ORA-27100: shared memory realm already exists

whenever you execute a command

SQL> startup nomount
ORA-27100: shared memory realm already exists

SQL> shutdown immediate
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist

Problem
====================
Some changes are made in sga_max_size or sga parameters.
After that database is abnormally shut down whenever you issue startup,
ORA-27100 is received as a result of changes made to parameters affecting the SGA size.

Solution
=====================

In most cases, ORA-27100 can be resolved by simply restarting Windows.

Otherwise

start the database with pfile .
SQL> startup pfile= loacation_of_pfile;

create spfile from pfile='location' ;