Abstract: Apparatus and method provide for tracking and maintaining precise state by assigning a unique identification tag to each instruction at the time of issue, associating the tag with a storage location in a first active instruction data structure, updating the data stored in the storage location in response to instruction activity status changes for each instruction, and maintaining a plurality of pointers to the storage locations that move in response to the instruction activity status. Status information includes an activity data item, such as an activity bit, that is set at the time the instruction is issued and cleared when execution completes without error. Pointers are established that point to the last issued instruction, the last committed instruction pointer, and reclaimed instruction pointer.

Abstract: In a microprocessor, an apparatus is included for coordinating the use of physical registers in the microprocessor. Upon receiving an instruction, the coordination apparatus extracts source and destination logical registers from the instruction. For the destination logical register, the apparatus assigns a physical address to correspond to the logical register. In so doing, the apparatus stores the former relationship between the logical register and another physical register. Storing this former relationship allows the apparatus to backstep to a particular instruction when an execution exception is encountered. Also, the apparatus checks the instruction to determine whether it is a speculative branch instruction. If so, then the apparatus creates a checkpoint by storing selected state information. This checkpoint provides a reference point to which the processor may later backup if it is determined that a speculated branch was incorrectly predicted.

Abstract: An instruction flow control circuit controls the selection and execution of instruction signals in a microprocessor having multiple execution units that can execute plural instructions at one time. The instruction flow control circuit compares a number of signals indicating how many execution units are available with a number of signals indicating how many execution units are required. The circuit is a matrix of transmission gates which propagate signals through, or shift signals between, various signal paths for available resources, depending on the signals requesting the executing units. A number of output gates suppress the execution of instruction signals where an execution unit is requested but none are available.

Abstract: A system and method providing ECC protection for all data block sizes for which a computer system supports store (ST) accesses includes an access control logic unit which converts store accesses for small data block (.ltoreq.64-bits) into read-modify (merge)-write accesses and a data merge logic unit which merges 64-bit data blocks retrieved by the load access with the small data block of the store access to create a new 64-bit data block. The data merge logic unit utilizes a merge code provided from a look-up table in a programmable logic array to perform the merging of the data blocks. An ECC generation logic unit processes the merged 64-bit data block, including the new small data block.

Abstract: A method and apparatus for performing address translation in a computer system supporting virtual memory by searching a translation lookaside buffer (TLB) and, possibly, a translation table held in memory and implemented as a B-tree data structure. The TLB is initially searched for a translation for a specified input address. If exactly one valid entry of the TLB stores a translation for the specified input address then the output address corresponding to the specified input address is determined from the contents of that entry. Otherwise, the translation table is searched for a translation for the specified input address. If two or more valid entries of the TLB store a translation for the specified input address, then these entries are invalidated.

Abstract: A data processor and associated method for taking and returning from traps speculatively. The data processor supports a predefined number of trap levels for taking nested traps each having a corresponding trap level. The data processor comprises means to form checkpoints, means to back up to the checkpoints, means to take a trap, means to return from a trap, registers, and a trap stack unit. The registers have contents that define the state of the data processor each time a trap is taken. The trap stack unit includes a trap stack data storage structure that has a greater number of trap slack storage entries than there are trap levels. It also includes a freelist unit that maintains a current availability list of the trap stack storage entries that are currently available for mapping to one of the trap levels. The freelist unit identifies, each time a trap is taken, a next one of the currently available trap stack storage entries for mapping to the corresponding one of the trap levels.

Abstract: An out of program control order execution data processor that comprises an issue unit, execution means, a floating point exception unit a precise state unit, a floating point status register, and writing means. The issue unit issues instructions in program control order for execution. The issued instructions include floating point instructions and non-floating point instructions. The execution means executes the issued instructions such that at least the floating point instructions may be executed out of program control order by the execution means. The floating point exception unit includes a data storage structure including storage elements. Each issued instruction corresponds to one of the storage elements. Each storage element has a floating point instruction identifying field and a floating point trap type field.

Abstract: Asynchronous combinatorial logic apparatus and method are provided that propagate data through a logic array at the speed of a raw combinational logic array and generate a functional output signal. The apparatus and method provide a minimum expected value of data propagation delay. In one embodiment, a particular data path is identified that has higher than average usage probability based on knowledge of the probabalistic distribution of data values, and the particular data path connecting devices located in the identified higher usage path are modified, such as by shortening the path, so that the path that is known to have a higher usage is made faster.

Abstract: Time-out checkpoints are formed based on a predetermined time-out condition or interval since the last checkpoint was formed rather than forming a checkpoint to store current processor state based merely on decoded instruction attributes. Such time-cut conditions may include the number of instructions issued or the number of clock cycles elapsed, for example. Time-out checkpointing limits the maximum number of instructions within a checkpoint boundary and bounds the time period for recovery from an exception condition. The processor can restore time-out based checkpointed state faster than an instruction decode based checkpoint technique in the event of an exception so long as the instruction window size is greater than the maximum number of instructions within a checkpoint boundary, and such method eliminates processor state restoration dependency on instruction window size. Time-out checkpoints may be implemented with conventional.

Abstract: Time-out checkpoints are formed based on a predetermined time-out condition or interval since the last checkpoint was formed rather than forming a checkpoint to store current processor state based merely on decoded instruction attributes. Such time-out conditions may include the number of instructions issued or the number of clock cycles elapsed, for example. Time-out checkpointing limits the maximum number of instructions within a checkpoint boundary and bounds the time period for recovery from an exception condition. The processor can restore time-out based checkpointed state faster than an instruction decode based checkpoint technique in the event of an exception so long as the instruction window size is greater than the maximum number of instructions within a checkpoint boundary, and such method eliminates processor state restoration dependency on instruction window size.

Abstract: A method and apparatus detects whether more than one object has been selected from a set of objects. A unique code and an error code is coupled to objects in the set. At least one object is selected and the unique codes from the selected object are logically summed, as are the error codes from the selected objects. A test code is generated from the logically summed unique code and tested for equality with the logically summed error code to determine if more than one object was selected.

Abstract: Time-out checkpoints are formed based on a predetermined time-out condition or interval since the last checkpoint was formed rather than forming a checkpoint to store current processor state based merely on decoded instruction attributes. Such time-out conditions may include the number of instructions issued or the number of clock cycles elapsed, for example. Time-out checkpointing limits the maximum number of instructions within a checkpoint boundary and bounds the time period for recovery from an exception condition. The processor can restore time-out based checkpointed state faster than an instruction decode based checkpoint technique in the event of an exception so long as the instruction window size is greater than the maximum number of instructions within a checkpoint boundary, and such method eliminates processor state restoration dependency on instruction window size.

Abstract: A high-performance processor is disclosed with structure and methods for: (1) aggressively scheduling long latency instructions including load/store instructions while maintaining precise state; (2) maintaining and restoring precise state at any instruction boundary; (3) tracking instruction status to maintain precise state; (4) checkpointing instructions to maintain precise state; (5) creating, maintaining, and using a time-out checkpoint; (6) tracking floating-point exceptions; (7) creating, maintaining, and using a watchpoint for plural, simultaneous, unresolved-branch evaluation; and (9) increasing processor throughput while maintaining precise state.

Abstract: A system and method for transferring data between clock domains operating at substantially the same frequency continuously compares the relationship between a delayed data phase signal driven by a non-delayed source domain clock and a data phase signal sampled by a delayed receiving domain clock. The result of the comparison determines along which one of multiple data paths within the synchronizing circuit the transfer of the data frame will take place. Several data paths with different delays (at least two) transfer the data frame and clock signals. An XNOR comparator responds to the relationship between a delayed data phase signal driven by a non-delayed source domain clock and a data phase signal sampled by a delayed receiving domain clock to determine which one of the multiple data paths transfers the data frame.

Abstract: Time-out checkpoints are formed based on a predetermined time-out condition or interval since the last checkpoint was formed rather than forming a checkpoint to store current processor state based merely on decoded Instruction attributes. Such time-out conditions may include the number of instructions issued or the number of clock cycles elapsed, for example. Time-out checkpointing limits the maximum number of instructions within a checkpoint boundary and bounds the time period for recovery from an exception condition. The processor can restore time-out based checkpointed state faster than an instruction decode based checkpoint technique in the event of an exception so long as the instruction window size is greater than the maximum number of instructions within a checkpoint boundary, end such method eliminates processor state restoration dependency on instruction window size.

Abstract: A method and apparatus for generating a zero flag (z-flag) status signal in a microprocessor includes a z-flag signal generator that generates a z-flag signal from unaligned data simultaneous to the load alignment of such data. The z-flag generator first performs a zero detect on each byte of data retrieved from memory. The zero detect results are next decoded according to bit selection signals generated from a data format code which corresponds to the specific format of the retrieved data.

Abstract: A system and method provides hardware support for fast software emulation of unimplemented instructions using issue trap logic that determines the instruction type and parameter fields of an unimplemented instruction when an exception is triggered and uses the fields to branch directly to emulation code specific to an unimplemented instruction having the determined instruction type and parameter fields.

Abstract: A pullup circuit having a limited voltage swing and fast pullup and pulldown times comprises a pullup structure, a pulldown structure and an internal node. The pullup circuit operates to limit the current of the pullup structure before the N-tree discharges the internal node, thereby reducing the pullup effect of the pullup structure to reduce fall time and power consumption. Then the pullup circuit maximizes the current of the pullup structure after the N-tree has pulled down the internal node to increase the pullup effect of the pullup structure to reduce rise time. As a result, the voltage of the internal node both charges more quickly when the N-tree becomes inactive and discharges more quickly when the N-tree becomes active.

Abstract: A differential sense amplifier includes positive feedback cross coupling to control operation in one mode as a differential sense amplifier and in another mode as a latch to control a data-latching load. Circuit nodes are precharged and equalized in response to applied enable signal.

Abstract: The gate of a P-channel pull-up transistor connected between an input node and a supply voltage in a buffer circuit is coupled to a test node. An N-channel pull-down transistor is connected between the input node and ground and has a gate connected to the test node. A logic low signal provided to the test node allows the circuit to operate normally. During test mode, a logic high signal is provided to the test node to turn off the P-channel pull-up transistor and thus prevent DC current flow in the circuit via the pull-up transistor. This logic high signal also turns on the pull-down transistor and, by shorting the input node to ground potential, prevents any other DC crossover currents from flowing in the circuit. Thus, during test mode, quiescent current flow resulting from small manufacturing defects in the circuit are obscured by larger DC currents and, as a result, may be readily measured to detect the presence of such small manufacturing defects.