Microprocessor having power management circuitry with coprocessor support
This Application is related to and commonly assigned U.S. applications Ser. No. 07/858,579 filed on Mar. 27, 1992, application Ser. No. 07/859,110 filed on Mar. 27, 1992 (U.S. Pat. No. 5,375,209), and application Ser. No. 07/859,022 filed on Mar. 27, 1992 (now abandoned). 
    
    
     This Application is related to and commonly assigned U.S. applications Ser. No. 07/858,579 filed on Mar. 27, 1992, application Ser. No. 07/859,110 filed on Mar. 27, 1992 (U.S. Pat. No. 5,375,209), and application Ser. No. 07/859,022 filed on Mar. 27, 1992 (now abandoned). 
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to integrated circuits, and more particularly to a microprocessor having power management circuitry with coprocessor support. 
     BACKGROUND OF THE INVENTION 
     Increasingly, electronic circuit manufacturers need to reduce the power consumption of their boards. The conservation of power is particularly important in portable electronic devices, such as laptop or notebook computers, where the product is specifically designed for use in situations where power outlets are not available. Since laptop and notebook computers must operate using internal batteries or rechargeable battery packs for extended periods of time, the conservation of battery power becomes a primary concern. 
     In a laptop or notebook computer, the largest consumer of power is the display. The proportion of power consumed by the display will vary depending on the technology used. Thus, laptop and notebook computer manufacturers have disabled the power to the display during periods of inactivity. Decoupling the display from the power supply can be accomplished with fairly simple circuitry. 
     The next largest consumer of power on a laptop or notebook computer is the CPU motherboard microprocessor. Heretofore, computer manufacturers have used one or two techniques for reducing power consumption of the microprocessor during periods of inactivity. One technique reduces the speed of the system clock to a fraction of the normal operating frequency during periods of inactivity. Since the power consumption of the microprocessor is proportional to the frequency, reducing the frequency of the system clock also reduces the power consumption of the microprocessor. In an Intel 80386DX microprocessor (manufactured by Intel Corporation of Santa Clara, Calif.), reducing the operating frequency from 33 MHz to 4 MHz reduces the typical operating current of the microprocessor from 400 to approximately 100 milliamps. Nevertheless, an operating current of 100 milliamps still poses a large power drain on the battery. 
     A second technique for reducing power turns off the system clock during periods of inactivity. Turning off the system clock affects all circuitry on the motherboard. Consequently, the circuitry which disables the system clock must also save all pertinent information in the microprocessor and associated board logic and restore the data upon resumption of activity such that the state of the computer after resumption of the system clock will be identical to the state of the computer prior to disabling the system clock. As a result, this technique for consuming power is both costly because of the complicated circuitry and slow because of the need to store and restore the state of the computer. 
     Therefore, a need has arisen in the industry to provide a method and apparatus for conserving power in an electronic device which significantly reduces the power drain of the microprocessor without the need for complicated external circuitry. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method and apparatus is provided which provides significant advantages in reducing the power consumption of a microprocessor. 
     The present invention comprises a processing unit including a plurality of subcircuits and circuitry for generating a clock signal to the subcircuits. Detecting circuitry detects the assertion of a first control signal indicative of a request for suspending operation of the processing unit for detecting a second control signal asserted by a coprocessor, indicating the state of the coprocessor. Disabling circuitry disables the clock signal to one or more of the subcircuits responsive to the first and second control signals. 
     The present invention provides significant advantages over the prior art. First, the processing unit monitors the operation of a option coprocessor, thereby relieving the external circuitry from monitoring the coprocessor when power reduction measures are desired. Further, an acknowledge signal output from the processing unit alerts the external circuitry that microprocessor operations having been suspended and that the coprocessor is currently in an idle state. Thus, the external circuitry can take additional power saving measures, such as disabling the system clock for a computer circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a block diagram of a computer system; 
     FIG. 2 illustrates a block diagram of the preferred embodiment of a microprocessor used in the computer system of FIG. 1; 
     FIG. 3 illustrates a detailed block diagram of portions of the microprocessor of FIG. 2 related to the power management circuitry; 
     FIG. 4 illustrates a flow chart describing a preferred embodiment of operation for reducing microprocessor power consumption; 
     FIGS. 5a and 5b illustrate circuitry for enabling and disabling pins providing power management control signals; and 
     FIG. 6 illustrates a flow chart of the operation of software controlled embodiment for conserving microprocessor power consumption. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiment of the present invention and its advantages are best understood by referring to FIGS. 1-6 of the drawings, like numerals being used for like and corresponding parts of the various drawings. 
     FIG. 1 illustrates a block diagram of a computer system. The computer system 10 comprises a microprocessor 12 coupled to a memory subsystem 14, BIOS ROM 16 and logic 18 (commonly referred to as the &#34;chipset&#34;). Microprocessor 12 is coupled to bus 20. Bus 20 is used to communicate with a number of devices, shown in FIG. 1 as keyboard controller 22, video controller 24, I/O circuitry 26 and disk controller 28. Keyboard controller 22 is coupled to keyboard 29. Disk controller 28 is coupled to hard disk 30 and floppy disk 32. Video controller 24 is coupled to display 34. An optional coprocessor 35 is coupled to microprocessor 12 and BIOS 16. 
     The computer system 10 shown in FIG. 1 is a general-purpose architecture common to personal computers such as the IBM Personal Computer and compatibles. The BIOS 16 (basic input/output system) is typically a read-only memory which contains a set of programs for performing the basic control and supervision operations for the computer system 10. The BIOS 16 acts as an interface between the computer circuitry and the application software being executed by the CPU 12. Importantly, for power consumption purposes, the BIOS 16 and logic 18 monitor the circuitry to determine whether power consumption reduction procedures may be invoked. For example, the BIOS 16 and/or logic 18 may monitor the display 34 to determine whether its output has changed over a predetermined time period. If not, the BIOS 16 may invoke procedures to disable power to the display 34 (assuming computer system 10 is a portable computer) to conserve energy. Further, BIOS 16 monitors microprocessor 12 to determine whether the microprocessor can be idled without affecting operation of the computer system 10. For example, the microprocessor 12 may be executing a routine to wait for a character from the keyboard. In this case, the operation of the microprocessor can be suspended until a key is pressed. 
     FIG. 2 illustrates a detailed block diagram of the various subcircuits of a preferred embodiment of the microprocessor 12. For purposes of illustration, the microprocessor 12 will be described in connection with a microprocessor which is pin-compatible and instruction-compatible with the 80×86 family of processors by Intel Corporation, specifically the 80386 microprocessor although the invention could be used in other processors as well. The microprocessor 12 comprises three main functional groups: the core circuitry 36, the memory circuitry 38 and the bus controller 40. The core circuitry 36 includes an instruction queue 42 coupled to an internal data bus 44. The output of the instruction queue 42 is coupled to a decoder 46 of the decode/sequence circuitry 47. The decode/sequence circuitry 47 also includes a sequencer 50 and an exception processor 86. The decoder 46 is coupled to a microcode ROM 48, exception processor 86 and sequencer 50. The sequencer 50 is also coupled to the microcode ROM 48 and to an execution unit 52. The execution unit includes a limit unit 54, a multiplier unit 56, an adder unit 58, a shift unit 60, and a register file 62. The execution unit 52 is coupled to the microcode ROM 48 and to multiplexer and I/O register circuitry 64. The memory circuitry 38 comprises a memory management unit 66 coupled to a linear address bus 68 which is also connected to the execution unit 52 and an instruction/data cache memory 70. Memory management unit 66 is further coupled to the internal data bus 44. A prefetch unit 72 is coupled between the memory management unit 66 and the cache 70. Bus controller 40 includes data buffers 74, address buffers 76 and control circuitry 78. The data buffers 74 are coupled to the data I/O pins D31-D0, the address buffers 76 are coupled to the address pins A31-A2 and BE3#-BE0#. A data address bus 80 couples the memory management unit 66, the cache 70 and the address buffer 76. An instruction address bus 82 couples the prefetch unit 72, cache 70 and address buffer 76. The data buffers 74 are coupled to the internal data bus 44. 
     Clock module 84 receives an external clock signal (CLK2) and generates CLKA (connected to the bus controller 40) and CLKB (coupled to the memory circuitry 38 and the core circuitry 36). CLKA and CLKB are both clock signals of one-half the frequency of CLK2. Clock module 84 receives control signals from bus controller 40. 
     In operation, instructions are received by the microprocessor 12 from external memory under control of the memory management unit 66. For enhanced performance, an instruction/data cache 70 caches instruction and data received through the bus controller 40. Instructions are stored in the instruction queue and are subsequently translated by the decode circuitry 46 into microcode. The sequencer points to the next address in the microcode ROM 48 under control of the decoder 46 and the execution unit 52. The execution unit 52 processes information under control of the microcode ROM 48. 
     In the preferred embodiment, the microprocessor 12 has a static design, i.e., retention of data in the internal memories and registers of the microprocessor 12 is not dependent upon the clock signal. As described in greater detail hereinbelow, the clock module 84, under control of the bus controller 40, can disable clocks to the subcircuits of the core circuitry 36 and the memory circuitry 38 while continuing to generate clock signals to the bus controller 40. Thus, during periods of inactivity, a large portion of the circuitry of the microprocessor may be suspended, thereby greatly reducing the power consumed by the microprocessor 12. 
     FIGS. 3 and 4 describe the power reduction circuitry in greater detail. FIG. 3 is a block diagram showing control signals between various portions of the microprocessor. The bus controller 40 controls signals from external pins of the microprocessor 12. A suspend (SUSP) signal is input to the bus controller 40 and a suspend acknowledge (SUSPACK) is output from the bus controller 40. A busy (BUSY) is received by the bus controller 40 from the coprocessor 35. The bus controller 40 also receives a maskable interrupt (INT) and a non-maskable interrupt (NMI). The bus controller 40 outputs an interrupt (or &#34;exception&#34;) F --  SUSP to the exception processor 86 and receives a control signal D --  SUSPACK. The exception processor 86 also monitors the microcode ROM 48, bus controller 40 and execution unit 52 to determine whether instructions are being executed. The exception processor 86 outputs a signal D --  EXCEPTION to the sequencer 50 and receives a control signal U --  AHALT from the microcode ROM 48. The bus controller 40 outputs a control signal F --  IDLE to the clock module 84. 
     In operation, an external circuit (typically the BIOS 16 in conjunction with the logic 18) detects conditions where microprocessor operations could be suspended. Upon detection of such a situation, the external circuit asserts the SUSP pin (for example, by driving the SUSP pin with a logical low voltage). In response to the assertion of the SUSP signal, the bus controller 40, in conjunction with the exception processor 86, asserts the F --  IDLE control signal to the clock module 84. In response to the assertion of the F --  IDLE signal, the clock module 84 disables the CLKB clock signals (by holding the disabled clock signal at a logical high or logical low voltage), while continuing to generating the CLKA clock signals. Since the design of the microprocessor is static, the memories do not require refreshing, and therefore suspending the clock will not result in a loss of data within the microprocessor 12. The SUSPACK signal is asserted to notify external circuitry that the microprocessor 12 is in the suspended state. To resume operation of the microprocessor 12, the SUSP signal is de-asserted (i.e., by applying a logical high voltage to the SUSP pin). 
     By suspending the clocks to the core circuitry 36 and memory circuitry 38, a significant reduction in the power consumed by the microprocessor 12 is realized. The bus controller 40 remains active to observe and control I/O signals between the microprocessor 12 and the external circuitry. 
     FIG. 4 illustrates a flow chart showing a more detailed operation of the suspend mode. In decision block 88, a loop is formed waiting for the SUSP signal to be asserted. In block 90, after the SUSP signal is asserted, the bus controller 40 asserts the F --  SUSP signal, which is coupled to the exception processor 86. In block 92, in response to the assertion of the F --  SUSP signal, the instruction queue 42 is prevented from advancing new instructions. In block 94, the decoder 46 ceases to advance new instructions to the microcode ROM 48 and any instructions currently being processed by the microcode ROM 48 or execution unit 52 (collectively, the &#34;pipeline&#34;) are completed, including any activity by the bus controller 40 related to the instructions in the pipeline. After all instructions in the pipeline have been executed, the control signal D --  EXCEPTION is asserted by the exception processor 86 in block 96. D --  EXCEPTION is received by the sequencer 50 which initiates a power-down microcode routine (block 98) responsive to D --  EXCEPTION. The power-down microcode routine prepares the microprocessor for suspend mode. In block 100, the microcode ROM 48 asserts the control signal U --  AHALT to the exception processor 86. In response to receiving U --  AHALT, the exception processor 86 asserts D --  SUSPACK to the bus controller 40 in block 102. In decision 104, the bus controller 40, after receiving D --  SUSPACK from the exception processor, checks the busy signal received from the coprocessor. So long as the busy signal from the coprocessor is asserted, the SUSPACK signal to the external circuitry will not be asserted and CLKB will not be disabled. Once, the busy signal is de-asserted by the coprocessor, the SUSPACK signal is asserted by the bus controller 40 to alert the external circuitry that the microprocessor 12 is in a suspended state and that the coprocessor is not currently performing any calculations, and may also be suspended. In block 108, F --  IDLE is asserted by the bus controller 40 to the clock module 84. In response to the assertion of the F --  IDLE signal, the clock module 84 disables the CLKB in block 109, thereby suspending operation of the core circuitry 36 and memory circuitry 38. The bus controller 40 then waits until the SUSP signal is de-asserted in decision block 110. Upon de-assertion of the SUSP signal, CLKB is resumed. 
     Most microprocessors, including the 80386, do not use all available pins on the chip package. Thus, the SUSP and SUSPACK signals may be communicated to and from the microprocessor 12 using unused pins, thereby maintaining compatibility with a pre-existing technology. Nonetheless, in the preferred embodiment, the pins for the SUSP and SUSPACK signals may be selectively enabled or disabled. In the preferred embodiment, the SUSP and SUSPACK pins are initially disabled, and the BIOS 16 must be configured to enable the pins in its start-up routine. To effect enabling or disabling of the SUSP and SUSPACK pins, a control bit is provided which may be written to or read from via preselected I/O ports. The preferred embodiment of this aspect is shown in greater detail in connection with FIGS. 5a-b. 
     In FIG. 5a, a plurality of control registers are accessible using INDEX and DATA signals input to the control registers 120. The majority of the registers (and bits thereof) are used for configuring the cache memory subsystem. For example, the control registers may be used to define non-cacheable regions of the main memory 14, to select the cache method (direct-mapped or set associative), and to enable flushing of the cache memory 70 via an external pin. Each control register is accessible by writing the address (referred to herein as the INDEX) of the register to an I/O port, shown in FIG. 5a as I/O port 22h. Another I/O port, shown herein as I/O port 23h, is used to read or write data from the specified control register. In the preferred embodiment, each I/O port 23h operation is preceded by an I/O port 22h operation, otherwise the second and later I/O port 23h operation would be directed off-chip. In the illustrated embodiment of FIG. 5a, the control registers each have an index between COh and CFh. 
     In FIG. 5b, the register having an index of COh uses its least significant bit to control tri-state devices 124 and 126. A bit equal to a logical high (i.e., a logical &#34;0&#34;) enables both tri-state devices 124 and 126 to provide transmission of the SUSP and SUSPACK signals. A logical &#34;1&#34; disables the SUSP and SUSPACK pins from the circuitry of the microprocessor 12. 
     This aspect of the preferred embodiment ensures pin-compatibility with an existing pin structure. 
     FIG. 6 illustrates another aspect of the present invention wherein the operation of the microprocessor 12 may be suspended responsive to a software command. 80×86 devices support a &#34;HALT&#34; operation (Opcode F4) which stops execution of all instructions and places the 80×86 in a HALT state. Execution is resumed responsive to a non-maskable interrupt (on the NMI pin) coupled to the bus controller 40, an unmasked interrupt (on the INT pin coupled to the bus controller 40) or a RESET. Normally, this instruction is used as the last instruction in a sequence which shuts down the system. 
     In the present invention, however, the HALT instruction has essentially the same consequence as asserting the SUSP pin. Thus, the BIOS 16 can issue a HALT instruction to the microprocessor 12, thereby disabling CLKB. Again, disabling CLKB will result in a significant reduction of power consumed by the microprocessor 12. 
     FIG. 6 illustrates a flow chart showing the operation of the HALT instruction in the preferred embodiment. Once a HALT instruction to the microprocessor 12 is received in decision block 130, U --  AHALT is asserted by the microcode ROM 48 in block 132. In response to the U --  AHALT signal from the microcode ROM, the exception processor 86 asserts D --  SUSPACK. After checking the busy signal from the coprocessor in decision block 136, the SUSPACK signal is asserted in block 140 by the bus controller 40 and the internal CLKB clock is disabled in block 142. In decision block 144, the microprocessor 12 remains in the suspended state until an interrupt is asserted in decision block 144. Once the interrupt is asserted, the CLKB clock is enabled and processing continues. 
     The HALT instruction allows the BIOS 16 to place the microprocessor 12 in a suspended state without any additional hardware connections to the microprocessor. 
     The present invention provides significant advantages over the prior art. By suspending the clocks to the core circuitry and memory circuitry, a current consumption of less than 10 milliamps has been demonstrated. Since most BIOS programs support power conservation measures, the additional coding for supporting the SUSP and SUSPACK signals is relatively simple. Alternatively, the chipset logic 18 can be modified to support the SUSP and SUSPACK signals. Further, since the SUSPACK, in the preferred embodiment, is not asserted until after coprocessor operations are completed, the BIOS does not have to provide additional circuitry or codes for monitoring the coprocessor. Further, the power saving circuitry may be provided on the microprocessor chip without sacrificing pin-compatibility. Additionally, by using the enhanced HALT command, the microprocessor may be operated in a suspended state without any hardware interaction, other than asserting an interrupt to bring the microprocessor 12 out of a suspended state. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.