CH 6 - Microprocessor

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University of Toronto St. George
Computer Science
Steve Engels

PROCESSOR COMPONENTSMicroprocessorsThe Arithematic side of ALU o S0 far weve been talking about making devices such as adders o Fundamentally this side is made of an addsubtractor unit counters and registers o The ultimate goal is to make a microprocessorA digital device that processes input can store values and produces output according to a set of onboard instructionsProcessoro ALU block diagramIn addition to data inputs and outputs this circuit also hasOutputs indicating the different conditionsInputs specifying the operation to perform similar to subo Deconstructing processorconsider them piece by piece o Beyond addition and subtraction many more operations can beperformed by manipulating what is addeded to input A o MicroprocessorsThese devices are a combination of the units that weve discussed so far Registers to store valuesAdders and shifters to process dataFinite state machines to control the processArithematic Logic Unit ALUthe Arithmetic Thing o The first microprocessor applications were calculatorsRecall the unit on adders and SubtractorsThese are part of a larger structure call the arithmetic logic unitALU o Arithemtic operations if the input logic circuit on the left side o This larger stricture is responsible for the processing of all data valuesSends B straight through to the adder in a basic CPUResult of addition operation GAB o The ALU performs all of the arithematic operations covered in thisReplace B with all ones course so far and logical operations as well AND OR NOT etcResult of addition operation GA1Input A and Bdata inputs for the operationsReplace B with BThe select bits indicate which operations is being performedResult of addition operation GAB1S2 is a mode select bit indicating whether the ALU is inReplace B with all zeros arithmetic or logic modeResult of addition operation GAThe carry bit C is used in operations such as incrementing an in Instead of sub singal the operation you want is signaled using input value or the overall result the select bits SSOutput signals V C NZ indicate special conditions in the 01o Full operation selection arithmetic results Select Input OperationV overflow condition the result of the operation could S S Y C 0 C 1 not be stored in the n bits of G meaning that the result is 10in in GA transfer GA1 increment 0 0 All 0s incorrect GAB1 GAB add 0 1 BC carryout bit 1 0 B GA1 GABAB1NAB subtract GA1 negative GA transfer 1 1 All 1s indicator decrement Z Zero Based on the values on the select bits and the carry bit we can condition perform any number of basic arithmetic operations by indicator manipulating what value is added to A
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