Chapter 4

1. Chapter 4 The Central Processing Unit: What Goes on Inside the Computer

2. CPU • CPU - central processing unit • electronic circuitry designed to execute instructions • composed of control unit and ALU • all computers must have at least one • control unit - coordinates and controls all parts of the computer system • manages the machine cycle • ALU (arithmetic logic unit) - performs required arithmetic and logic operations • registers - special-purpose, high-speed, temporary storage associated with CPU

3. Terms associated with the CPU • system unit - case that houses the electronic components of the computer • motherboard - circuit board that typically contains the CPU, support chips, RAM, and expansion slots • microprocessor - an integrated circuit that contains the control unit and ALU of a CPU

4. Terms associated with the CPU • machine cycle - steps performed by the CPU to execute an instruction • fetch, decode, execute, store • system clock - internal clock that pulses at a fixed rate to synchronize computer operations • instruction set - specific group of instructions a CPU is designed to understand • bus - set of electrical paths that transport signals from one part of the system to another • bus width - number of bits that can be carried at one time

5. Processor Speeds • microprocessor speeds - usually determined by clock speed • measured in megahertz (MHz) and gigahertz (GHz) • may also be expressed in MIPS or megaflops • cache - very high speed memory used to store instructions that have been recently or frequently used

6. Strategies to Improve Speed • RISC - reduced instruction set approach to processor design • parallel processing - multiple processors working simultaneously • capable of processing speeds in teraflops • strategy used for designing and building supercomputers

7. Data Representation • Everything stored in the computer is represented using the binary system (base 2) - sequences of 0’s and 1’s • bit - an single binary digit (0 or 1) • byte - a group of 8 bits • needed to store a character • word - number of bits processed by the CPU as a unit

8. Larger measures of storage • Kilobyte (KB) - about 1,000 bytes • megabyte (MB) - about 1 million bytes • gigabyte (GB) - about 1 billion bytes • terabyte (TB) - about 1 trillion bytes

9. Collating Codes • collating code (coding scheme) - a common scheme for assigning a specific series of bits to a specific character • ASCII - most widely used code • used by most PC’s • EBCDIC - used mostly on IBM mainframes • Unicode - uses 2 bytes/character • can handle alphabets used throughout the world

10. Memory • Memory - part of the computer that holds program instructions and data currently being used • also referred to as - • primary storage, primary memory, main memory, internal memory, internal storage • closely associated with the CPU • composed of RAM and ROM

11. RAM - random access memory • used to store instructions and data for currently active programs • can be accessed very quickly by the CPU • usually volatile - power must be on to retain contents • can be erased and written over • 128MB common for current PC’s

12. ROM - read only memory • contains programs and data that are permanently stored • can be accessed, but cannot be changed • used to store boot routine (activated when computer is turned on) • nonvolatile - does not lose its contents when the power is turned off