Ch.11 The System Unit of a Computer System
The Components of a computer
system unit
power supply
storage
motherboard
main circuit board
sockets
Central Processing Unit (CPU)
memory chips
expansion slots
peripherals
hardware device (connect to the system unit)
input
output
storage
communication
communicates with other devices through bus lines
processing power determines the oeverall performance of a computer
input -> processing -> output
Arithmetic and Logic Unit (ALU)
Perform arithmetic and logical operations
Use registers to hold data during calculations
Results are usually stored in a register called an accumulator
Control Unit (CU)
Keep track of the sequence of instructions being processed
Monitors and coordinates all I/O operations and system units
Registers
Memory units inside a CPU
Provide storage space for ALU and CU.
general purpose registers
Load data from main
memory into a register
Store data temporarily in arithmetic operations
Store data from a register into main memory
Example
Accumulator(AX)
Base register (BX)
Counter (CX)
Data register (DX)
control registers
Provide temporary memory for CU to control the operation of instructions.
Example
Instruction Register (IR)
Contains the instructions to be executed by the CPUs
Program Counter (PC)
Program Counter (PC)
Contains the memory address of the next instruction to be executed
Memory Address Register (MAR) and Memory Data
Register (MDR)
acilitate the communication of the CPU with the main memory through the system bus
MAR holds the address of the memory location.
MDR contains the data to be written into or read from the location specified in MAR.
Status registers
Contain a number of flag bits indicating the status after the execution of instructions
Used in instructions of conditional testing and program branching
Example
Zero flag (Z)
Sign flag (S)
Parity flag (P)
Carry flag (C)
Overflow flag (O)
System Bus (Bus line)
Data bus (Transfer data and instructions)
Address bus (Transfer the source address or the destination address of data)
Control bus (Indicate the direction of the data transfer and coordinate the timing of the event during transfer)
size of bus = bus width
bus width determines the number of bits of data the computer can transmit at one time
Machine Cycle
Process of executing an instruction in a CPU
sub-cycle
fetch
decode
Identify the operation code and operands in an instruction.
execute
Interpret the operation code and perform the required operation.
Read the next instruction from the main memory into instruction register (IR)
End of execution of an instruction
CPU saves the
current process status or handles the interrupt immediately
CPU resumes the operation of the next instruction of the active process
Measurement of CPU Speed
Clock rate
Measure speed of CPU
Measurement unit
megahertz (MHz)
gigahertz (GHz)
l 1 hertz = 1 clock cycle per second
Word length
Number of bit of data and instruction can handle at one time
more data can be processed in one time
more instructions can be included in the instruction set
more complicated instructions can be included in the instruction set
Main Memory
Accessible to the CPU of a computer without using other I/O channels.
The memory chips are installed on the motherboard
Used to store data or instruction that CPU will execute
Random access memory (RAM)
Read-only memory (ROM)
Temporarily hold the data and instructions of application software and the operation system
Volatile (all data stored in RAM disappears instantly after the computer is turned off)
Access time is just hundredth of hard disk
Measurement units: bytes (B), kilobytes (KB), megabytes (MB), gigabytes (GB), terabytes (TB)
dynamic random access memory（DRAM）
static random access memory（SRAM）
Computer main memory, game console, etc
CPU cache, hard disk buffer, printer buffer, PDA
Store the permanent information supplied by the manufacturer
Store basic input/output system (BIOS)
Non-volatile
Erasable programmable ROM (EPROM)
Electrically erasable programmable ROM (EEPROM)
cache memory
high-speed memory
store data and instructions
Integrated into a CPU or resided close to a CPU
Latest Development of CPU and Main Memory
The future development of the CPU in the future has focused on the integration of more cores into the CPU