Question

Given below are two statements :
one is labelled as
Assertion (A) and the other is labelled as
Reason (R).
Assertion (A) :
During memory interfacing, address decoding is required to select the desired memory chip.
Reason (R) :
Each memory chip has unique address range that must be enabled using chip select signals.
In the light of the above statements, choose the most appropriate answer from the options given below :

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A)
• C. (A) is correct but (R) is not correct
• D. (A) is not correct but (R) is correct

Hint:

Address decoding is like the operator in an old telephone exchange—they look at the "number" (address) you dialed and connect your call to the specific "house" (memory chip) meant for that range.

Answer 1

The Correct Option is A

Concept:
• A computer system typically contains multiple memory chips (RAM, ROM) and I/O devices.
• The processor has a single address bus that is shared by all these components.
• Address decoding is the process of generating a unique 'Chip Select' (CS) or 'Device Enable' signal for each individual component so they don't clash on the bus.

Step 1: Evaluate the Assertion (A)
When a processor wants to read or write data, it puts an address on the bus. To ensure that only the intended memory chip responds, we must "decode" that address to activate the specific chip's enable pin. Without this, multiple chips might attempt to drive the bus simultaneously, causing a bus contention. Thus, (A) is true.

Step 2: Evaluate the Reason (R)
In a memory map, different physical chips are assigned different ranges (e.g., Chip 1 handles \(0000H - 3FFFH\), Chip 2 handles \(4000H - 7FFFH\)). The address decoder looks at the higher-order bits to see which range is being accessed and then pulls the corresponding chip's CS pin low (or high). Thus, (R) is true.

Step 3: Determine the relationship
The physical requirement of having unique address ranges (R) is exactly why we need a decoder (A) to translate those logical addresses into physical selection signals. Hence, (R) explains (A).