The Central Processing Unit

• The central processing unit (CPU) is a highly complex, extensive set of electronic circuitry.
• The CPU consists of two parts: the control unit and the arithmetic/logic unit.

• The control unit directs the entire computer system to carry out stored program instructions.
• The control unit communicate with both the arithmetic/logic unit and memory.

• The arithmetic/logic unit (ALU) executes arithmetic operations, including addition, subtraction, multiplication, and division.  It executes logical operations by comparing numbers, letters, and special characters.
• Logical operations test for three conditions:

equal to conditions in which two values are the same;
less than conditions in which one value is smaller than another;
greater than conditions in which one value is larger than another.
• Relational operators (=, <, >) are used to describe the comparison operations used by the ALU.

• Registers are temporary storage areas used by the CPU for instructions or data.
• There are a number of different registers with different jobs, including:

accumulator is a register that collects the results of computations;
address register keeps track of where a given instruction or piece of data is stored in memory;
storage register temporarily holds data taken from or about to be sent to memory;
general-purpose register is used for several functions, such as arithmetic operations.

• Memory is the part of the computer that holds data and instructions for processing.  Although closely associated with the CPU, it is separate from it. Memory allows the CPU very fast access to instructions and data.
• Memory associated with the CPU is also called primary storage, primary memory, main storage, internal storage, and main memory.  This type of memory stores items only while the computer is turned on.

• Many types of personal computers can execute instructions in less than one millionth of a second; super computers can execute instructions in less than one trillionth of a second.
• The CPU performs four steps in executing an instruction:
1. The control unit gets the instruction from memory.
2. The control unit decides what the instruction means and directs the necessary data to be moved from memory to the arithmetic/logic unit.
3. The arithmetic/logic unit performs the actual operation on the data.
4. The result of the operation is stored in memory or a register.

The first two instructions make up what is called the instruction time, or I-time.
The last two instructions make up what is called the execution time, or E-time.
• The combination of the I-time and E-time is called a machine cycle.
• Each central processing unit has an internal clock, which produces pulses at a fixed rate to synchronize all computer operations.  A single machine cycle instruction is made up of a number of sub instructions, each of which must take at least one clock cycle.
• Each type of central processing unit is designed to understand a specific group of instructions called the instruction set.  Therefore one CPU - say, for an IBM PS/2 - cannot understand the instruction set from another CPU - say, for a Macintosh.

See figure 2 on page 86 and figure 3 on page 88.

• The location in memory for each instruction and each piece of data is identified by an address, or a number that stands for a location in computer memory.
• An address may be compared to a mailbox in everyday life, except that the address can hold only one item - a fixed amount of data, a number, or a word - at any one time.
• Programmers using programming languages do not have to worry about the actual address number, since each address is a symbolic address, using a name to identify its location.

See figure 4 on page 89

• A computer basically knows only two things: on and off.  This on/off, yes/no two state system is called a binary system.
• The binary system has a base of 2.  This means it contains only two digits, 0 and 1, which correspond to the two states off and on.

• Each 0 or 1 in the binary system is called a bit (for binary digit).
• The bit is the basic unit for storing data in computer memory (0 means off, 1 means on).  Since a bit is always either on or off, the computer is always storing some kind of data.
• Single bits cannot store all the numbers, letters, and special characters (such as $ and ?) that must be processed by a computer.  The bits are put together in a group called a byte (pronounced "bite").
• A byte usually holds 8 bits and represents one entity, like a character of data such as a letter, digit, or special character.
• Bytes are usually expressed by the thousands, millions, and billions: kilobytes (KB or K), megabytes (MB), and gigabytes (GB), respectively.  A kilobyte is 1,024 bytes (2¹⁰).  A megabyte is 1,024,000 bytes (2²⁰) or 1,024 KB.  A gigabyte can be 1,024,000,000 bytes (2³⁰), 1,024,000 KB, or 1,024 MB.
• Personal computer memory may have 64 MB and more.
• A computer word is defined as the number of bits that constitute a common unit of data, as defined by the computer system.  The size is from 8 bits to 32 or 64 bits.  The larger the word, the more powerful the computer.
• The parts of a computer are connected by collections of wires called buses or bus lines.  Each has a certain number of data paths along which bits can travel; the number of data paths is usually related to the number of bits in the word size.

• See figure 5 on page 89

• We used coding schemes to define which particular set of bits is equivalent to which character of data.
• ASCII (pronounced "AS-Key") stands for American Standard Code for Information Interchange.  ASCII uses 7 bits for each character.
• ASCII-8, also called extended ASCII, which uses 8 bits per character and can represent 256 different characters.

See figure 6 on page 90.

• Basically there are two kinds of chips: one kind holds the central processing unit, and one kind holds memory.  A chip that holds the central processing unit, or processor, is called a microprocessor.  It may be called a logic chip when it is used control specialized devices (such as a watch or fuel system in a car).

• Microprocessors usually contain four key components: the control unit and arithmetic/logic unit, registers, buses, and a clock. (On personal computers, the clock is usually on a separate chip.)
• Microprocessors contain tiny transistors, electronic switches that may or may not allow current to pass through.  If current passes through, the switch is on, representing the 1 bit.  Otherwise, it is off, and 0 bit.

• Historically, memory components evolved from vacuum tubes to magnetic cores to semiconductors.
• Most modern computers use semiconductor storage because it has several advantages:
• reliability
• compactness
• speed
• low cost
• low power usage
• Semiconductor storage has one major disadvantage: It is volatile, meaning it requires continuous electrical current to work.  If the current is interrupted, the data is lost.
• There are two basic types of memory chips: RAM and ROM
• RAM stands for random-access memory.  It is usually volatile.
• SRAM (pronounced "s-ram") - static RAM.  It will retain its contents without intervention from the CPU.  It is faster than DRAM.
• DRAM (pronounced "d-ram") - dynamic RAM.  It is used in most PC memory because of its size and cost advantages.
• ROM stands for read-only memory.  This one is still a random-access storage.  However, data and programs on ROM are printed at factory and cannot be changed.  It is nonvolatile. (i.e. When power is off, the information is still on ROM)
• ROM burners can change the data and instructions in PROM (programmable read-only memory chips.

• Microprocessor speed
• The execution of an instruction on a very slow computer may take less than a millisecond, one thousandth of a second.
• Most computers can execute an instruction in one millionth of a second, a microsecond.
• Some modern computers have reached the nanosecond range- one billionth of a second.
• Still to be broken is the picosecond barrier - one trillionth of a second.
• The speed of a computer system can be measured in many ways.  Measuring speed on personal computers is usually done in clock speed, expressed in megahertz, representing one million ticks per second.
• Another measure of computer speed is MIPS, which stands for one million instructions per second.
• Bus line size
• the available of cache
• cache is pronounced "cash".
• It is a selective memory.  CPU gets data from it will be faster than from main memory.
• There are two kinds: internal cache (8KB) and external cache (512KB).

Read page 96 for cache.
• flash memory
• nonvolatile
• Flash chips are currently being used in cellular phones and cockpit flight recorders, and they are replacing disks in some handheld computers.
• RISC computers
• Reduced instruction set computers
• Parallel processing
• It is using many processors, each with its own memory units, all working at the same time.