If we see RAM physically then we notice that RAM is made up of a few chips. We can design the required RAM size using basic RAM chips.

**We can observe a basic RAM chip as follows: –**

**CS1: –**

For chip select 1, the value should be 1**CS2: –**

For chip select 2, the value should be 0.**Read and Write: –**

Used

If we have n bit address and m bit words then our RAM size will be** 2 ^{n }x m.**

**Example:**

n=7 bit, m=8 bit RAM size= 128 x 8

**Given: – **

Basic RAM size = 128 x 8 Required RAM size = 512 x 8

**To design a RAM size of 512×8 from 128×8, here are some calculations we need to do first –**

**1. Number of chips required:**

Number of chips required = Desired RAM Size/ Basic RAM Size =512x8/128x8 =4 chips

**2. Address Bits:**

Required Size is 512 x 8 512 x 8= 2^{9 }x 8 Therefore, 9 bit address is required

**3. Decoder Size:**

If the number of words is increasing them there will be a need for a decoder.

In this case number of words is increasing (from 128 to 512).

Size of decoder= number of times words increasing 128⇢512 words increased by 4 times Decoder Size= 2x4

**Pictorial Description (Design of 512×8 RAM):**

From the above calculations, we see that we need four **128 x 8** RAM chips to design a **512 x 8** RAM size. If only the number of words is increasing then all chips will be arranged vertically. In this case, only the number of words is increasing (128-512) so all chips will be arranged vertically.

- In a
**512 x 8**RAM, there is a 9-bit address, we will divide this 9-bit address into 2 and 7-bit addresses. - The 2-bit address will be input as a
**2 x 4**decoder, this decoder will have 4 output and the input will have 2 bits. - The 4 output of the decoder will enable every RAM of
**128 x 8**individually. - Also, we can write 128 x 8 RAM chip as
**2**every RAM chip will need a 7-bit address. We will connect the remaining 7-bit address line to every RAM.^{7 }x 8, - This 7-bit address will select any word from the 128 words of that RAM, the selected word will be the output as the 8-bit word in the output bus.