Microprocessor Logic

Photo courtesy Intel CorporationIntel Pentium 4 processor Rosetta Stone To understand how a microprocessor works, it is helpful to look inside and learn about the logic used to create one. In the process you can also learn about assembly language -- the native language of a microprocessor -- and many of the things that engineers can do to boost the speed of a processor.A microprocessor executes a collection of machine instructions that tell the processor what to do. Based on the instructions, a microprocessor does three basic things:Using its ALU (Arithmetic/Logic Unit), a microprocessor can perform mathematical operations like addition, subtraction,multiplication and division. Modern microprocessors contain complete floating point processors that can perform extremelysophisticated operations on large floating point numbers.A microprocessor can move data from one memory location to another.A microprocessor can make decisions and jump to a new set of instructions based on those decisions.There may be very sophisticated things that a microprocessor does, but those are its three basic activities. The following diagram shows an extremely simple microprocessor capable of doing those three things:This is about Rosetta Stone Software as simple as a microprocessor gets. This microprocessor has:An address bus (that may be 8, 16 or 32 bits wide) that sends an address to memoryA data bus (that may be 8, 16 or 32 bits wide) that can send data to memory or receive data from memoryAn RD (read) and WR (write) line to tell the memory whether it wants to set or get the addressed locationA clock line that lets a clock pulse sequence the processorA reset line that resets the program counter to zero (or whatever) and restarts executionLet's assume that both the address and data buses are 8 bits wide in this example.Here are the components of this simple microprocessor:Registers A, B and C are simply latches made out of flip-flops. (See the section on edge-triggered latches in How Boolean Logic Works for details.)The address latch is just like registers A, B and C.The program counter is a latch with the extra ability to increment by 1 when told to do so, and also to reset to zero when told to do so.The ALU could be as simple as an 8-bit adder (see the section on adders in How Boolean Logic Works for details), or it might be able to add, subtract, multiply and divide 8-bit values. Let's assume the latter here.The test register is a special latch that can hold values from comparisons performed in the ALU. An ALU can normally compare two numbers and determine if they are equal, if one is greater than the other, etc. The test register can also normally hold a carry bit from the last stage of the adder. It stores these values in flip-flops and then the instruction decoder can use the values to make decisions.There are six boxes marked 3-State in the diagram. These are tri-state buffers. A tri-state buffer can pass a 1, a 0 or it can essentially disconnect its output (imagine a switch that totally disconnects the output line from the wire that the output is heading toward). A tri-state buffer allows multiple outputs to connect to a wire, but only one of them to actually drive a 1 or a 0 onto the line.The instruction register and instruction decoder are responsible for controlling all of the other components.Helpful ArticlesHow Bytes and Bits WorkHow Boolean Logic WorksHow Electronic Gates WorkAlthough they are not shown in this diagram, there would be control lines from the instruction decoder that would:Tell the A register to latch the value currently on the data busTell the B register to latch the value currently on the data busTell the C register to latch the value currently output by the ALUTell the program counter register to latch the value currently on the data busTell the address register to latch the value currently on the data busTell the instruction register to latch the value currently on the data busTell the program counter to incrementTell the program counter to reset to zeroActivate any of the six tri-state buffers (six separate lines)Tell the ALU what operation to performTell the test register to latch the Chinese Learning Software ALU's test bitsActivate the RD lineActivate the WR lineComing into the instruction decoder are the bits from the test register and the clock line, as well as the bits from the instruction register.