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3-Instruction Processing

3-Instruction Processing - Instruction processing...

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Instruction processing Instruction processing Tathagata Bhattacharjee
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Tathagata Bhattacharjee The von Neumann Model The von Neumann Model Memory Processing Unit Input Output MAR MDR ALU TEMP Control Unit PC IR (keyboard) (monitor) Memory: holds both data and instructions Processing Unit: carries out the instructions Control Unit: sequences and interprets instructions Input: external information into the memory Output: produces results for the user
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Tathagata Bhattacharjee The von Neumann Model The von Neumann Model z Memory Each location has an address and contents Address: bit pattern that uniquely identifies a memory location Contents: bit pattern stored at a given address. analogy: p.o. boxes have fixed numbers, but changing contents. Address Space: The total number of memory locations (“boxes”) available. eg. a 28 bit address provides an address space of 2 28 locations.
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Tathagata Bhattacharjee The von Neumann Model The von Neumann Model z Memory (continued) Addressability (Byte vs. Word): a word is the basic unit of data used by the processing unit, often multiple bytes; an instruction must store or retrieve an entire word with a single memory access. Addressability refers to the number of bytes of memory referenced by a given address. Extending the p.o. box analogy with an ISA whose word size is 2 bytes: ± if we have to deliver wide envelopes, we could convert pairs of the original single-width boxes into new double-wide boxes. ± we then have the choice of retaining the original numbering scheme, with each of the new boxes keeping both their original addresses (Byte Addressability); ± or we could renumber them all, giving a single address to each of the wider boxes (Word Addressability).
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Tathagata Bhattacharjee The von Neumann Model The von Neumann Model z Memory (continued) Accessing memory via the MAR & MDR The Memory Address Register sets up the decoder circuitry in the memory. Read: the contents of the specified address will be written to the Memory Data Register.
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