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Question

A Processor supports 256KW of memory and uses memory mapping I/O  for I/O port. Address to the I/O port is assigned when 3 MSB bits of the address is high. The number of memory address available for I/O port is ____?

Answer 1

Number of memory words can be addressed = 256 KW = 2^18.

18 bits to address a word.

Given that when MSB is 111, then the address corresponds to I/O.

Using, 3 bits, 8 combinations are possible.

We give "111" to I/O and "000,001,010,011,100,101,110" to memory address.

So, I/O addresses = 1 * 2^(18-3) = 2^15

Memory addresses = (8-1) * 2^(18-3) = 7 * 2^15

Comments

But, These guys have not considered remaining 7 positions for the address.