private address 70 constitute the low order bits of the address to common memory 16. Bit (0) of private address 70 is used to identify which byte of the identified word is to be accessed. 
     When the location in common memory 16 is accessed, the contents of the window address register that was used in the conversion process are loaded into window bus register 54. Window bus register 54 thus receives the value 410 (octal). This may be used for diagnostic purposes if an error is detected. 
     The invention can be further shown by another example, which also illustrates the operation of the address conversion unit in converting addresses in the common memory address space to addresses in the private memory space, thereby allowing the processor 20 to use or identify locations in common memory 16 when processing its internal programs. 
     In one specific embodiment, the slave processors 12 and 14 load information into common memory 16 that may be used by master processor 10 for control purposes. This information may be, for example, status information indicating the status of the slave processor, information useful in diagnostic or maintenance processing, certain control information, and so forth. The slave processor loads the information for master processor into a particular location of common memory 16, for example, starting at common memory location (001020, octal). The starting location (001020) is addressed using seventeen-bit addresses in this example. 
     After loading the information into the common memory, the slave processor informs the master processor 10 of the location. In this example, the master processor has a sixteen-bit data path, and so the slave processor then transfers the upper sixteen bits of the address (001020, octal) to the master processor 10; the upper sixteen bits have the value (000410, octal). 
     The master processor 10 receives the upper sixteen bits of the common bus address from the slave processor and loads it into one of the window address registers, for example window address register WADR1 61. The master processor, specifically the address conversion unit 24, then constructs a private sixteen-bit address for the information for internal addressing and processing purposes. The address conversion unit constructs this address, which is designated 72 in FIG. 3, as follows: 
     (a) the four most significant bits (1110 binary, or 16 octal) are constant and identify the address conversion unit; 
     (b) the next three bits (001 binary, or 1 octal) identify the window address register containing the information, here window address register WADR1 61; 
     (c) the next seven bits (0001000) are the seven least significant bits from the window address register WADR1 61; and 
     (d) the two least significant bits (00) are constants added by the address conversion unit 24. 
     The address conversion unit thus returns (161040, octal) when processor 20 reads window address register WADR1 61. This is the private address that the processor 20 uses for processing the information that has been loaded into common memory 16 by the slave processor. 
     When processor 20 desires to retrieve the information that the slave processor loaded into common memory 16, it transmits the internal address (161040, octal) onto private bus 28. The address is in the range of the common memory window region 34 (FIG. 2) and so the address conversion unit 24 recognizes this as an address for conversion. The address conversion unit then intercepts this private address and generates an address for the common memory as described above. 
     In particular, as shown in FIG. 3, when the address conversion unit receives the address (161040, octal) from private memory bus 28, it discards the four most significant bits (16, octal), which merely indicate that the address is in the common memory window region 34 (FIG. 2) and is to be converted. The next three bits (001 binary, or 1 octal), indicate that the contents of window address register WADR1 61 are to be used in the conversion. The address conversion unit then retrieves the contents of the window address register WADR1 61 and loads them into window bus register 54, where they may be available to processor 20 for diagnostic purposes in the event of error. Additionally, the nine most significant bits (15:7), identified by reference numeral 68, from window address register WADR1 form the most significant bits, that is, bits (16:8), of the common memory address 74 generated by the address conversion unit. Bits (7:0) of the common memory address 74 form bits (8:1) of the internal address from processor 20. Bit (0) of the private address is used to control signals transmitted over common bus 18 during byte transfer operations over the bus. 
     As a further feature of the invention, address conversion unit 24 can generate common memory addresses in an entire block of addresses shown in FIG. 3 based on the contents of window address register WADR1 61. As shown in FIG. 3, the nine most significant bits of the address which are retrieved from window address register WADR1 can be used to address a block of two hundred and fifty-six common memory locations having octal addresses (001000) through (001377). Therefore, a slave processor can load information for master processor 10 into a block of the common memory 16 and transmit one address to the master processor, which can then retrieve the information from the common memory 16 without further communication from the slave processor. 
     The foregoing description is limited to a specific embodiment of this invention. It will be apparent, however, that this invention can be practiced in data processing systems having diverse basic constructions or in systems that use different internal circuitry that is described in this specification with the attainment of some or all of the foregoing objects and advantages of this invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of this invention.