PURPOSE: To preliminarily define the minimum RAM capacity necessary for printer function by providing a RAM control means and a monitoring means in order that each of RAM regions controlled by the RAM control means may be used for a separate purpose.

CONSTITUTION: A printer 1 such as laser beam printer has CPU 2 for controlling the operation of the components of the printer and ROM 3 for programming and storing the operation sequence. An engine part 6 in the printer 1 has two side printing function and, when the user sets operation panel controls to the both side printing function, a both side printing function setting means 8 gives an instruction of the both side printing and, at the same time, a region determining means 10 defines the RAM region of an extended RAM 9 to realize the both side printing. The region thus defined is exclusively used for the both side printing.