Register

The register keyword is used to specify that a certain variable is going to be used a lot frequently and should be placed in a cpu register.

void foo (register int var) {
    register int test;
}

Both var and test are supposed to be placed in cpu registers but there is no surety that they will be placed in cpu registers or not.
This is because the excessive use of the keyword is ignored and it is also ignored due to disallowed declarations.
The keyword has been deprecated since c++17.