Serial Access Memory

Consider the organization of memory cells shown in Figure 1. In this organization, the output of a cell is the input to the next cell. A read-signal places the contents of each of the cells on the respective output lines. A write-signal following this read-signal will store these bits in the respective “next” cells. One read-write pair of signals would thus “shift” the contents of the cells right by one cell position and the bit stored in the rightmost cell will appear on the output line. A sequence of read-write signals will serially shift the contents of the register. After three read-write (namely, shift) Signals, the output will be 0. As the bits stored in the cells appear serially (i.e., one after another) at the output, this memory is called a serial access memory. The structure shown in Figure 1 is known as a shift register. If the output of the shift register is connected to its input, each shift-signal will circularly shift the contents of the memory. A series of shift-signals will keep the contents of the memory circulating.

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                                                    Figure 1. A serial access memory

Another model of memory cells is shown in Figure 2. a. In this model, one bit is stored in each cell. The Read- head reads the content of the placed below it and places it on the output line. The cells are moved physically from left to right and as each cell appears below the read-head. Its content is placed on the output. The bits appear on the output as shown in Figure 2.b.

Figure 2. Illustration of reading with read – head.

Writing in the cells is done using a Write - head as shown in Figure 3 a. As the cells move below the write-head, the bits fed to the write-head are stored in the cells as shown in Figure 3b. This model of serial access memory is appropriate to explain the operation of memories using magnetic surface recording.

Figure 3. Illustration of writing with write-head.

A serial access memory is a non- addressable memory. In other words, sets of bits stored in the memory cannot be selectively retrieved by specifying their locations in the memory. The bits stored  can be retrieved only in strict serial order.