32 



Information Storage and Neural Control 



the even numbered positions. We shall write the word in the form 



101010 . . . 101010, 



where the meaning of the three dots is obvious, and their use 

 enables us to avoid writing" all 48 bits. 



A left shift of n bits causes the individual digits to be shifted 

 to the left n places in an "end-around" fashion, which means 

 that bits shifted off the left end are carried around and introduced 

 into the right end of the word. Thus, if n is an even integer, the 

 pattern displayed above will not appear to have changed following 

 the shift. On the other hand, if n is odd, the pattern will have the 

 appearance 



010101 . . . 010101 

 following the shift. 



This shifting operation can be quite useful. To illustrate this 

 we need to mention that the machine is capable of performing 

 branchmg operations, i.e., it can be made to do one thing if the first 

 bit of a word is a "one" and another thing if the first bit is a 

 "zero." This means that the machine is capable of performing 

 each of two sequences of operations depending on the nature of 

 the first bit of a word. Figure 1 will aid us in this discussion. 



Fig. 1 



If lines represent sequences of operations then we traverse the 

 path AB if the first bit of our word is a "one" and AC if the first 

 bit is a "zero." If we assume that we shall begin at point A many 

 times and if we desire to traverse the paths AB and AC on alter- 

 nate occasions, then we can use the word 



101010 



101010 



and the left shifting operation to do this. All we need to do upon 

 arrival at either of the points B or C is to command the machine 



