Design of 512x8 RAM using 128x8 RAM - GeeksforGeeks

23 Feb, 2021
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 1CS2: –For chip select 2, the value should be 0.Read and Write: –Used for upcoming signals.
CS1: –For chip select 1, the value should be 1
CS2: –For chip select 2, the value should be 0.
Read and Write: –Used for upcoming signals.
If we have n bit address and m bit words then our RAM size will be 2n 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= 29 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 27 x 8, every RAM chip will need a 7-bit address. We will connect the remaining 7-bit address line to every RAM.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.
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 27 x 8, every RAM chip will need a 7-bit address. We will connect the remaining 7-bit address line to every RAM.
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.
Digital Electronics & Logic Design
GATE CS
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