18-RAM - Random access memory Sequential circuits all...

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Random Access Memory 1 Random access memory Sequential circuits all depend upon the presence of memory. A flip-flop can store one bit of information. A register can store a single “word,” typically 32-64 bits. Random access memory , or RAM , allows us to store even larger amounts of data. Today we’ll see: The basic interface to memory. How you can implement static RAM chips hierarchically. This is the last piece we need to put together a computer!
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Random Access Memory 2 Introduction to RAM Random-access memory , or RAM , provides large quantities of temporary storage in a computer system. Remember the basic capabilities of a memory: It should be able to store a value. You should be able to read the value that was saved. You should be able to change the stored value. A RAM is similar, except that it can store many values. An address will specify which memory value we’re interested in. Each value can be a multiple-bit word (e.g., 32 bits). We’ll refine the memory properties as follows: A RAM should be able to: - Store many words, one per address - Read the word that was saved at a particular address - Change the word that’s saved at a particular address
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Random Access Memory 3 Picture of memory You can think of computer memory as being one big array of data. The address serves as an array index. Each address refers to one word of data. You can read or modify the data at any given memory address, just like you can read or modify the contents of an array at any given index. If you’ve worked with pointers in C or C++, then you’ve already worked with memory addresses.
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Random Access Memory 4 Block diagram of RAM This block diagram introduces the main interface to RAM. A Chip Select, CS , enables or disables the RAM. ADRS specifies the address or location to read from or write to. WR selects between reading from or writing to the memory. To read from memory, WR should be set to 0. OUT will be the n-bit value stored at ADRS. To write to memory, we set WR = 1. DATA is the n-bit value to save in memory. This interface makes it easy to combine RAMs together, as we’ll see. 2 k x n memory ADRS OUT DATA CS WR k n n
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5 Memory sizes We refer to this as a 2 k x n memory. There are k address lines , which can specify one of 2 k addresses. Each address contains an n-bit word. For example, a 2 24 x 16 RAM contains 2 24 = 16M words, each 16 bits long. The RAM would need 24 address lines.
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This note was uploaded on 10/19/2011 for the course CS 231 taught by Professor - during the Spring '08 term at University of Illinois at Urbana–Champaign.

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18-RAM - Random access memory Sequential circuits all...

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