The Heart of the Hardware

  C P U   a n d   M e m o r y  

CPT 105 · Franklin College · Erich Prisner · 2002-2007


CPU: Central Processing Unit

Consists essentially of two parts:

plus registers, ...

Memory is not part of the CPU, but closely connected to it. Both CPU and memory are usually integrated circuits (chips).

Control unit

Controls everything. Moves data from and to and inside of memory or registers.

ALU (arithmetic/logic unit)

width: How many operations can be performed in parallel. This is also called a "word".


temporary storage for data (or instructions)
special-purpose or general-purpose


(=primary storage, primary memory, main storage, internal memory, main memory, RAM)
volatile, limited, very quick access.
semiconductor memory
1: a given circuit conducts current,
0: not.

Data is send from secondary storage device to memory by control unit (and also back).
SRAM or DRAM (static or dynamic), DRAM must be refreshed constantly and is therefore slower, but it is also cheaper and mostly used. see here Dell's explanation

CPU and Memory ard connected (on the motherboard) by the

System Bus

parallel wires between CPU and memory.
bus width indicates the number of those wires.
bus speed in MHz

Executing Instructions

  1. Put instructions and data from external sources (input devices or secondary storage) into memory.
  2. For each instruction the CPU does the following:
    1. control unit puts instruction from memory into register
    2. control unit decodes instruction and finds memory location of data required
    3. control unit moves data from memory to registers, ALU does the step required by instruction
    4. control unit stores result in memory or register.
    steps 1 and 2 are called I-time (instruction time). steps 3 and 4 E-time (execution time). Together the form the machine cycle.
  3. Control unit puts memory content to secondary storage device, or output device, as required.

Example of such a "program"

Data set: 10, 50

How data is stored

storage units have "address" and "content". The "symbolic address" used in programs also refers to some address, like in "the house where Prisner has his office"---it is usually a "name".

the smallest units ("rooms") are again binary, but we have "floors" ("bytes") and buildings ("words") where we can store larger things.


clock speed
MIPS one million instructions per second, more accurate, since task related
megaflop: one million floating point operations per second (for mathematics/science)

increase speed by use of caching (reserve part of the memory for recently/most often used data)
increase speed by using flash memory (nonvolatile)
increase speed by using RISC (reduced instruction set computing)
increase speed by parallel computing --- use several processors, say 64 at the same time. A lot of difficult problems arise!