Abstract:
A partitioned decoder circuit responds to an address signal supplied at a decoder circuit input by providing a result data signal that corresponds to the address signal. Selection signal decoder circuitry asserts one or more of a plurality of decoder enable signals based upon the value of a decoder selection signal. A plurality of decoder circuit elements are each connected to receive a separate one of the asserted decoder enable signals. Each decoder circuit element includes a first clock input coupled to receive a precharge clock signal, an address input coupled to receive the address signal, address latching circuitry that latches the address signal in response to a polarity transition of the precharge clock signal, and a second clock input. Significantly, gated discharge clock signal generation circuitry of each decoder circuit element generates a gated discharge clock signal in response to the asserted decoder enable signal. The gated discharge clock signal is provided to the second clock input. A data output responds to the gated discharge clock signal being provided to the second clock input by providing an evaluation signal that corresponds to the latched address signal such that the result data signal includes the evaluation signal.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to decoder circuits, and in particular, to a decoder circuit which is partitioned for power conservation. 
     BACKGROUND OF THE INVENTION 
     Conventional processor architectures include table-based control circuits for implementing processor &#34;macro&#34; instructions in microcode. Specifically, the microcode execution datapath is specified by a finite-state table-based control unit, implemented, for example, in a read only memory (&#34;ROM&#34;) or Programmable Logic Array (&#34;PLA&#34;). The control unit, in response to a control input (e.g., a macroinstruction opcode), provides a state designation that tells the datapath what needs to be done to execute the &#34;macro&#34; instruction to which the control input corresponds. 
     Similarly, processor macroinstructions are typically stored in a ROM. A macroinstruction is provided from a data output of the ROM by strobing a read input concurrently with the ROM address of the macroinstruction being provided on an address bus. 
     With the quest for greater processor miniaturization, it is becoming necessary to further decrease the amount of power that processors consume. Thus, there is a need for table-based control circuits that can be optimized for lower-power consumption. 
     SUMMARY OF THE INVENTION 
     The present invention is a partitioned decoder circuit. The partitioned decoder circuit responds to an address signal supplied at a decoder circuit input by providing a result data signal that corresponds to the address signal. Selection signal decoder circuitry asserts one or more of a plurality of decoder enable signals based upon the value of a decoder selection signal. A plurality of decoder circuit elements are each connected to receive a separate one of the asserted decoder enable signals. Each decoder circuit element includes a first clock input coupled to receive a precharge clock signal, an address input coupled to receive the address signal, address latching circuitry that latches the address signal in response to a polarity transition of the precharge clock signal, and a second clock input. 
     Significantly, gated discharge clock signal generation circuitry of each decoder circuit element generates a gated discharge clock signal in response to the asserted decoder enable signal. The gated discharge clock signal is provided to the second clock input. A data output responds to the gated discharge clock signal being provided to the second clock input by providing an evaluation signal that corresponds to the latched address signal such that the result data signal includes the evaluation signal. 
     A better understanding of the features and advantages of the invention will be obtained by reference to the following detailed description and accompanying drawings which set forth an illustrative embodiment in which the principles of the invention are utilized. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 schematically illustrates a partitioned decoder circuit in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a partitioned decoder circuit 100 in which a decoder selection signal SEL is provided to a decoder selector 102. The decoder selector 102 asserts those ones of four decoder enable signals 104a-104d indicated by the decoder selection signal SEL. 
     For example, if the decoder selection signal SEL is ostensibly be a processor macroinstruction opcode of a variable length instruction set, the decoder selector 102 could determine the &#34;type&#34; of the instruction from its length. As another example, the decoder selector 102 could determine the &#34;type&#34; as either floating point or fixed point. As yet another example, the &#34;type&#34; could be determined by the decoder selector 102 as one of reg/reg (for register to register data move instructions), reg/mem (for register to memory data move instructions), and mem/mem (for memory to memory data move instructions). 
     In any event, the decoder selector 102 asserts a decoder enable signal (one of 104a-104d) that corresponds to the determined instruction type. If the decoder selection signal SEL does not represent a valid instruction type, then none of the decoder enable signals 104a-104d are asserted. Circuitry suitable for implementation of decoder selector 102 is disclosed, for example, in commonly-assigned U.S. patent application Ser. No. 08/445,563, entitled &#34;TAGGED PREFETCH AND INSTRUCTION DECODER FOR VARIABLE LENGTH INSTRUCTION SET AND METHOD OF OPERATION&#34; (atty. docket no. NSC1-64100), filed on even date herewith and which is hereby incorporated by reference in its entirety. 
     As another example, the decoder selection signal SEL may represent an address, in a processor linear address space, of a macroinstruction to be fetched and decoded, with the decoder 102 being conventional address decoding logic circuitry. In this case, the decoder selector 102 asserts a decoder enable signal (one of 104a-104d) that corresponds to a block in the processor linear address space in which the macroinstruction resides. 
     As is discussed in detail below, the decoder enable signals 104a-104d function to selectively enable decoders 106a-106d, respectively, to provide a n evaluation signal at an output terminal Q. In particular, the decoders 104a-104d are circuits, such as a read-only memory (&#34;ROM&#34;) or programmable logic array (&#34;PLA&#34;) which latch an address signal ADR, presented to a data input D, in response to a clock signal CK provided to a &#34;precharge&#34; input, PRECHG. (In the case of the decoder circuit for providing macroinstructions from a ROM in a processor linear instruction space, the address signal ADR is the same signal as the select signal SEL provided to the decoder selector 102. This is shown by the dashed line 105 in FIG. 1.) The decoders 104a-104d provide the evaluation signal, corresponding to the latched address signal, to a data output Q in response to a clock signal CK --  D provided to a &#34;discharge&#34; input, DISCHG. Examples of such circuits are well-known in the art and are discussed, for example, in Chapter 5 of CMOS Digital Circuit Technology, by Masakazu Shoji (Prentice Hall, 1988). 
     Referring still to FIG. 1, the clock signal CK is continuously provided to the &#34;precharge&#34; inputs of the decoders 106a-106d. However, a significant feature of the present invention is that the clock signals provided to the &#34;discharge&#34; inputs DISCHG of the decoders 106a-106d are &#34;gated&#34; by the decoder enable signals 104a-104d, respectively, via AND devices 108a-108d, respectively. Thus, only the required one or ones of decoders 106a-106d is discharged, with the result that the power consumed by the decoder circuitry as a whole is much less than the power that would otherwise be consumed if the clock signal CK --  D provided to the &#34;discharge&#34; inputs DISCHG was not gated and, furthermore, if all of the decoding were provided in a single decoder circuit, discharged by a single clock. 
     In some applications of the present invention, more than one of the decoder enable signals 104a-104d may be asserted at any one time. That is, the decoder selection signal SEL may indicate more than one &#34;type&#34;. However, in many other applications (e.g., decoding processor macroinstructions), only one (if any) of the decoder enable signals 104a-104d is asserted at any one time. Thus, in this case, at most one of the decoders 106a-106d is used at any one time, and at most one of the data outputs Q of the decoders 106a-106d need be enabled at any one time. Thus, as further shown in FIG. 1, in accordance with an embodiment of the present invention, the data outputs Q of the decoders 106a-106d are enabled to drive a data output bus D --  OUT only in response to the decoder select signals 104a-104d, respectively. In particular, the data outputs Q of decoders 106a-106d drive the data output bus D --  OUT via bus driver circuitry 110a-110d, respectively, which are enabled by decoder select signals 104a-104d, respectively. 
     As an example, referring still to FIG. 1, when decoder select signal 104a is not asserted, then bus driver circuit 110a presents a high impedance to data output bus D --  OUT. When decoder select signal 104a is asserted, then data output Q of decoder 106a is allowed to drive the data output bus D --  OUT, via bus driver circuit 110a. 
     While a particular embodiment of the invention has been described, it should be understood that various alternatives to the embodiment of the invention described herein may be employed in practicing the invention. 
     Moreover, the invention embodiments described herein have been implemented in an integrated circuit which includes a number of additional functions and features which are described in the following co-pending, commonly assigned patent applications, the disclosure of each of which is incorporated herein by reference: U.S. patent application Ser. No. 08/451,319, entitled &#34;DISPLAY CONTROLLER CAPABLE OF ACCESSING AN EXTERNAL MEMORY FOR GRAY SCALE MODULATION DATA&#34; (atty. docket no. NSC1-62700); U.S. patent application Ser. No. 08/451,965, entitled &#34;SERIAL INTERFACE CAPABLE OF OPERATING IN TWO DIFFERENT SERIAL DATA TRANSFER MODES&#34; (atty. docket no. NSC1-62800); U.S. patent application Ser. No. 08/453,076, entitled &#34;HIGH PERFORMANCE MULTIFUNCTION DIRECT MEMORY ACCESS (DMA) CONTROLLER&#34; (atty. docket no. NSC1-62900); U.S. patent application Ser. No. 08/452,001, entitled &#34;OPEN DRAIN MULTI-SOURCE CLOCK GENERATOR HAVING MINIMUM PULSE WIDTH&#34; (atty. docket no. NSC1-63000); U.S. patent application Ser. No. 08/451,503, entitled &#34;INTEGRATED CIRCUIT WITH MULTIPLE FUNCTIONS SHARING MULTIPLE INTERNAL SIGNAL BUSES ACCORDING TO DISTRIBUTED BUS ACCESS AND CONTROL ARBITRATION&#34; (atty. docket no. NSC1-63100); U.S. patent application Ser. No. 08/451,924, entitled &#34;EXECUTION UNIT ARCHITECTURE TO SUPPORT x86 INSTRUCTION SET AND x86 SEGMENTED ADDRESSING&#34; (atty. docket no. NSC1-63300); U.S. patent application Ser. No. 08/451,444, entitled &#34;BARREL SHIFTER&#34; (atty. docket no. NSC1-63400); U.S. patent application Ser. No. 08/451,204, entitled &#34;BIT SEARCHING THROUGH 8, 16, OR 32-BIT OPERANDS USING A 32-BIT DATA PATH&#34; (atty. docket no. NSC1-63500); U.S. patent application Ser. No. 08/451,195, entitled &#34;DOUBLE PRECISION (64-BIT) SHIFT OPERATIONS USING A 32-BIT DATA PATH&#34; (atty. docket no. NSC1-63600); U.S. patent application Ser. No. 08/451,571, entitled &#34;METHOD FOR PERFORMING SIGNED DIVISION&#34; (atty. docket no. NSC1-63700); U.S. patent application Ser. No. 08/452,162, entitled &#34;METHOD FOR PERFORMING ROTATE THROUGH CARRY USING A 32-BIT BARREL SHIFTER AND COUNTER&#34; (atty. docket no. NSC1-63800); U.S. patent application Ser. No. 08/451,434, entitled &#34;AREA AND TIME EFFICIENT FIELD EXTRACTION CIRCUIT&#34; (atty. docket no. NSC1-63900); U.S. patent application Ser. No. 08/451,535, entitled &#34;NON-ARITHMETICAL CIRCULAR BUFFER CELL AVAILABILITY STATUS INDICATOR CIRCUIT&#34; (atty. docket no. NSC1-64000); U.S. patent application Ser. No. 08/445,563, entitled &#34;TAGGED PREFETCH AND INSTRUCTION DECODER FOR VARIABLE LENGTH INSTRUCTION SET AND METHOD OF OPERATION&#34; (atty. docket no. NSC1-64100); U.S. patent application Ser. No. 08/450,153, entitled &#34;PARTITIONED DECODER CIRCUIT FOR LOW POWER OPERATION&#34; (atty. docket no. NSC1-64200); U.S. patent application Ser. No. 08/451,495, entitled &#34;CIRCUIT FOR DESIGNATING INSTRUCTION POINTERS FOR USE BY A PROCESSOR DECODER&#34; (atty. docket no. NSC1-64300); U.S. patent application Ser. No. 08/451,219, entitled &#34;CIRCUIT FOR GENERATING A DEMAND-BASED GATED CLOCK&#34; (atty. docket no. NSC1-64500); U.S. patent application Ser. No. 08/451,214, entitled &#34;INCREMENTOR/DECREMENTOR&#34; (atty. docket no. NSC1-64700); U.S. patent application Ser. No. 08/451,150, entitled &#34;A PIPELINED MICROPROCESSOR THAT PIPELINES MEMORY REQUESTS TO AN EXTERNAL MEMORY&#34; (atty. docket no. NSC1-64800); U.S. patent application Ser. No. 08/451,198, entitled &#34;CODE BREAKPOINT DECODER&#34; (atty. docket no. NSC1-64900); U.S. patent application Ser. No. 08/445,569, entitled &#34;TWO TIER PREFETCH BUFFER STRUCTURE AND METHOD WITH BYPASS&#34; (atty. docket no. NSC1-65000); U.S. patent application Ser. No. 08/445,564, entitled &#34;INSTRUCTION LIMIT CHECK FOR MICROPROCESSOR&#34; (atty. docket no. NSC1-65100); U.S. patent application Ser. No. 08/452,306, entitled &#34;A PIPELINED MICROPROCESSOR THAT MAKES MEMORY REQUESTS TO A CACHE MEMORY AND AN EXTERNAL MEMORY CONTROLLER DURING THE SAME CLOCK CYCLE&#34; (atty. docket no. NSC1-65200); U.S. patent application Ser. No. 08/452,080, entitled &#34;APPARATUS AND METHOD FOR EFFICIENT COMPUTATION OF A 486™ MICROPROCESSOR COMPATIBLE POP INSTRUCTION&#34; (atty. docket no. NSC1-65700); U.S. patent application Ser. No. 08/450,154, entitled &#34;APPARATUS AND METHOD FOR EFFICIENTLY DETERMINING ADDRESSES FOR MISALIGNED DATA STORED IN MEMORY&#34; (atty. docket no. NSC1-65800); U.S. patent application Ser. No. 08/451,142, entitled &#34;METHOD OF IMPLEMENTING FAST 486™ MICROPROCESSOR COMPATIBLE STRING OPERATION&#34; (atty. docket no. NSC1-65900); U.S. patent application Ser. No. 08/452,659, entitled &#34;A PIPELINED MICROPROCESSOR THAT PREVENTS THE CACHE FROM BEING READ WHEN THE CONTENTS OF THE CACHE ARE INVALID&#34; (atty. docket no. NSC1-66000); U.S. patent application Ser. No. 08/451,507, entitled &#34;DRAM CONTROLLER THAT REDUCES THE TIME REQUIRED TO PROCESS MEMORY REQUESTS&#34; (atty. docket no. NSC1-66300); U.S. patent application Ser. No. 08/451,420, entitled &#34;INTEGRATED PRIMARY BUS AND SECONDARY BUS CONTROLLER WITH REDUCED PIN COUNT&#34; (atty. docket no. NSC1-66400); U.S. patent application Ser. No. 08/452,366, entitled &#34;SUPPLY AND INTERFACE CONFIGURABLE INPUT/OUTPUT BUFFER&#34; (atty. docket no. NSC1-66500); U.S. patent application Ser. No. 08/451,744, entitled &#34;CLOCK GENERATION CIRCUIT FOR A DISPLAY CONTROLLER HAVING A FINE TUNEABLE FRAME RATE&#34; (atty. docket no. NSC1-66600); U.S. patent application Ser. No. 08/451,206, entitled &#34;CONFIGURABLE POWER MANAGEMENT SCHEME&#34; (atty. docket no. NSC1-66700); U.S. patent application Ser. No. 08/452,350, entitled &#34;BIDIRECTIONAL PARALLEL SIGNAL INTERFACE&#34; (atty. docket no. NSC1-67000); U.S. patent application Ser. No. 08/452,094, entitled &#34;LIQUID CRYSTAL DISPLAY (LCD) PROTECTION CIRCUIT&#34; (atty. docket no. NSC1-67100); U.S. patent application Ser. No. 08/450,158, entitled &#34;DISPLAY CONTROLLER CAPABLE 0F ACCESSING GRAPHICS DATA FROM A SHARED SYSTEM MEMORY&#34; (atty. docket no. NSC1-67500); U.S. patent application Ser. No. 08/450,726, entitled &#34;INTEGRATED CIRCUIT WITH TEST SIGNAL BUSES AND TEST CONTROL CIRCUITS&#34; (atty. docket no. NSC1-67600); U.S. patent application Ser. no. 08/445,568, entitled &#34;DECODE BLOCK TEST METHOD AND APPARATUS&#34; (atty. docket no. NSC1-68000). 
     It is intended that the following claims define the scope of the invention and that methods and apparatus within the scope of these claims and their equivalents be covered thereby.