DIGITAL COMPUTERS — McCORMICK 287 



Yet the set of operations resulting from the above instructions would 

 go on indefinitely unless the procedure is modified. The modification 

 would start with clianging the instruction in 004 to "Put the number 

 in drawer 022 on the pad." (The number in 022 will be when the 

 problem starts.) Next in 005 the instruction would be, "Add the num- 

 ber on the pad to the number in 023 and leave only the answer on the 

 pad." Since the number in 023 is 1, the sum will be 1. In OOC the 

 instruction is, "Store the number on the pad in location 022, leaving 

 the pad blank." Thus a 1 is stored in 022 in place of the which 

 was there. The number in 022 thus indicates the number of interest 

 calculations that have been made. 



To use tliis to determine when to stop, the mstruction in 007 is, 

 "Take the number in 022 and write it on the pad." Then in 008, 

 "Subtract from the number on the pad the number in drawer 024, 

 leaving only tlie result on the pad." Since the number in 024 is 20, 

 the first time this instruction is obeyed the result will be — 19. Now 

 assume that in 009 there is a decision instruction of this nature: "If 

 the number on the pad is or positive, go to the next instruction 

 in order; if the number is negative, erase it and go to drawer 000." 

 Thus the clerk would in this case go back to 000 and repeat the entire 

 process. However, the next time he came to 022 he w^ould find a 1 in 

 it which would be clianged to a 2, leaving —18 after executing the 

 instruction in 008 the second time. Thus in response to instruction 

 in 009 the process would repeat again. The third time the result 

 would be —17, and so on. However, after the value of principal plus 

 accumulated interest has been computed for the 20th time, the result 

 of executing the instruction in 008 will be a on the pad. Now 

 when tlie instruction in 009 is encountered, the result will be that tlie 

 clerk will go for the first time to 010 for his next instruction. Thus 

 after 20 iterations the program of activity indicated by the instnic- 

 tions in the drawers results in a "branch" to an alternate course of 

 action. Tlie instruction in 010 can be simply "Stop" or it could be 

 the first instruction of a sequence which will solve some other problem. 



A digital computer generally solves problems in just this manner. 

 The "storage" of a computer takes the place of the group of file 

 cabinets. Each drawer is an individual storage location containing 

 a "word" w^hich is a sequence of numbers which may be data or an 

 instiniction. Each storage location is identified by an "address" 

 much as houses are identified by different addresses. Instead of the 

 pad of paper, a computer has an electronic storage device called an 

 "accumulator." (This corresponds to tlie row of dials on the top of 

 many manually operated calculators.) The equipment for storage 

 and for performing the duties of the clerk are electronic and operate 

 automatically at high rates of speed. 



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