Display controller with a variable scrolling speed, and method for operating same
BACKGROUND OF THE INVENTION 
     The present invention relates to a data display-controlling device, and more particularly to a data display-controlling device which is used for a data-processing apparatus including a keyboard with a control key, and a display unit for displaying data, and which can vary the speed at which data is displayed on the display unit. 
     In a conventional data-processing apparatus, data (e.g., a character string) is displayed on the display unit at such a speed that the operator can visually comprehend the data. 
     With recent developments in microprocessors, the basic clock frequency of the microprocessors has been increased considerably. As a result, the speed at which data is displayed on the display unit has also been increased, resulting in difficulties in comprehending a character string as it is displayed. The comprehension of the character string becomes particularly difficult when a large amount of data is displayed in a scroll mode. 
     In addition, if the data is scrolled too fast, the operator finds it very difficult to temporarily stop the data at an appropriate line. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a data display-controlling device which solves the above problems and which can switch the display speed between &#34;high&#34; and &#34;low&#34;. 
     A data display-controlling device of the present invention is used for a data-processing apparatus which operates under an operating system program and includes a keyboard for entering data and a display unit for displaying data. To achieve the above object, the data display-controlling device comprises: 
     instruction means for generating an instruction signal which reduces the speed at which data is displayed on the display unit, by delaying the time at which data to be displayed is supplied to the display unit; 
     timer means for measuring current time; and 
     data output-controlling means, which operates in response to the instruction signal, for reading the current time measured by the timer means and determining whether a predetermined time has passed on the basis of the read current time, effecting data supply to the display unit if it is determined that the predetermined time has passed, and suspending data supply to the display unit so as to execute the operation system program if it is determined that the predetermined time has not yet passed. 
     The present invention also comprises a method of carrying out these steps. 
     According to the data display-controlling device and method of the present invention, the display speed can be changed between &#34;high&#34; and &#34;low&#34; by use of a control key. If the amount of data to be displayed is small (for instance, if the data can be displayed within the display area of the screen at one time), the data is displayed in a high-speed display mode. If the amount of data to be displayed is so large as to exceed the display area of the screen, the data is displayed in a low-speed display mode. If data is displayed in this manner, it can be easily comprehended even in the scroll mode, and the scrolling of data can be stopped at a desirable time. If it is not time to supply the data to be displayed to the display unit, the central processing unit (CPU) immediately executes the operating system program even in the low-speed display mode. The CPU does not remain in the data display routine, waiting for the data-supply time to come. Therefore, the selection of the low-speed display mode does not adversely affect the other processing. 
    
    
     BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Other objects and features of the present invention will be apparent from the following description taken in connection with the accompanying drawings in which: 
     FIG. 1 is a block diagram illustrating a data-processing apparatus to which the data display-controlling device according to one embodiment of the present invention is applied; and 
     FIG. 2 shows a flow chart illustrating the processings for reducing the data display speed. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a data-processing apparatus to which a data display-controlling device of one embodiment of the present invention is applied. As illustrated, the data-processing apparatus comprises: keyboard 11, which is connected to common bus 15 via keyboard controller 13; timer 17 for measuring current time, random access memory (RAM) 19 for storing the current time measured by timer 17, data to be displayed, and an operating system program; refresh memory 21 for storing data to be displayed; parallel-serial converter 23 for performing parallel-serial conversion of the data supplied from refresh memory 21; and display unit 25 for displaying the data supplied from parallel-serial converter 23. Multiplexer 27 has an output terminal connected to refresh memory 27, and two input terminals connected to common bus line 15 and display controller 29, respectively. Either display control information from CPU 31 or that from display controller 29 is selected by multiplexer 27 on the basis of a selection signal supplied from CPU 31, and the selected display control information is supplied to refresh memory 21. Display controller 29 can be formed, for example, by HD 46505, which is commercially available from Hitachi Ltd., Japan. Display controller 29, multiplexer 27, refresh memory 21, keyboard controller 13, timer 17, and RAM 19 (which are all directly connected to common bus 15) operate under the control of CPU 31. CPU 31 includes ROM 31a, which stores the display control program shown in FIG. 2. CPU 31 can be formed, for example, by Intel 80286, which is commercially available from U.S. Intel Corporation. Display unit 25 may be of a plasma type, a liquid crystal type, or a CRT type. Keyboard 11 includes a control key for commanding the reduction of the data display speed. 
     The processing performed in the above embodiment will be described with reference to the flow chart shown in FIG. 2. In Step 41, CPU 31 reads keyboard status information from keyboard controller 13. In step 43, it is determined, on the basis of the keyboard status information, whether or not the control key is depressed. If it is determined that the control key is not depressed, the flow advances to step 49, in which the control of the data display is given over to display controller 29. If it is determined that the control key is depressed, the flow advances to step 45. In this step, CPU 31 reads the current time which is then measured by timer 17 and the current time which is stored in RAM 19, and subtracts the value of the latter from that of the former. Further, CPU 31 compares the difference of the subtraction with a predetermined value (e.g., 50 msec). This comparison is made so as to determine whether the control key has been depressed for more than 50 msec. When these steps are effected for the first time, an initial value of &#34;0&#34; is stored in RAM 19, so that the difference of the subtraction is not larger than 50 msec. Therefore, CPU 31 exits from the display control routine and executes the operating system program. After effecting other processing (e.g., the processing for entering data from keyboard 11) for a certain length of time, the flow is returned to the display control routine shown in FIG. 2. This procedure is repeated until it is determined that the control key has been depressed for more than 50 msec and the data to be displayed is supplied to display unit 25. If it is determined in step 45 that the difference of the subtraction exceeds 50 msec, step 47 is effected, in which data on the current time read from timer 17 is stored in a predetermined area of RAM 19. Next, step 49 is effected, in which the data to be displayed is supplied to display controller 29. Therefore, the data to be displayed is supplied to display unit 25 with a time delay of 50 msec. In other words, one character is displayed at least 50 msec after the preceding character is displayed. As a result, the data display speed is reduced, enabling the operator to easily comprehend the data on the display unit. When the data is being scrolled in this fashion, it may happen that the operator wants to stop the scrolling of the data. In such a case, the operator depresses a certain key (i.e., &#34;S&#34; key) while simultaneously pressing the control key. By so doing, the scrolling of the data is stopped. Since the data can be scrolled slowly, the operator can easily stop the scroll display at any line he wants. 
     The present invention is not limited to the above embodiment. In the above embodiment, the time period during which the control key is depressed is 50 msec, but this time length can be determined arbitrarily. In addition, the data display-controlling device can be applied also to a data-processing apparatus which can display data in a scroll mode, such as a personal computer, a word processor, and a work station.