A digital signal processor for determining the maximum and minimum values of a plurality of data items wherein operations of an arithmetic logic unit and data memories are controlled by micro-instructions, including a device for decoding specified bits of an operand of the micro-instruction, a device for detecting a value of a condition code which has been designated by an output of the decoding device, and a control device for executing a logical operation between the output of the detection device, which becomes "1" if the value of the condition code is true, and a decoded value of an operation code of the micro-instruction and to generate a control signal for the arithmetic logic unit on the basis of a result of the logical operation.

A word oriented data processing system includes a plurality of system units all connected in common to a system bus. Included are a central processor unit (CPU), a memory system and a high speed buffer or cache system. The cache system is also coupled to the CPU. The cache includes an address directory and a data store with each address location of directory addressing its respective word in data store. The CPU requests a word of cache by sending a memory request to cache which includes a memory address location. If the requested word is stored in the data store, then it is sent to the CPU. If the word is not stored in cache, the cache requests the word of memory. When the cache receives the word from memory, the word is sent to the CPU and also stored in the data store.

A cache unit includes a cache store organized into a number of levels to provide a fast access to instructions and data words. Directory circuits, associated with the cache store, contain address information identifying those instructions and data words stored in the cache store. The cache unit has at least one instruction register for storing address and level signals for specifying the location of the next instruction to be fetched and transferred to the processing unit. Replacement circuits are included which, during normal operation, assign cache locations sequentially for replacing old information with new information. The cache unit further includes apparatus operative in response to a first predetermined type of command specifying the fetching of data words to set an indicator flag to a predetermined state. The apparatus conditions the replacement circuits in response to each subsequent predetermined type of command to bypass storage of the subsequently fetched data words when the indicator flag is in the predetermined state preventing the replacement of extensive numbers of data and instruction words already stored in cache during the execution of the instruction.

A computer has its programs in instructions and operand descriptors to specify the operands of the instructions. Apparatus for identifying data coherency and encaching requirements and providing access control in a computer system with operands of its instructions specified by operand descriptors is described hereby. In a computer system wherein data items (operands) are represented by operand descriptors that can comprise object numbers, addresses, data types and sizes, vector information and other relevant information concerning the operands, with one bit to identify if the data coherency is to be maintained, another bit to identify if the data is cached, and a field to provide information on the privilege of Read, Write and Execute, and Supervisor or User mode. When an operand is accessed, the respective access control code is checked to validate if any protection is violated, whether caching is activated and whether it is required to maintain data coherency.

A method and apparatus for improving the efficiency of executing arithmetic and logical operations performed on arguments provided during the execution of computer instructions in which operands include a variable type argument or data portion accompanied by a tag identifier which defines the data type. The processing of the data is enhanced by the addition of two condition codes derived from the values of the pre-ALU tag identifiers and the post-ALU results. The condition codes allow rapid determination of data types without additional execution cycles or hardware overhead, resulting in enhanced execution of the instructions.