Basic Computer Organization Background for CS260

Basic Components CPU Main Memory External Memory I/O devices Bus

CPU Main Memory VideoDisk Drive All devices communicate through the bus with each other and with the CPU. CPU access of the bus takes precedence. Main Bus

CPU – a very simple one! Accumulator Instruction Reg ALU Stack Pointer Flags external bus internal bus

a b MEMORY addresses values

EXAMPLE OPERATION Changing the value of a memory location (decrement by 2) a b 104

a b LOAD value from memory into the CPU’s accumlator 106 accumulator 1.

a b SUBTRACT 2 from accumulator. Goes through the ALU 106 accumulator

a b STORE the result in the AC into the proper location in main memory Accumulator (AC)

What’s the connection? The operation in c would look like –I = I – 2; The operation in assembly language is –LOAD 101 –SUB 105 –STORE 101 More details involved More hardware knowledge required Not as clear to read/write/understand

We want to see the connection! C++ compiler assembly

Why? In order to better understand what we are actually instructing the computer to do when we write in the higher level language.