General purpose calculator having selective data storage, data conversion and time-keeping capabilities
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation of application Ser. No. 364,570 filed May 29, 1973, now abandoned. Allowed U.S. application Ser. No. 566,144, now U.S. Pat. No. 4,001,569 and also a division of U.S. Patent Application Ser. No. 364,570, is hereby incorporated by reference as amended as if fully set forth herein. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     Calculators using delay line memories and serial data processing are well known in the art, as shown by U.S. Pat. No. 3,328,763 granted to H. M. Rathburn et al. While these calculators can perform some complex calculations involving more than one mathematical function or operation, several, even redundant, key strokes are usually required. For example, to multiply sine X times cosine X, the argument X must be entered into prior art calculators twice. A calculator constructed according to the present invention reduces the number of key strokes required in many calculations by including a capability to readily recall the last number (Last X) utilized in an immediately preceding calculation or the result of such a calculation if that result would be destroyed by the next calculation to be performed by the calculator. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of a scientific calculator according to the preferrred embodiment of the invention. 
     FIG. 2 is a block diagram of the calculator of FIG. 1. 
     FIG. 32 is a flow diagram of the last X function of the calculator of FIGS. 1 and 2. 
     FIGS. 3-31 inclusive and FIGS. 33-37 inclusive may be found in the above incorporated allowed U.S. patent application. 
     Refer to allowed U.S. Patent Application Ser. No. 566,144. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIGS. 1, 2, and 32, a calculator according to the present invention includes a Last-X function which automatically stores the currently displayed number (for example, the last number entered from keyboard input unit 12 or the result of the last function performed by the calculator) in a Last-X shift register of auxiliary data storage circuit 25 whenever the next function to be performed by the calculator would destroy the currently displayed number. That stored number may then be immediately recalled and displayed by simply pressing a prefix key 110 and a Last-X key 112 of the keyboard input unit. 
     In order to perform this Last-X function, a Last-X subroutine is stored in ROM&#39;s 3 and 6, as shown in the detailed listing given below for those ROM&#39;s. In ROM 3 this subroutine begins on line 4 and ends on line 91, and in ROM 6 begins on line 99 and then jumps to line 165 and continues through line 187 where the subroutine terminates and the recalled number is then displayed. ROM&#39;s 3 and 6 are connected by IS instruction line 28 to auxiliary data storage circuit 25 where the Last-X function to be saved is stored. The latter circuit is connected to Start line 40 to synchronize operation of this circuit with the rest of the calculator and is also connected to arithmetic and register circuit 20 by BCD line 35. Referring to FIG. 32, the Last-X subroutine determines whether the currently displayed number would be destroyed by the next function to be performed by the calculator. The number is automatically stored in the Last-X register of auxiliary data storage circuit 25 if the key code of the next key actuated (i.e., the state of the system counter as described above with reference to FIG. 5) corresponds to any key except SCI/FIX, x y, R↓, STO, RCL, ENTER, CHS, EEX, CLX and any numeric key or alternate function associated a numeric key. 
     The Last-X function is useful for correcting errors, such as pressing the wrong arithmetic operator key or entering the wrong number. For example, suppose one were performing a long calculation where the number 3 must be subtracted from the number 12, but instead of pressing the subtraction key, the division key is pressed and as a result the number 4 appears in the display. This error can be corrected by pressing the prefix key 110 and the Last-X key 112 to recall the number 3; by pressing the multiplication key to reverse the division and cause the number 12 to appear in the display; by again pressing the prefix key 110 and the Last-X key 112, to again recall the number 3 to the display; and, finally, by pressing the subtraction key to accomplish the operation originally intended. The convenience afforded by the Last-X function will be appreciated even more if instead of a single number like 3, the number destroyed by pressing the wrong function key were, for instance, 3.56789. The Last-X function is also useful for calculations involving the same number more than once. For example, sine N is multiplied by cosine N, where N equals 3.15672, by consecutively entering N, pressing the sine key, pressing the prefix key 110 and the Last-X key 112, pressing the cosine key, and pressing the multiplication key to obtain the final result. While the value of N was used twice in this calculation, it was entered into the calculator only once.