Computer Design 
Datapaths 
The term datapath refers to a series of devices that perform calculations. A control distributes program instructions to the datapath, memory, and I/O devices. We discussed control units previously in Chapters 1 and 3. Standard datapaths consist of a PC (program counter) and various small registers. Arithmetic logic units (ALUs) and simple adders perform （ 1 ） arithmetic tasks. In some cases, there are multiple adders and ALUs performing calculations simultaneously. However, it is impractical to wire every possible I/O （ 2 ）. Many datapaths solve this problem with a multiplexer, or data （ 3 ）. Multiplexers transfer data from the correct input source to its destination. 
The address of the current instruction is stored in an instruction register. Be careful not to confuse the （ 4 ） register with the PC. The PC stores the address of the next planned instruction,like a bookmark. 
In order to understand implementation,we must understand the three instruction classes: 
memory-reference instructions read from memory or write data to （ 5 ） 
arithmetic-logical instructions perform calculations 
branch instructions provide the PC with a new instruction address 
Note that all three instruction classes make use of ALUs! Don't let the term 'arithmetic-logical' fool you.