NIELIT 2018-39

Question

A RAM chip has a capacity of $1024$ words of $8$ bits each $(1K \times 8)$. The number of $2 \times 4$ decoders with enable line needed to construct a $32 K \times 8$ RAM from $1K  \times 8$ RAM is

• A. $4$
• B. $5$
• C. $6$
• D. $7$

Answer 1

BASIC CAPACITY= $1K*8$

TARGET CAPACITY= $32K*8$

#CHIPS REQUIRED= TARGET/GIVEN

$\Rightarrow\frac{ 32K*8}{1K*8}$$\Rightarrow32$

$\Rightarrow 2^{5}$

$\Rightarrow \log {_{2}} {2^5}$

therefore 5 DECODERS are required.

HENCE OPTION B

Comments

you missed last one 2÷2 =1(keep on dividing upto that point where you can't divide it any more)

8+2+1=11

ok so you're considering 32×1 configuration leading upto 10 decoder but recall that

REALISING LARGE DECODER USING SMALL ALWAYS GIVES ODD NUMBER OF DECODER

that means you have misinterpreted this. Also recall that computer science is all about efficiency, reducing overheads/cost, minimization , optimization and all those things that eases life maintaining standards.

 

I can also use a 1x2 decoder ( for example not gate) instead of 2×4 that will lead to 31.

i request you please brush up this topic again.

 

KEEP IN MIND REALISING LARGE CONFIGURATION WITH SMALL ONE WILL ALWAYS YIELD ODD NUMBER OF DECODER.

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Yes I forgot, 2/4=1 than total 11 decoder required.but how 5 will be answer.if we draw circuit , just look at this link.

 

https://gateoverflow.in/194518/number-of-decoders

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Plz answer this

RAM chip has a capacity of 1024 words of 8 bits each (1K×8). The number of 2×4 decoders with enable line needed to construct a 16 K×16 RAM from 1K×8 RAM ???

Answer 2

→ The capacity of the RAM needed = 16K
→ Capacity of the chips available = 1K
→ No. of address lines = 16K/1K = 16
→ Hence we can use 4×16 decoder for this. But we were only given 2×4 decoders.
→ So, 4 decoders are required in inner level as from one 2×4 decoder we have only 4 output lines whereas we need 16 output lines.
→ To point to these 4 decoders, another 2×4 decoder is required in the outer level.
→ Hence no. of 2×4 decoders to realize the above implementation of RAM = 1+4 = 5

Comments

@topper98 you have considered capacity as 16K.

 

Answer 3

None of the options is correct here.

Ref: https://gateoverflow.in/1555/gate2013-46

Try this question from GATE ‘13 and you will come to know why options are wrong.