Computer Organization or Computer representation in Computer Memory

Memory cells fabricated using current technology can be placed in one out of two stable states. We call these cells binary cells and each cell can store a binary digit. One of the two stable states is used to represent the binary 0 and the other binary 1. Figure 1 shows storage cells. The cell has an input data line on which the symbol to be written is sent to the cell. In order to write this symbol in the cell, a write-control signal is sent to the cell via the write –line. If the content of a cell is to be read, a read-control signal is sent on the read-line and the content of the cell may be sensed on the output data line.

Figure 1.  A memory cell

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By appropriate variation in the interconnection of binary memory cells, it is possible to organize different types of memories. We will assume that individual cells are non-volatile and reading is non-destructive. The simplest organization of a set of cells is shown in Figure 2. In this organization, three cells are interconnected in such a way that the write-control lines of all the cells are connected together. The read-control lines are also connected together. The bits to be written in each cells are fed to the appropriate input data lines. When the write -signals is applied to the write-control line, these bits are written in the individual cells. The previous contents in cells are automatically erased when the new data is written. In order to read the contents of the cells, a read- signal is applied to the common read-line. The contents of the cells appear on the respective output data lines. The contents of individual cells are not erased by the read operation as reading from these cells is assumed to be non- destructive. The interconnecting of the cells is called a resister. The resister in Figure 2 stores three bits.

     

  Figure  2. A 3-bit register

A group of registers may be interconnected as shown in Figure 3 to form a memory. This memory is said to store four words of three bits each. In this organization, the input data lines of the first bit of all four registers are connected together. Similarly, the input data lines of the first bit of all four registers are connected together. Similarly, the input data lines of the second bits are connected together and those of the third are also connected together as shown in figure 3. The output lines of the bits of the registers are also connected together.

                                                                              Figure 3. A 3-bit per word 4- word Memory

Observe that there are four write lines, one for each 3-bit register and four read-lines. In order to write data in this memory, the resister in which the data is to be written is specified. If data is to be written in the second resister, a signal is sent on the write-line of this register. The data to be written is placed of the input lines. If the data in, say, the third resister, is to be red, a read-signal is sent on the read-line corresponding to the register. The contents of the third register appear on the output lines.

Each of the four registers has one read- line and one write-line. At a time we may either read from or write in a register. Each register in the memory should have a unique identification so that the appropriate register may be selected for writing or reading. In Figure 3. Each pair of read/ write lines corresponding to a register is identified by the binary codes 00, 01,10 and 11. The memory has 4 words and each word stores 3 bits. The identification code of each register corresponding to a word in the memory is known as its address. If a memory has eight words, the binary codes 000, 001, 010,011,100,101,110 and 111 will uniquely address these lines. In general, with n bits we can address 2^n words of a memory. In order to select an appropriate word in a memory, for either reading ot writing, it is necessary to specify the address of the word. This address is usually specified as a binary number and is placed in a register called a Memory Address Register (MAR). A selector circuit is connected to MAR which selects the appropriate read- or write-control line based on the contents of MAR and on whether read or write is specified. For the memory shown in Figure 3, MAR will be 2-bit register.

The data read from a memory or that to be written in a memory is placed in a register called a Memory Data Register (MDR). This register receives the output of memory cells during a read operation. For write- operation, the data to be stored in memory cells is placed in this register. In the example, memory of Figure 3, MDR will be 3-bit register.

Block Diagram of the Memory

Figure  4 is a  block diagram of the memory of Figure 3 including the registers used to specify the address for address for reading/ writing and the register for storing the data read or to be written.

                  Figure 4. Block Diagram of a 4-word 3-bit per word memory

In practice, the number of bits stored in memories is much larger than that shown in this example. The block diagram of memory which capable of storing 64K (K=1024) bytes is shown in Figure 5. Sometimes the notation Ki is used to represent 1024 and K to represent 1000. Observe that 64K= 2^16. Thus, the number of bits in MAR is 16. The memory will be made up of 64K words with each word storing eight bits, that is one byte. The MDR will be an 8-bit register. The selector block in Figure 5 will select a read- or write-line corresponding to the word to be read or written. Observe that the address of the word to be read or written will be placed in MAR. The data read from the memory or to be written will be placed in MAR. the data read from the memory or to be written in the memory will be placed in MDR.

If the number 173 = (10101101)2= (AD)16 is to be written in the memory in address (1111111010101011010)2= (FD5A)16, then the number (AD)16 is entered in MDR and the address (FD5A)10 in MAR. The write-signal is initiated. The number in MDR, namely, (AD)16. will be written in location (FD5A )16. This address (101010101100110100)2 is placed in MAR.

                                                 Figure 5. Block diagram of a 64 KB memory

A read- signal is initiated. A copy of the data stored in (AB34)16 is retrieved and placed in MDR. If the cells used in the memory are destructive read-out, then the contents in the specified address will be cleared to zero. In this case, whatever is read from the memory would, however, be available in MDR to be written back in the memory.

Assume that at time to the address from which data is to be retrieved is placed in MAR and a read – signal is applied. If at time t1 the required data is available in MDR, the elapsed time (t1-t0) is known as the access time of the memory (Figure 6.a). The access time is usually slightly larger than the read-time of individual cells in the memory. If data is to be written in the memory, the data is placed in MDR and the address in MAR at time t`2 (see Figure 6 b.) and a write- signal is initiated. If writing operation is completed at time t3, then the elapsed time (t3-t2) is called Write-time.

The time interval between the time t0 at which a read/ write command is given to the memory and the time t4 when the next such instruction can be issued to the memory (namely, t4-t0) is known as the cycle time of the memory (see Figure 6 a.).

If the time taken to access a word in a memory is independent of the address of the word the memory is called Random Access Memory(RAM). The main memory used to sore program and data in a computer is a random access memory. In this article the memory which is illustrated is Random Access memory.

Summary:

•          A number of cells, with each cell storing a bit, constitute a word. A Word is a group of bits which are stored and retrieved as a unit. The main memory system of a computer is organized to store a number of words. A word may store one or more bytes. The length of a word may be specified as ‘n bits per word’.
•          Each word of the main memory of a computer is stored in an addressable location. A Memory Address register (MAR) stores the address of the location in the memory from where a word is to be retrieved or where a word is to be stored. The data retrieved from the memory or the data to be stored in memory are placed in a Memory Data Register (MDR).
•          In order to store a word in memory, it is placed in MDR and the address where it is to be stored is entered in MAR, and a write command is given. The time taken to write a word is known as the write-time.
•          In order to retrieve a word from memory, the address of the word is placed in MAR and a read command is issued.
•          The retrieve data is placed in MDR by the memory. The time to retrieve data is called the access time of the memory.