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Memory selection circuit for selecting one of various memory areas in a memory unit based on the capacity and the starting address of each area

  • US 5,394,538 A
  • Filed: 03/23/1994
  • Issued: 02/28/1995
  • Est. Priority Date: 03/29/1990
  • Status: Expired due to Term
First Claim
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1. A memory selection circuit of a computer provided with a central processing unit and a memory unit divided into a plurality of memory areas of an equal capacity to be accessed by common lower bits of an address sent from the central processing unit, comprising:

  • memory capacity storing means for storing capacity data corresponding to said equal capacity of each of said plurality of memory areas, said capacity data being rewritable by signals from the central processing unit;

    decoder means for decoding said stored capacity data to generate bit selecting signals;

    start address storing means for storing upper bits of a predetermined start address assigned to a beginning of said memory unit, said upper bits being rewritable by signals from central processing unit;

    means for comparing bit by bit, upper bits of an address output from said central processing unit and the stored upper bits of said predetermined start address and generating a comparison signal representing a result of each comparison; and

    means for generating a selection signal for selecting one of said plurality of memory areas on the basis of the stored capacity data and said comparison signal, said generating means including;

    bit selecting means for selecting which address bits to decode based on said bit selection signals,logic gate means connected to receive said comparison signal and said bit selecting signals for generating a signal representing that the address sent from the central processing unit is in a valid address range starting with said predetermined start address, anddecoding gate means connected to receive said selected address bits and said signal from said logic gate means for decoding said selected address bits to generate said selection signal if the address is in said valid address range.

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