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## An Adder

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**An Adder**A Subtractor**Arithmetic Logic Unit**A and B are the inputs of the adder/ subtractor R is the output of the adder/ subtractor F is the control to tell it to add or subtract D is the status to tell us when it is done (or maybe something else?)**A Flip Flop**A 4 bit register**Decoder**Add Subtract AND OR To ALU Add Subtract AND OR**Von Neumann Architecture**Harvard Architecture**Atmel Registers**Note: R0 to R15 will not work with some instructions**Status Register**I – Global Interrupt enable flag, must be set for interrupts to work T – Bit copy storage, used with BST and BLD H – Half carry flag, detects carry from lower nibble S – Sign flag, exclusive OR between N flag and V flag V – Two’s complement overflow flag N – Negative flag, result of an operation is negative Z – zero flag, result of an operation is zero C – Carry flag, indicates an overflow after count reaches 255