1.  Field of the Invention
The present invention relates to a data processing apparatus provided with a display device. 
2.  Description of the Prior Art
Among compact and lightweight microcomputers, portable type computers powered by batteries are now used extensively.  Particularly, one of them known as a note-size computer is lighter in weight and smaller in size, yet provides equal capabilities to those of a desktop or laptop computer.  The note-size computer powered by batteries is handy for use in a place where a power supply facility is rarely available, e.g. a meeting room or a lecture hall. 
However, the disadvantage of such handy use is that the life of batteries is short and limited.  When used to record a business meeting or a college lecture, the service duration of such a note-size computer with fully charged batteries is preferably 10 hours nonstop; more preferably, 20 to 30 hours.  If possible, more than 100 hours-a standard of hand calculators-is most desired. 
So far, the service operation of a commercially available note-size computer lasts 2 to 3 hours at best.  This results in battery runout in the middle of a meeting or college lecture causing an interruption during input work.  As a result, troublesome replacement of batteries with new ones will be needed at considerable frequency. 
Such a drawback of the note-size computer tends to offset the portability in spite of its light weight and compactness. 
It is understood that known pocket-type portable data processing apparatuses including hand calculators and electronic notebooks are much slower in processing speeds than common microcomputers and thus, exhibit less power requirements.  They are capable of servicing for years with the use of a common primary cell(s) of which life will thus be no matter of concern.  The note-size computer, however, has a processing speed as high as that of a desktop computer and consumes a considerable amount of electric energy-namely, 10 to 1000 times the power consumption of any pocket-type portable data processing apparatus.  Even with the application of up-to-date high quality rechargeable batteries, the serving period will be 2 to 3 hours at maximum.  This is far from a desired duration demanded by the users.  For the purpose of compensating the short life of batteries, a number of techniques for energy saving have been developed and some are now in practical use. 
The most well known technique will now be explained. 
A “resume” function is widely used in a common note-size computer.  It works in a manner that when no input action continues for a given period of time, the data needed for restarting the computer with corresponding information is saved in a nonvolatile IC memory and then, a CPU and a display are systematically turned off.  For restart, a power switch is closed and the data stored in the IC memory is instantly retrieved for display of the preceding data provided before disconnection of the power supply.  This technique is effective for extension of the battery servicing time and suitable in practical use. 
However, a specified duration, e.g. 5 minutes, of no key entry results in de-energization of the entire system of the computer and thus, disappearance of display data.  Accordingly, the operator loses information and his input action is interrupted.  For reviewing the display data or continuing the input action, the power switch has to be turned on each time.  This procedure is a nuisance for the operator.  The resume technique is advantageous in saving energy of battery power but very disadvantageous in operability of the note-size computer. 
More specifically, the foregoing technique incorporates as a means for energy saving a system which de-energizes all the components including a processing circuit and a display circuit.  The operator is thus requested to turn on the power switch of the computer at considerable frequencies during intermittent data input action because each no data entry duration of a given length triggers automatic disconnection of the switch.  In particular, the data input operation with a note-size computer is commonly intermittent and thus, the foregoing disadvantage will be much emphasized.