Resetting multiple processors in a computer system

[0002] This is a continuation of U.S. patent application Ser. No.: 09/200,276 filed Nov. 24, 1998, abandoned, which is a continuation of U.S. patent application Ser. No.: 09/014,154 filed Jan. 27, 1998, U.S. Pat. No. 5,867,703, which is a continuation of U.S. patent application Ser. No.: 08/700,300 filed Aug. 20, 1996, U.S. Pat. No. 5,729,675, which is a continuation of U.S. patent application Ser. No.: 08/538,779 filed Oct. 3, 1995, U.S. Pat. No. 5,596,759, which is a continuation of U.S. patent application Ser. No.: 08/051,601 filed Apr. 22, 1993, U.S. Pat. No. 5,497,497, which is a continuation of U.S. patent application Ser. No.: 07/431,743 filed Nov. 3, 1989, abandoned.

[0003] 1. Field of the Invention

[0004] The present invention relates to multiple processors in computer systems and more particularly to multiple processors having a common reset using a single ROM.

[0005] 2. Description of the Prior Art

[0006] The personal computer industry is a vibrant and growing field that continues to evolve as new innovations occur. The driving force behind this innovation has been the increasing demand for faster and more powerful personal computers. New and increasingly more complex applications for personal computers are continually being developed, and the computer programs required to implement these new applications have experienced a corresponding increase in size and complexity, requiring greater amounts of time for the computer to be able to run them properly. As a result, personal computers have been burdened with lengthier amounts of software instructions that take increasing amounts of time for them to execute.

[0007] To meet the challenge of these new software applications, computer designers have used various methods to increase the speed with which personal computers can process instructions. Historically, the personal computer has developed as a system utilizing a single microprocessor to handle all instruction execution. The microprocessor is the key working unit or “brains” of the personal computer, and its task is to handle all of the instructions that programs give it in the form of computer software. The methods that have been used to increase speed in the personal computer have generally centered around maximizing the efficiency with which this single microprocessor can handle instructions.

[0008] Limits are being reached, however, on the amount of speed that can be obtained from a system based on a single microprocessor. The obvious choice to remove this constraint has been the incorporation of multiple microprocessors operating in parallel into a computer system. With the use of multiple processors, or multiprocessing, each microprocessor can be working on a different task at the same time. The use of multiprocessing has generally increased computer performance, but it has also resulted in numerous difficulties that were not found in a single processor environment.

[0009] A problem that has arisen in multiprocessing has been how to start up 6r initialize each of the processors without them interfering with each other. In a single processor personal computer environment, the microprocessor generally initializes itself by performing a POST (power on self test). The POST program is located in the computer's ROM, which is a special type of memory that is permanently recorded into the computer. This special memory cannot be written to or changed by software and is nonvolatile, meaning that turning off the computer will not disturb it. The ROM holds a key set of programs that provide essential support for the operation of the computer, among these the test and initialization programs known as the POST, which make sure that the computer is in good working order at power on. The POST program usually includes the microprocessor placing the memory in a known state, testing any memory, placing peripherals in a tested, reset and ready condition and loading or booting up the operating system, among other things.

[0010] The problem that arises in multiprocessing is that, in a system where multiple processors coexist sharing a common bus and a common memory, it would be catastrophic for more than one processor to perform a complete POST. For example, if one processor had completed a POST and was up and running, a subsequent processor attempting to perform a POST would re-initialize the peripherals and memory that the previous processor was now using to run its code, resulting in a potential error for the first processor.

[0011] For this reason it would seem necessary in a multiprocessing environment for each processor to have its own ROM with which to bring itself to a working state so that one processor's initialization would not interfere with another processor that may already be up and running. However, this scheme would require that numerous ROM's be present in the computer, one associated with each processor, resulting in an unnecessarily large amount of nonvolatile memory being used for this purpose. A further problem that would be associated with the use of multiple ROM's would be the resultant complexity and loading effects on the bus that these ROM's share with the processor. Therefore, it would be highly desirable for these multiple processors to be able to start or “boot” up using a single, common ROM as opposed to each processor containing a different ROM in memory.

[0012] For further background on the present invention, it is necessary to examine some of the special features and considerations involving the Intel Corporation (Intel) family of microprocessors, which are used in personal computers compatible with those manufactured and sold by International Business Machines Corp. (IBM). The Intel 80286 microprocessor introduced a new feature that allowed it to operate in two different modes: real mode and protected mode. In real mode, the 80286 behaves very much like the 8088 microprocessor that was in the original IBM PC, thus allowing for full compatibility with these older systems. In protected mode, there are no compatibility considerations, and the 80286 is allowed to utilize all of its special features for maximum capability.

[0013] A problem soon became apparent, however, in that, while provisions were made for the 80286 microprocessor to switch from real to protected mode, no provisions were made for the 80286 to return from protected to real mode. This problem was corrected in the Intel 80386 microprocessor, but could only be remedied in the 80286 through the use of the reset operation to return the processor to real mode. The reset operation, however, generally required the computer to perform a complete reset and reboot, and this was found to be unnecessary for the purpose of simply returning the computer from protected to real mode. Therefore, it was determined that some method was needed to indicate whether each reset operation was simply a software reset used for protected to real mode switching or a true system reset, and, as a result, a byte was provided in the CMOS nonvolatile memory available in the computer system to reflect whether or not a full reboot of the system was necessary. At power on, this byte reflects a “normal POST” status, informing the processor that a protected mode reset is not occurring and that a full boot is necessary to begin operation. When the processor is up and running, the status of this byte is changed to reflect a “vector on reset” status when a protected to real mode change is desired, thus informing the processor that a complete reboot is unnecessary.

[0014] The processor generally polls the status of this reset byte during its POST program so that the protected to real mode switch can be made with a minimum of lost time. At power on, the normal POST boot status of the reset byte directs the microprocessor to perform a complete POST, whereas, when the processor has been up and running and has its RESET pin toggled, the vector on reset status of the reset byte directs the processor away from the remainder of the POST program to an alternate memory location which is contained in a reset vector location. This memory location is the location desired upon entry into real mode to continue operation of the computer.

[0015] The present invention includes two design variations which allow multiple processors to start up using a single, common ROM. These designs, or techniques, are intended for use in a multiprocessing environment and utilize a system whereby each processor is directed to begin a normal POST, but only a single processor is allowed to perform a complete POST, whereas the remaining processors are directed very early in their POST procedure to perform a limited initialization sequence. The first design is intended for more general applications and is adaptable to any system incorporating multiple processors. This design can be implemented either by the operating system of the particular machine, or by software specifically written for this purpose, with minimal hardware requirements. The second technique is intended to be more particularly adapted to a specific computer and is executed from the POST procedure that is stored in the computer's ROM. This design has a hardware requirement in that an identity register is used to differentiate between the microprocessor performing the complete POST and the remaining microprocessors. This second design also includes a method for allocating tasks to the secondary processors once they are running.

[0016] In each of these designs, at power on the processor which performs the complete POST, referred to as the primary processor or processor P₁, begins a POST while the remaining processors which perform alternate initialization sequences, referred to as secondary processors or in the singular as Processor P_Z, are held in an inactive state. In the first design, an initialization procedure for each processor P_Zcommences when the processor P₁has completed its POST program. This initialization procedure includes the processor P₁placing a vector in the reset vector memory location pointing to initialization code that processor P_Zwill execute, changing the reset byte in the CMOS non-volatile memory to reflect a vector on reset status, and enabling address line A20. When the processor P_Zis activated, it begins a POST as the processor P₁did, but the vector on reset status in the CMOS reset byte directs the processor P_Zto vector off to a different location in ROM containing its initialization code, which it executes. When the processor P_Zhas been successfully dispatched, it notifies the processor P₁, which then restores the CMOS reset byte and the vector at the reset vector memory location to their original values as well as the original state of A20. This technique is repeated until all the processors have been initialized.

[0017] In the second design the initialization process of each processor P_Zoccurs during the POST program being executed by the processor P₁. A register in the computer's hardware is used to differentiate between the processor that performs the complete POST, the processor P₁, and the remaining processors, represented by the processor P_Z, which utilize a more limited initialization scheme. Early in the course of the POST sequence, each processor is directed to the identity register, which informs it as to whether it is a primary or secondary processor. If the processor is determined to be the processor P₁, it is allowed to continue with its POST. If the processor is determined to be the processor P_Z, it is instructed by the POST program to use a vector placed in the reset vector memory location by the processor P₁to transfer operation to a different location in the ROM containing the more limited initialization code.

[0018] Generally, at power on the processor P₁is activated to begin a POST while the processor P_Zremains in a held state. The activation and initialization of the processor P_Zis directed from the latter part of the POST procedure that is executed by the processor P₁. The processor P₁releases the processor P_Zand then polls a semaphore location to determine if the processor P_Zhas completed its initialization. When the processor P_Zhas completed its initialization, it notifies the processor P₁and places itself on hold, at which time the processor P₁completes the initialization of any other processors P_Zand then completes the POST.

[0019] Upon completion of the POST, the processor P₁is running, and subsequently the operating system begins allocating various tasks to the processor P_Z. When the operating system decides to give the processor P_Za task, the processor P₁takes the processor P_Zout of hold and provides it with a vector pointing to the code that the processor P_Zis to execute. The processor P₁and the processor P_Zcommunicate back and forth through a semaphore in memory as to when the processor P_Zhas begun execution of its task and when it has completed it. When the processor P_Zhas finished its task, it generally places itself in a held state to make it ready to receive other tasks from the operating system. This cycle of task allocation then repeats itself. The processor P₁may optionally direct the processor P_Zto execute a reset code at any time, this having the utility of resetting the processor P_Zto a known state prior to starting an operation.

[0020] A better understanding of the invention can be obtained when the following detailed description of the preferred embodiment is considered in conjunction with the following drawings, in which:

[0021] FIG. 1 is a schematic block diagram of a computer system incorporating the present invention;

[0022] FIG. 2 is a flow chart diagram of a first design for initializing the multiple processors using a single ROM according to the present invention;

[0023] FIG. 3 is a circuit diagram of an identity register called the Who-Am-I port according to the present invention;

[0024] FIG. 4 is a flow chart diagram of a second design for initializing the multiple processors using a single ROM according to the present invention;

[0025] FIG. 5 is a flow chart diagram of a design for dispatching tasks to secondary processors according to the present invention; and

[0026] FIG. 6 is a flow chart diagram of a design for resetting secondary processors after dispatching according to the present invention.

[0027] A computer system incorporating the present invention can utilize one of several design variations that allow multiple processors to start up using a single, common ROM with minimal changes to the ROM from single processor ROM's. The following designs will be discussed with specific reference to Intel 80386 or 80486 microprocessors being the microprocessors utilized in the multiprocessor system, but the use of other processors is also contemplated. Throughout the course of this description, the secondary processors will be referred to in the singular as the processor P_Z, but it is understood that multiple secondary processors may coexist in this environment.

[0028] Referring now to FIG. 1, the letter C designates generally a computer system incorporating either of the two designs according to the present invention. Many of the details of a typical computer that are not relevant to the present invention have been omitted for the purpose of clarity. The system C generally includes a primary processor P₁, a secondary processor P_Z, and various interprocessor logic circuitry 30 that is connected between the primary processor P₁and the secondary processor P_Z. Both the primary processor P₁and the secondary processor P_Zeach generally include a cache subsystem (not shown). The interprocessor logic 30 generally includes various option and status registers and circuitry relating to the operation of the processors. The primary processor P₁, the secondary processor P_Z, and the interprocessor logic circuitry 30 are each connected to a system bus 40 which generally includes an address bus and a data bus as well as various control and status lines that allow for the proper functioning of the computer C. Also attached to the system bus is read only memory (ROM) 50, which includes the start-up program that initializes the multiple processors according to the present invention called the power on self test (POST); random access memory (RAM) 52 which forms the main memory of the system C; and CMOS memory 54 which is used to provide nonvolatile, random access memory for use by the system C.

[0029] In each of the designs according to the present invention, at power on the primary processor, processor P₁, is activated and begins a POST (power on self test) which is located in the ROM 50 of the computer system, while the secondary processor P_Z, is kept in a held state. This activation of the processor P₁and holding of the processor P_Zis accomplished in slightly different manners in the two designs. In the first design a reset bit, which is located preferably in a register in the interprocessor logic circuitry 30 referred to as the Processor Option Register is used. The reset bit generally operates similarly for all processors such that a setting of the reset bit, which occurs at power up by hardware control, results in the RESET input of the respective processor being pulsed and causes the respective processor to be placed in a held state. A subsequent clearing of the reset bit releases the respective processor from its held state and allows the respective processor to begin the POST. Therefore, at power on the reset bit of the processor P₁is toggled, allowing the processor P₁to begin the POST, while the reset bit of the processor P_Zis set but not cleared, thereby keeping it in a deactivated or held state.

[0030] The second design utilizes a sleep bit located in the Processor Option Register in the interprocessor logic 30 associated with each processor P_Z. The sleep bit operates such that, when it is set for a respective processor, requests for the bus 40 by the respective processor are blocked. Therefore, in the second design, the processors P₁and P_Zeach have their reset bit toggled and a sleep bit is set on the processor P_Z. The toggling of the reset bit of the processor P₁allows it to begin the POST, while the setting of the sleep bit on the processor P_Z, which occurs at power up by hardware control, causes any requests for the bus by the processor P_Zto be blocked, thus effectively placing the processor P_Zin a held state. A subsequent clearing of the sleep bit of the processor P_Zby the processor P₁allows bus requests by the processor P_Zto be passed, thereby allowing the processor P_Zto begin the POST. Therefore, in each of these designs, at power on the reset bit of the processor P₁is generally toggled by the power on circuitry (not shown) of the computer system C, allowing it to begin a POST, while the processor P_Zis kept in a deactivated or held state.

[0031] In the first design according to the present invention, as shown in FIG. 2, when the processor P₁(FIG. 1) has finished a sufficient portion of its POST routine in step 212 to allow start up of the processor P_Z(FIG. 1), the software implementing this design performs an initialization procedure I₁to bring the processor P_Zinto an active state. First, in step 216, the vector at memory location 40:67 in the ROM 50 (FIG. 1), which is the reset vector memory location, is replaced with a new vector pointing to initialization code located in the ROM 50 that will be executed by the processor P_Zwhen it is enabled. The vector that was previously stored in this location is saved for later restoration.

[0032] In step 218, the processor P₁performs a similar replacement procedure with the CMOS non-volatile memory reset byte, which is preferably located in the CMOS 54. The CMOS reset byte is located at address location 0Fh in the CMOS memory 54 and is accessed through ports 070h and 071h, as is standard for addressing the CMOS memory in the IBM PC compatible computers. The status of this byte reflects whether a normal boot or vector on reset is necessary when the processor P_Zreads this value in the POST sequence. The processor P₁will have previously interrogated this location and have found the normal boot value present, enabling it to begin a normal POST. The current value of the CMOS reset byte is saved in step 218 to a temporary location, preferably to a register in the processor P₁. The CMOS reset byte is then changed to value 0Ah, this value signifying that the computer system C is or has been running and that only a vector on reset is necessary instead of the normal POST routine that a processor would normally follow at power on. This change, in effect, fools the processor P_Zinto thinking that it has already performed the POST operations. The 0Ah value written to the CMOS reset byte is the reset without EOI value and is utilized to prevent the Processor P_Zfrom clearing the interrupt controller, as this is unnecessary and might inadvertently cause an error.

[0033] Address line A20 of the system address bus 40 (FIG. 1) is enabled in step 220 to allow the processor P_Zto properly access high memory in the ROM 50 where the POST program is located. Address line A20 had previously been disabled in the POST routine of the processor P₁(step 212) for software compatibility reasons, these stemming from the use by previous programmers of a feature of the 8086 microprocessor whereby the program counter rolled over to 0000h after FFFFFh due to the maximum of twenty address lines available in that microprocessor. This roll over was incorporated into software written for these older 8086 microprocessor-based systems for various purposes, the end result for current purposes being that, in order to maintain compatibility with this older software, address line A20 was disabled during the POST of the processor P₁(step 212). Consequently, address line A20 must be re-enabled in step 220 to allow the processor P_Zto address the bootstrap program which is preferably located in high memory in the ROM 50.

[0034] In step 222, the processor P₁clears the reset bit in the Processor Option Register of the Processor P_Z, thereby activating the processor P_Zto begin the POST routine in step 226. The processor P_Zcomes out of reset and vectors to the reset location in the ROM 50 where the POST program is located. This is the same location where processor P₁vectored to after reset, this being a general function of the microprocessors used in the present invention. Thus, both the processors P₁and P_Zoperate from the same ROM 50 immediately after reset execution. Very early in the execution of the POST routine, the processor P_Zpolls the CMOS reset byte to determine its status. As the CMOS reset byte was previously changed by the processor P₁in step 218 to value 0Ah, reflecting a vector on reset status, the processor P_Zin step 228 is directed to the reset vector memory location 40:67. This location contains the vector which was placed there earlier by the processor P₁in step 216, and this vector is used in step 230 to direct the processor P_Zto its alternate initialization code, preferably located in the ROM 50 but alternatively located in RAM 52 after being loaded by the processor P₁. This initialization code generally includes the processor P_Zexecuting any specific reset code, testing its cache memory and performing other processor P_Zdependent features.

[0035] After activating the processor P_Zin step 222, the processor P₁awaits the successful dispatch of the processor P_Zin step 224 by polling a semaphore bit which is preferably located in the RAM 52. A final step 232 in the initialization sequence performed by the processor P_Zinvolves the processor P_Zaltering the semaphore bit to signal to the processor P₁that the initialization sequence has been successfully completed. When step 232 is completed, the processor P_Zbegins performing a software loop in step 234, waiting until it is directed by the operating system to perform a task.

[0036] When the processor P₁receives notification by way of the changed semaphore bit that the processor P_Zhas completed its initialization, the processor P₁proceeds to step 236 where it restores the original value of the CMOS reset byte from its temporary location. The processor P₁then proceeds to step 238 where the original vector is returned to the reset vector memory location 40:67 and the initial state of address line A20 is restored. The processor P₁then continues with its operation.

[0037] Referring again to FIG. 1, the second design according to the present invention is similar in many respects to the first, but is tailored for a two processor system C (FIG. 1) incorporating one primary processor, referred to as the processor P₁, and one secondary processor, referred to as the processor P₂. The expansion of this design to incorporate multiple secondary processors, however, is also contemplated.

[0038] This second design is also different from the first in that it utilizes an identity register located in the interprocessor logic 30 called the Who-Am-I register to differentiate between primary and secondary processors. The Who-Am-I register resides preferably in the system I/O port space and is used by software to determine whether the processor P₁or the processor P₂is currently active. The possible contents of this register include a value 00h to represent that the processor P₁is currently active, a value F0h to represent that the processor P₂is currently active, and a value FFh to represent that neither the processor P₁nor the processor P₂are active.

[0039] Referring now to FIG. 3, the contents of the Who-Am-I register are determined by the hold acknowledge (HLDA) outputs from the processor P₁and the processor P₂, the HDAKP1 and HDAKP2 signals, respectively. The hold acknowledge signal is generally asserted high when the respective processor is in a held or inactive state and is generally negated low when the respective processor is currently active. In the preferred embodiment if a processor P₁or P₂is reading the location it is active and the other processor must be inactive to allow the active processor access to the register, thus allowing the determination. The two hold acknowledge signals HDAKP2 and HDAKP1 are inverted by inverters 106 and 104, respectively. The output of the inverter 104 and the HDAKP2 signal are inputs to a two input AND gate 108, whose output is the P1ON signal. The P1ON signal is connected to the input of an inverter 116, whose output provides the upper four bits of the Who-Am-I register. The output of the inverter 106 and the HDAKP1 signal are inputs to a two input AND gate 110, whose output is the P20N signal. The P1ON and P2ON signals are inputs to a two input NOR gate 118, whose output provides the lower four bits of the Who-Am-I register. Therefore, by polling the status of the Who-Am-I register, the processor is able to determine whether it is the processor P₁or the processor P₂.

[0040] The Who-Am-I register can be readily expanded to indicate the active status of a greater number of processors than the two utilized in the present invention. This expandability can be achieved by reducing the numbers of output bits driven for each processor and incorporating the respective hold acknowledge signals of each of the additional processors through the appropriate logic, allowing for up to 8 processors to be identified in each byte.

[0041] Referring now to FIG. 4, the second design begins similarly to the first in that in step 252 the processor P₁is allowed to begin a POST, while in step 254 the processor P₂is held inactive. At power on the processor P₁generally has its reset bit toggled, allowing it to begin a POST. The processor P₂also has its reset bit toggled, but the sleep bit of the processor P₂is set, and this has the utility of blocking any bus requests by the processor P₂, thereby keeping the processor in a held state. Early in the execution of the POST sequence in step 252, the processor P₁is directed to the Who-Am-I register (FIG. 3), which informs it that it is in fact the processor P₁, thereby allowing it to resume execution of the POST program. During the remaining course of the POST, the processor P₁is directed by the POST program to execute the following initialization procedure for the processor P₂.

[0042] In step 256 the processor P₁initializes a processor available bit register, preferably located in the interprocessor logic 30 (FIG. 1) of the computer C, from configuration information contained in the CMOS memory 54.

[0043] The processor available register contains a sequence of bits that serve as an indicator to the various processors as to what other processors are available to be given tasks to perform. In general, each secondary processor preferably has a representative availability bit in the processor available register that reflects whether or not that processor is available for dispatching. If the respective availability bit of a secondary processor is set, then that processor is available for dispatching, and if the respective availability bit of a secondary processor is cleared, then that processor is not available for dispatching and is assumed to be either currently dispatched or not present within the system. If the CMOS memory 54 is found to be invalid during initialization, all of the availability bits in the processor available register are cleared. The processor available register is initialized in step 256 to reflect the present configuration of the system, this being which secondary processors, represented by the processor P₂, coexist in this environment with the primary processor P₁. The processor available register in the second design according to the present invention is configured to include two microprocessors, one primary processor P₁and one secondary processor P₂, but a configuration of the processor available register to incorporate a greater number of microprocessors than two is also contemplated.

[0044] In step 258, the processor P₁proceeds with the initialization procedure by storing the current vector from the reset vector memory location 40:67 into a temporary location and replacing it with a new vector pointing to initialization code that the processor P₂will execute when it is enabled. In step 260, the processor P₁activates the processor P₂by clearing the sleep bit in its Processor Option Register, enabling the processor P₂to begin the POST program in step 264. After enabling the processor P₂in step 260, the processor P₁begins polling the sleep bit associated with the processor P₂. The sleep bit is used here as a handshake between the processor P₁and the processor P₂to indicate when the processor P₂has finished its initialization procedure and has been placed on hold. Preferably, if processor P₁does not see the sleep bit set by the processor P₂after a certain period of time, the processor P₁sets the reset bit of the processor P₂, effectively placing the processor P₂in a held state, and then continues with the POST. A different Processor Option Register is preferably used for each additional secondary processor incorporated into the multiprocessing environment.

[0045] The processor P₂begins the POST sequence in Step 264 after the processor P₁clears its sleep bit. The processor P₂is directed in step 266 to the Who-Am-I register (FIG. 3) to determine its identity, just as the processor P₁was in step 252 during its execution of the POST program. The Who-Am-I register informs the processor P₂that it is a secondary processor and directs it to vector off based on the value in the reset vector memory location 40:67. This memory location contains the vector previously placed there by the processor P₁in step 258, and this vector points to the alternate initialization code which the processor P₂executes in step 268. The alternate initialization code is preferably located in the ROM 50 but may be alternatively located in RAM 52 after being loaded by the processor P₁. This initialization code generally includes the processor P₂executing any specific reset code, testing its cache memory, and programming its noncacheable address map, as well as any other processor P₂dependent features. Upon completion of its initialization code in step 268, the processor P₂places itself on hold in step 270 by setting the sleep bit in its own Processor Option Register. The setting of the sleep bit places the processor P₂in a hold state as soon as the bus 40 must be requested and serves as notification to the processor P₁that the processor P₂has finished its initialization procedure. Completing step 270, the processor P₂performs a jump or branch instruction requiring that the value at the reset vector 40:67 be obtained. Because this results in a bus request, the processor P₂goes into a held state. When the processor P₁receives this notification in step 272, it resumes execution of the remainder of its POST.

[0046] Upon completion of the POST, the primary processor, processor P₁, is running, and the operating system subsequently begins an allocation of various tasks to each secondary processor. The following allocation procedure will be discussed with reference to a computer system C incorporating a particular secondary processor referred to as the processor P₂, but the incorporation of multiple secondary processors in the task allocation scheme is also contemplated. Referring now to FIG. 5, when the operating system has a task for the processor P₂to perform, it directs the processor P₁to perform the following dispatch procedure for the processor P_2.

[0047] The processor P₁first checks the status of the respective availability bit of the processor P₂in the processor available register in the interprocessor logic in step 282 to determine if the processor P₂is available for dispatching. This availability bit was initially set by the processor P₁in step 256 (FIG. 4) according to the configuration information held in the CMOS memory 54 and this bit is subsequently cleared whenever the processor P₂is given a task to perform. If the availability bit of the processor P₂is not set, then the operating system knows that either the processor P₂has not yet finished the task that it was previously given or that the processor P₂is not presently configured to the system. In either case, the processor P₂is determined to be unavailable. If the availability bit of the processor P₂is set, then the operating system commences with the allocation of the task in step 284 by placing a vector in memory location 40:67 pointing to software which generally includes the task that the operating system wishes the processor P₂to execute. The processor P₁saves the previous value from memory location 40:67 for later restoration.

[0048] The processor P₁then activates the processor P₂in step 286 by clearing the sleep bit in the processor P₂'s Processor Option Register located in the interprocessor logic 30 in step 286, causing the processor P₂in step 290 to obtain the vector at memory location 40:67 to begin operation of the new task. An attempt to obtain the vector at memory location 40:67 was actually the last instruction executed by the processor P₂in the POST procedure in step 270. This vector fetch is by definition a cache miss so that the processor P₂must use the bus. However, the sleep bit was set in step 270, disabling any bus request, so the vector fetch is pending, waiting for the sleep bit to be cleared. Once the sleep bit is cleared, the processor P₂can properly access the bus and obtain the vector at memory location 40:67 pointing to the task that it is to execute. The first instruction in this new task is step 292 which directs the processor P₂to clear its respective availability bit in the available bit register to indicate to the operating system that it is no longer available for dispatching and that it has commenced with the task. After this, the processor P₂begins executing the task in step 294.

[0049] The clearing of the availability bit by the processor P₂serves as notification to the processor P₁that the processor P₂has begun execution of the task that the oprating system has given it. Upon receiving this notification, in step 296 the processor P₁restores the previous vector to the reset vector memory location 40:67 and then resumes the execution of its own code. When the processor P₂has finished executing its task in step 294, it sets the sleep bit in its Processor Option Register in step 298, blocking any further bus requests, and then attempts to obtain the reset vector at memory location 40:67. In this way, operation of the processor P₂is again held at the vector fetch operation as described previously in step 270, and processor P₂is available to receive a new task from the operating system.

[0050] In certain cases, it may be necessary to reset the processor P₂to a known state in order to make it available for further dispatching. In these cases, the processor P₁performs the following Reset-After-Dispatch routine to reset the processor P₂to a known state. Referring now to FIG. 6, in step 312, the processor P₁first places a vector in memory location 40:67 pointing to the reset code that the processor P₂is to execute, saving the previous value in this memory location for later restoration. The processor P₁then activates the processor P₂from its hold or operating state in step 310 by toggling the processor P₂'s reset bit in step 314, forcing the processor P₂to reset and request the system reset memory location, which is the beginning of the POST program. The POST sequence is started in step 318. Early in the course of the POST, the processor P₂is directed in step 320 to the Who-Am-I register (FIG. 3), which informs it that it is a secondary processor and directs it to the reset vector memory location 40:67. This location contains the vector placed there previously by the processor P₁in step 312 pointing to the reset code, which the processor P₂executes in step 322. This reset code is generally similar to the reset code that processor P₂executes during its initialization in step 268 (FIG. 4) at power on and includes the setting of the processor to a known state. Note, however, that the reset code that the processor P₂executes in step 322 may not include the other procedures that the processor P₂performed during its original initialization such as testing the cache and programming the noncacheable address map. When the processor P₂has finished executing its reset code in step 322, it notifies the processor P₁by setting the sleep bit in its Processor Option Register in step 324. The processor P₂also initiates a request to transfer control to the vector contained at memory address 40:67 in step 324, but since its sleep bit is set, all bus accesses by the processor P₂are blocked, and it is therefore essentially in a held state waiting for its sleep bit to be cleared. In this way, the processor P₂is available for further dispatching in the manner described above in that the processor P₂obtains directions to its new task from memory location 40:67 when its sleep bit is cleared.

[0051] After resetting the processor P₂in step 314, the processor P₁proceeds to poll the processor P₂'s sleep bit in step 316, waiting for the Processor P₂to indicate that the reset operation has been completed. When the processor P₁sees that the sleep bit of the processor P₂is set, it proceeds to step 326 where the processor P₁sets the availability bit of the processor P₂in the processor available register. This signifies that the processor P₂is now available for dispatching. The processor P₁then returns to the execution of its code in step 328. When the operating system has another task for the processor P₂to perform, it repeats the allocation cycle of FIG. 5 and, in some instances, the Reset-After-Dispatch routine of FIG. 6, as necessary.

[0052] The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the procedures, components, and circuit elements, as well as in the details of the illustrated circuitry and method of operation may be made without departing from the spirit of the invention.