Semiconductor memory

A semiconductor memory comprising:

• a memory core having a memory cell that requires a refresh for data retention;
• a refresh control circuit that generates at predetermined intervals a refresh command to refresh said memory cell;
• a sub state machine that issues a refresh permission, a read permission, and a write permission to operate said memory core, in accordance with the refresh command, and a read command and a write command supplied from exterior, respectively;
• and
• a main state machine that makes said memory core perform a refresh operation according to the refresh permission, perform a read operation according to the read permission, and perform a write operation according to the write permission, wherein:
• said sub state machine has a ready state and a reserve state, the ready state being a state to which the sub state machine makes a transition when no read command is supplied the reserve state being a state to which the sub state machine makes a transition from the ready state in response to the read command;
• said main state machine has an idle state in which it puts said memory core in nonoperation, a read state in which it makes said memory core perform a read operation, a write state in which it makes said memory core perform a write operation, and a refresh state in which it makes said memory core perform a refresh operation.