Block Diagram of RAM and ROM

Block Diagram of RAM

In Random Access Memory (RAM), the memory cells can be accessed for information transfer from any desired random location. That is the process of locating a word in memory is the same and requires an equal amount of time no matter where the cells are located physically in memory: thus the name “Random Access“. Communication between memory and its environment achieved through data input and output lines, address selection lines, and control lines that specify the direction of transfer. A block diagram of a RAM unit is shown below figure:

The two operations that a random-access memory can perform are the write and read operations. The write signal specifies a transfer-in operation and the read signal specifies a transfer-out operation. On accepting one of these control signals, the internal circuits inside the memory provide the desired function. The steps that must be taken to transfer a new word to be stored into memory are as follows:

1. Apply the binary address of the desired word to the address lines.
2. Apply the data bits that must be stored in memory into the data input lines.
3. Activate the write input.

The memory unit will then take the bits presently available in the input data lines and store them in the word specified by the address lines. The steps that must be taken to transfer a stored word out of memory are as follows:

1. Apply the binary address of the desired word to the address lines.
2. Activate the read input.

The memory unit will then take the bits from the word that has been selected by the address and apply them to the output data lines. The content of the selected word does not change after reading.

Block Diagram of ROM

Read Only Memory (ROM) is a memory unit that performs the read operation only, it does not have a write capability. This implies that the binary information stored in a ROM is made permanent during the hardware production of the unit and cannot be altered by writing different words into it. Whereas RAM is a general-purpose device whose contents can be altered during the computational process.

A ROM is restricted to reading words that are permanently stored within the unit. The binary information to be stored, specified by the designer, and then embedded in the unit to form the required interconnection pattern. ROMs come with special internal electronic fuses that can be “programmed” for a specific configuration. Once the pattern is established, it stays within the unit even when power is turned off and on again. An m x n ROM is an array of binary cells organized into m words of n bits each. The block diagram of ROM is shown below figure:

A ROM has k address input lines to select one of 2^k = m words of memory, and n output lines, one for each bit of the word. An integrated circuit ROM may also have one or more enable inputs for expanding the number of packages into a ROM with a larger capacity. The ROM does not need a read-control line since, at any given time. The output lines automatically provide the n bits of the word selected by the address value. Because the outputs are a function of only the present inputs (the address lines).