What statement is incorrect regarding memory$-$mapped $\frac{\text{I}}{O}?$

a$.$ Registers of an on$-$chip peripheral are mapped to the same address space as the program

and data memory. In other words, the address space is shared between memory and I$/$O

devices or peripherals.

b$.$ Processors can use load and store instructions to access peripheral registers as if they were

part of the data memory.

c$.$ Memory$-$mapped I$/$O requires fewer logic circuits on the memory bus to decode memory

addresses.

d$.$ Memory$-$mapped $\frac{\text{I}}{O}$ often requires a $C$ program to use volatile keywords to prevent the

compiler from making false optimizations and mistakenly caching peripheral data in

registers.