United States Patent  [in Patent Number: 4,722,047
Chan et al.  Date of Patent: Jan. 26, 1988
 PREFETCH CIRCUIT AND ASSOCIATED METHOD FOR OPERATION WITH A VIRTUAL COMMAND EMULATOR
 Inventors: Kasun K. Chan, Del Mar; True D. Y.
Nguyen, San Diego; Chiman R. Patel,
Escondido, all of Calif.
 Assignee: NCR Corporation, Dayton, Ohio
 Appl. No.: 770,459
 Filed: Aug. 29,1985
 Int O* G06F 12/08
 UJS. O. 364/200
 Field of Search ... 364/200 MS File, 900 MS File
 References Cited
U.S. PATENT DOCUMENTS
3,618,041 11/1971 Horikoshi 364/200
3,768,080 10/1973 Bogeretal 364/200
4,218,743 8/1980 Hoffman et al 364/200
4,371,924 2/1983 Schaefer et al 364/200
4,422,144 12/1983 Johnson et al 364/200
4,502,110 2/1985 Saito 364/200
Primary Examiner—Gary V. Harkcom
Assistant Examiner—David L. Clark
A prefetch circuit for use with a memory including a storage register for receiving a command from the memory, a decoding circuit for decoding the command to determine the identification of an index register contained within the command, and a fetch circuit for fetching the contents of the index register from the command. The prefetch circuit also includes a virtual address storage register for receiving and storing the virtual address of the command, an adding circuit for adding a predetermined offset to the virtual address of the command to obtain a new virtual address, a comparison circuit for determining if the new virtual address from the adding circuit has crossed a virtual page boundary, a transfer circuit responsive to the comparison circuit for transferring the real address in the real address storage register to the adding circuit for adding the offset thereto, thereby obtaining a new real address. The fetch circuit then fetches a prefetched command from the memory at the new real address. Data registers are also included for prefetching data.
12 Claims, 12 Drawing Figures