##### How many RAM chips needed & what is the size of decoder used?

How many 256 X 4 RAM chips are required to organize a memory of capacity 32KB ? What is the size of decoder required in this implementation to select a row of chip?

Options :

(a) 128 , 7 X 128

(b) 256 , 7 X 128

(c) 512 , 7 X 128

(d) 256 , 8 X 256.

Option D should be correct.

target size = 32KB = 32K x 8 (1 Byte = 8 bits) = 2

^{15}x 8 bitsbasic size = 256 x 4 = 2

^{8}x 4 bitsNo of chips = target size/ basic size^{7}x 2 =256Size of decoder can be calculated from the row chips of basic size = 8 x 2

^{8}=8 x 256Note- If basic size is2then size of decoder is^{M}x NM x 2^{M}answer is B.

Please explain.

No of chips=32K*8/256*4=256

1 chip 4 bits

And for byte (8 bits) we need 2 chips in row

Total no of rows =128

So 7:128 decoder is required to select a row

the answer is a ,

we know that decoders used to distinguish between the RAMs so , first of all the Q said "memory of capacity 32KB " we need to convert from KB to Bytes , this gives whats 32*2^10 =32.768 Bytes . then we divdie those bytes on the size of RAMs to know how many of RAMs it can fit , 32.768\265= 128 which is the number of RAMs we need , now we just have to know the size of decoder and since there's 128 RAMs we should connect each one with a wire , but how we can differntiate between those RAMS ? 2^7 =128 , then all we need is 7 bits to differintaie , so the decoder should be 7*128 .