referring to fig1 through 4 , a preferred embodiment of the invention will be described in detail . referring first to fig1 there is shown a typewriter having word processing capability , which incorporates a printer according to one embodiment of the invention for performing a printing operation based on a text data stored in a text memory 49a ( which will be described ). the typewriter is provided with a keyboard 3 at its front , and a display 5 and a printing mechanism 14 disposed behind the keyboard 3 . the keyboard 3 has a multiplicity of character keys 3a for entering characters such as alphabetic letters , numerals and various symbols , and many function keys such as a printing key 3b , a space bar 3c , and a carriage return key 3d . the display 5 uses a liquid crystal display ( lcd ) 5a . as shown in fig1 and 3 , this lcd 5a is connected to a lcd control device 5b which is operated under the control of a main controller 72 . the main controller 72 incorporates a cpu 45 ( central processing unit ), a ram 49 ( random - access memory ), a rom 47 ( read - only memory ) and a data bus 70 . the rom 47 of the main controller 72 stores control programs for controlling the operation of the present typewriter . the lcd 5a controlled by the lcd control device 5b is adapted to provide a menu which lists various modes of operation of the typewriter that are selectable by the operator through the keyboard 3 . further , the lcd 5a is adapted to display a portion of a text which is entered through the keyboard 3 and stored in the above indicated text memory 49a provided in the ram 49 . the lcd 5a also displays various messages from the typewriter to the operator . the ram 49 is also used to store text data which are supplied from an external device through an interface 76 . the printing mechanism 14 has various drive sources including a type - wheel drive motor 80 , a hammer drive solenoid 82 , a carriage drive motor 84 , a paper feed motor 15 and a ribbon feed motor 87 . these drive sources for the printing mechanism 14 are controlled according to control commands from main controller 72 , via the corresponding control devices , i . e ., a type - wheel control device 88 , a hammer control device 90 , a carriage control device 92 , a paper feed control device 15a and a ribbon feed control device 96 . thus , the printing operation is effected . a type - wheel 100 shown in fig1 is mounted on the output shaft of the type - wheel drive motor 80 mounted on a carriage 85 . the type - wheel 100 has 96 radial arms which carry at their ends respective type fonts . with the type - wheel drive motor 80 rotated by a suitable angle according to the text data , the type - wheel 100 is indexed to bring the desired one of the type fonts into the printing position aligned with a line of printing on a platen 9 . a hammer 104 is provided on the carriage 85 such that the hammer 104 is disposed in front of the type font at the printing position . the hammer 104 is activated by the above - indicated hammer drive solenoid 82 , thereby impacting the selected type font against a cut sheet 106 supported on the platen 9 , via a ribbon 108 which is fed by the ribbon feed motor 87 . in this way , the character corresponding to the impacted type font is printed on the sheet 106 , as is well known in the art . the type - wheel drive motor 80 , the type - wheel 100 and the hammer 104 cooperate with other elements to constitute a print head 110 which is mounted on the carriage 85 . the carriage 85 is reciprocated along the axis of the platen 9 by the above - indicated carriage drive motor 84 , whereby the print head 110 is moved right and left relative to the sheet 106 , in the longitudinal direction of the platen 9 . the sheet 106 is fed up and down with the platen 9 rotated by the paper feed motor 15 . with the feeding movements of the sheet 106 and the print head 110 , the printing position on the sheet 106 can be changed or shifted . a tractor feeder 7 ( fig2 ) used to continuously feed a continuous - page sheet is detachably mounted on this type of typewriter as a printer . the tractor feeder 7 is mounted upwardly of the platen 9 of the printing mechanism 14 . as shown in fig2 the platen 9 is rotatably supported on a frame 11 within the body of the typewriter and a rotary shaft for the platen 9 is provided with a gear 13 . in addition , the gear 13 is interlocked with a rotary shaft of the paper feed motor 15 secured to the frame 11 via an intermediate gear 17 and the turning force produced by the paper feed motor 15 is transmitted to the platen 9 . a paper pan - 9 , which is movable between a paper pressure position adjacent to the bottom face of the platen 9 and a paper release position distant from the bottom face of the platen 9 , is provided at the bottom of the platen 9 . a plurality of rollers 21 , which are brought into contact with a printing sheet when the paper pan 19 is placed in the paper pressure position are rotatably supported in the paper pan 19 . the paper pan 19 is interlocked with a release lever 23 by a known mechanism , and is also changed over to either one of the paper pressure position and the paper release position by controlling the release lever 23 . in addition , the release lever 23 is controlled to change over the paper pan 19 to the paper release position with a view toward performing the feeding movement of the printing sheet only by the tractor feeder 7 when the tractor feeder 7 is mounted on the printing mechanism 14 . a photosensor 25 for sensing whether or not the printing sheet exists on the platen 9 is provided frontwardly of the bottom face of the platen 9 . when photosensor 25 senses that the printing sheet does not exist on the platen 9 , the cpu 45 controls the lcd control device 5b and a message for alarming the sheet absence or for urging an operator to insert the printing sheet , is displayed on the lcd 5a . thus , the operator can know that a printing sheet is needed . on the other hand , the tractor feeder 7 includes two parallel support shafts 29 , 31 connectably supported between a pair of tractor frames 33 and 35 , and two pin tractors 37 , 39 mounted on the support shafts 29 , 31 . in addition , a gear portion 41 is provided at one end of one of the support shafts , i . e ., the support shaft 29 , and is brought into engagement with an interlocking gear 43 rotatably supported within the tractor frame 33 . when the tractor feeder 7 is mounted on the printing mechanism 14 , the interlocking gear 43 is maintained in engagement with the gear 13 provided on the side of the typewriter , and the rotational force of the motor 15 is transferred to the tractor feeder 7 , whereby the pin tractors 37 and 39 are rotated . the operation of the present invention will now be described with reference to fig4 . it is first practiced in step 410 ( hereinafter abbreviated &# 34 ; s410 &# 34 ; in the present description and the accompanying drawing ) to input text data such as for example documents , etc . based on the control of the keyboard 3 by the operator . next , in s420 , the number of lines corresponding to one page to be printed is established by the control of the character keys 3a on the keyboard 3 performed by the operator , in view of the size of a cut sheet or the interval of perforations defined on the continuous - page sheet . the number of lines corresponding to one page to be printed is stored in a line memory 49b . the printing key 3b for making a start in the printing of the input text data is operated in s430 . when a start in printing movement is made , the sheet feeding operation is executed up to a position where the printing is actually made on the printing sheet in s440 . then , the routine proceeds to s450 . data corresponding to one line which has not yet been printed is read out from the text data and the read data corresponding to one line is thereafter printed . next , it is performed in s460 to judge whether or not the data corresponding to the number of lines to be printed , which has been established in s420 and stored in the line memory 49b , has been printed . if the judgment is no , the routine returns to s450 . then , s450 and s460 are repeatedly executed . if it is found to be yes in s460 , the routine proceeds to s470 . here , the sheet feeding is made by a predetermined number of lines , e . g ., five lines . as the result of this sheet feeding in s470 , if the tractor feeder 7 has been mounted on the printer , the sheet feeding is made until a printing start position reaches a position to be printed . when the printed sheet is a cut sheet , a printed cut sheet is discharged from the printing mechanism 14 . it is next judged in s480 whether or not data to be printed on the next page exists in the text memory 49a . if it is no , the routine is terminated . on the other hand , if it is yes , the routine proceeds to s490 . the photosensor 25 is driven in s490 . it is then performed in s500 to determine whether or not the photosensor 25 has detected a sheet for printing data corresponding to the following page . if it is yes , it is determined that the tractor 7 has been mounted , and hence the routine proceeds to s510 . in order to cause the operator to shift the release lever 23 to a release position , a message &# 34 ; release the release lever &# 34 ; is displayed on the lcd 5a in s510 . then , the routine returns to s450 , and the printing operation is continued . if it is no in s500 , it is judged that the tractor feeder 7 has not been mounted on the printer . the routine therefore proceeds to s520 . in order to prevent the cut sheet from being inclined by interference between the left and right longitudinal edges thereof and the carriage 85 on the platen 9 when the cut sheet is inserted into the printing mechanism 14 , the carriage 85 is controlled so as to be disposed in the center of a printing line width and the printing operation is prevented in s520 . a message &# 34 ; insert sheet &# 34 ; is displayed on the lcd 5a . at this time , the routine returns to s430 . the operator can , however , supply the following cut sheet during stoppage of the printing operation . when the cut sheet is supplied by the operator and the printing key 3a is operated by the operator again , a start in the printing operation is made . upon completion of the printing operation of one page in the above - described manner , the typewriter according to the present embodiment will automatically feed the sheet a predetermined number of lines . after feeding of the sheet , it is performed to determine whether or not the printing sheet for the following page is detected by the photosensor . based on this detection result , a judgment is made as to whether or not the tractor feeder 7 has been mounted on the printer . according to a consequence of this judgement , the optimal printing control can be done correspondingly . moreover it is unnecessary to provide a switch for detecting the tractor feeder 7 , or the like . accordingly , the increase in the number of components and that in the manufacturing cost can be avoided . a description has been made on the typewriter in which the printer 1 is provided with the keyboard 3 in the present embodiment . the present invention can , however , be applied in the same manner even to a singly - provided printer used as a terminal device for the connection to a host computer , a word processor , etc . having now fully described the invention , it will be apparent to those skilled in the art that many changes and modifications can be made without departing from the spirit or scope of the invention as set forth herein .