Abstract:
A latch transfers fetched opcode to PLA for execution at the earliest opportunity following execution of a prior single cycle opcode.

Description:
FIELD OF THE INVENTION 
     The field of the invention is the design of the internal architecture of a microprocessor, particularly as it relates to the speed of execution of program instructions. 
     BACKGROUND OF THE INVENTION 
     Program instructions generally comprise one to three bytes. The first byte is known as the opcode; it specifies the operation to be performed. Instruction bytes following the opcode, if any, may be data or address bytes (or both). 
     An instruction is first fetched and then executed. In the fetch phase, the instruction is fetched from microprocessor memory and transferred to the microprocessor control unit (CU). The CU is implemented, for example, by microprogramming a programmable logic array (PLA). The CU sequences the operation of the entire microprocessor. It generates synchronization signals and manages the commands exchanged between the microprocessor unit (MPU), input/output device (I/O) and memory. 
     The CU must perform a memory fetch for each instruction byte. The CU is generally programmed to &#34;know&#34; the number of bytes in each instruction; that is, the CU knows whether to go back to memory to fetch additional bytes after fetching the first byte or opcode. 
     A program instruction requires one or more machine cycles to execute. In the 6500 family of microprocessors, a machine cycle corresponds to the period of a microprocessor clock (φ 2 ), and a typical instruction can be executed in 1-7 machine cycles. The last machine cycle of any instruction includes the opcode fetch for the next instruction. During the opcode fetch, the appropriate program instruction is read from memory, and the instruction is gated over the data bus to the CU. Appropriate signals, including a SYNC pulse, are generated by the CU to execute a fetched instruction during the second half of a machine cycle. In the 6500 family of microprocessors, SYNC pulses cannot be generated every machine cycle due to the PLA characteristics. Thus, the SYNC pulse can at best be generated every other machine cycle. As a result, an instruction following a single byte instruction, i.e., an instruction which takes only one machine cycle to execute (referred to hereafter as a &#34;single cycle&#34; instruction), effectively occupies two machine cycles so that the next fetched instruction waits an extra machine cycle before it can be executed. 
     The present invention is directed to a technique for speeding up execution of program instructions, wherein at least one instruction can be executed in a single machine cycle, so that execution of the instruction following the single cycle instruction will begin in the very next machine cycle. 
     SUMMARY OF THE INVENTION 
     The invention is a system for accelerating the execution of program instructions by a microprocessor. A latch is coupled to the data bus and the PLA. A first opcode is fetched, gated over the data bus, and presented at the latch output for execution under control of the PLA. If a bit pattern in the first opcode indicates that the opcode can be executed in a single machine cycle, a pre-decode circuit generates a PRESYNC signal. While the first opcode is being executed, a second opcode is fetched and gated to the latch input. The second opcode is immediately transferred to the latch output during the next machine cycle based on the PRESYNC signal. Thus, where an instruction can be executed in a single machine cycle, the execution of the next instruction will begin in the very next machine cycle. 
     For the purpose of illustrating the invention, there is shown in the drawings forms which are presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the system of the present invention. 
     FIG. 2 is a logic diagram of an exemplary pre-decode circuit. 
     FIG. 3 is a group of waveforms showing conventional microprocessor operation for a single byte instruction which requires one machine cycle to execute (single cycle instruction). 
     FIG. 4 is a group of waveforms showing operation of the present invention for a single cycle instruction. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, wherein like numerals indicate like elements, there is shown in FIG. 1 a system 10 according to the present invention for accelerating execution of program instructions wherein at least one instruction is a single cycle instruction. The instruction contains a single byte or opcode which is fetched from microprocessor memory and gated over a data bus 12 to the input (IN) of a latch 14. A latch input is transferred to the latch output (OUT) in response to a microprocessor clock pulse φ 2  and a &#34;high&#34; signal appearing at the gate (G) of the latch. The latch output is fed to the PLA 16 and a pre-decode circuit 18. The PLA generates various control signals, utilized in executing an instruction, which are fed to various random logic components in conventional manner. In the 6500 series of microprocessors, for which the invention is particularly suited, the PLA generates a SYNC pulse during the last machine cycle of an instruction, i.e., the machine cycle during which an opcode fetch is performed for the next instruction to be executed. 
     Although in the 6500 microprocessor an opcode fetch can take place during execution of a prior instruction, the fetch does not begin until a SYNC pulse is generated by the PLA. In the present invention, a PRESYNC pulse is combined with the SYNC pulses so that a fetch is performed during a PRESYNC pulse or a SYNC pulse. If the prior opcode can be executed in a single machine cycle, then a fetch of the next instruction can begin during that machine cycle and execution of that instruction can begin at the very next machine cycle. 
     In the present invention, a bit pattern in the opcode indicates whether the opcode is a single cycle opcode, i.e., whether it can be executed in a single machine cycle. The single cycle opcode bit pattern is decoded by the pre-decode circuit 18. In general, the pre-decode circuit may assume the form shown in FIG. 2 wherein the latch output lines Dφ-D7 and Dφ-D7 decoded to generate the PRESYNC signal. The SYNC and PRESYNC signals are logically combined by an OR gate 20 which drives the latch gate (G). 
     Referring to FIG. 3, there are shown the waveforms corresponding to conventional microprocessor operation for a single cycle opcode, i.e., the microprocessor clock φ 2 , the data bus (latch IN), the SYNC signal, and the latch output (OUT). While the single cycle opcode is being fetched, the prior opcode is at the latch OUT (PLA IN). When the prior instruction has been executed, the single cycle opcode is gated from latch IN to latch OUT, i.e., to PLA IN. Execution of the single cycle opcode can then begin on the rising edge of the next clock pulse φ 2 . However, the next opcode cannot be fetched until the occurrence of the next SYNC pulse. Thus, two machine cycles must pass before the next opcode can be fetched and transferred to the latch outputs for execution. 
     Referring to FIG. 4, there is shown the waveforms corresponding to operation according to the present invention. A single cycle byte opcode contains a bit pattern which indicates that the instruction can be executed in a single machine cycle. The bit pattern is decoded by the pre-decode circuit 18 which generates the PRESYNC pulse in the very next machine cycle, i.e., between successive SYNC pulses. The microprocessor fetches the next opcode during the PRESYNC pulse as indicated by the output of OR gate 20. The PRESYNC pulse conditions the latch gate G so that the next clock pulse φ 2  transfers the fetched opcode from latch IN to latch OUT (PLA IN) for execution by the PLA during the very next machine cycle. 
     In the example described herein, the latch gate (G) must be &#34;high&#34; for the falling edge of the clock pulse (φ 2 ) to transfer information from latch IN to latch OUT. It should be appreciated, however, that other logic levels and pulse transitions can be employed to provide the same result. 
     The present invention guarantees the presence of a PRESYNC pulse between successive SYNC Pulses, i.e., during the machine cycle in which a single cycle instruction is executed, whereby the next instruction can be fetched during that machine cycle and executed during the very next machine cycle. The speed at which program instructions can be executed is thereby significantly increased by the present invention. The invention requires few additional logic components; and space demands on the microprocessor chip are slight. 
     The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.