Which storage class is used for the variables that are often required in a program ?
REGISTER
The register class exists precisely for variables accessed very frequently — option 2.
register int i;
What the keyword asks for. Ordinary variables live in memory, so every use costs a load and a store across the data bus. register requests that the variable be kept in one of the CPU's internal registers instead, where access is immediate. For a loop counter touched thousands of times, the saving is real:
| Location | Access time |
|---|---|
| CPU register | One cycle — no bus access |
| Cache | A few cycles |
| Main memory | Tens of cycles |
Two consequences follow from being in a register. First, it is a request and not a command: registers are few, so the compiler may ignore it, in which case the variable behaves as an ordinary automatic. Second — and this part is enforced — the address-of operator cannot be applied to it. &i on a register variable is a compile error, because a CPU register has no memory address. That restriction is the one guaranteed effect of the keyword.
The four storage classes compared.
| Class | Storage | Lifetime | Default value |
|---|---|---|---|
auto | Stack | Block | Garbage |
register | CPU register | Block | Garbage |
static | Data segment | Whole program | Zero |
extern | Data segment | Whole program | Zero |
Why the other options do not fit. static concerns lifetime and visibility — the variable keeps its value between calls — not speed of access, and being in memory it is no faster than auto. auto is the default for locals and says nothing about frequency of use. And EXTENT is not a C keyword at all; the real one is extern, which declares a variable defined in another file.
A modern footnote. Optimising compilers now allocate registers far better than a programmer's guess, using liveness analysis across the whole function, so the keyword is largely obsolete and is deprecated in current C++. Its lasting use is as documentation — and as the exam question's subject.
Hence, the storage class is REGISTER.
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