Chapter 2: Computer-System Structures n Computer System Operation n I/O Structure n Storage Structure n Storage Hierarchy n Hardware Protection n Network Structure
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A Modern Computer System
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Computer-System Operation n I/O devices and the CPU can execute concurrently n Each device controller is in charge of a particular device type n Each device controller has a local buffer n CPU moves data from/to main memory to/from local buffers n I/O is from the device to local buffer of controller n Device controller informs CPU that it has finished its operation by
causing an interrupt
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Common Functions of Interrupts n Interrupt transfers control to the interrupt service routine
generally, through the interrupt vector, which contains the addresses of all the service routines n Interrupt architecture must save the address of the interrupted
instruction n Incoming interrupts are disabled while another interrupt is being
processed to prevent a lost interrupt n A trap is a software-generated interrupt caused either by an error
or a user request n An operating system is interrupt driven
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Hardware Interrupts interrupt
CPU branch General Interrupt Handler Interrupt 1 service routine
...
Device causing interrupt
e.g., I/O devices memory unit, OS Code timer, etc.
Interrupt n service routine User code
Main Memory
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Interrupts n General Interrupt Handler: l save context (contents of all registers including Program Counter) of
running process. l determine type of interrupt and execute specific interrupt service
routine. l select a process to be run next. l restore the context to the saved context of the process selected to
execute next.
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Software Interrupts (traps) n Generated by user programs through system calls. n System calls are requests to the OS to perform specific actions
on behalf of the user. n Examples of system calls include: l I/O requests l process management l memory management
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Software Interrupts (traps) ... r = fwrite (...); ...
fwrite: ---generate trap to fwrite
1 2
OS
System call library
trap handler: 3 fwrite trap service routine
Trap Handler
User Program Operating System Concepts with Java
OS
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Interrupt Handling n The operating system preserves the state of the CPU by storing
registers and the program counter n Determines which type of interrupt has occurred: l polling l vectored interrupt system
n Separate kernel routines determine what action should be taken
for each type of interrupt
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Interrupt Time Line For a Single Process Doing Output
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I/O Structure n Synchronous I/O - After I/O starts, control returns to user
program only upon I/O completion l Wait instruction idles the CPU until the next interrupt l Wait loop (contention for memory access) l At most one I/O request is outstanding at a time, no simultaneous
I/O processing n Asynchronous I/O - After I/O starts, control returns to user
program without waiting for I/O completion l System call – request to the operating system to allow user to wait
for I/O completion l Device-status tablecontains entry for each I/O device indicating its
type, address, and state l Operating system indexes into I/O device table to determine device
status and to modify table entry to include interrupt
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Two I/O Methods Synchronous
Operating System Concepts with Java
Asynchronous
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Device-Status Table
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Direct Memory Access Structure n Used for high-speed I/O devices able to transmit information at
close to memory speeds n Device controller transfers blocks of data from buffer storage
directly to main memory without CPU intervention n Only on interrupt is generated per block, rather than the one
interrupt per byte
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Input/Output 1,4
1,2,3,4
CPU
Disk Controller Memory Controller
3
1. OS programs disk controller with parameters of required I/O operation.
Memory
2. Disk controller starts I/O (without CPU intervention). Block read is stored in the disk controller buffer. 3. Disk controller transfers block to memory via DMA (direct memory access) without CPU intervention. 4. Disk controller interrupts CPU to indicate end of operation.
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Storage Structure n Main memory – only large storage media that the CPU can
access directly n Secondary storage – extension of main memory that provides
large nonvolatile storage capacity n Magnetic disks – rigid metal or glass platters covered with
magnetic recording material l Disk surface is logically divided into tracks, which are subdivided
into sectors l The disk controller determines the logical interaction between the
device and the computer
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Moving-Head Disk Mechanism
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Storage Hierarchy n Storage systems organized in hierarchy l Speed l Cost l Volatility
n Caching – copying information into faster storage system; main
memory can be viewed as a last cache for secondary storage
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Storage-Device Hierarchy
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Caching n Use of high-speed memory to hold recently-accessed data n Requires a cache management policy n Caching introduces another level in storage hierarchy. l This requires data that is simultaneously stored in more than one
level to be consistent
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Migration of Integer “A” From Disk to Register
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Hardware Protection n Dual-Mode Operation n I/O Protection n Memory Protection n CPU Protection
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Dual-Mode Operation n Sharing system resources requires operating system to ensure
that an incorrect program or poorly behaving human cannot cause other programs to execute incorrectly n OS must provide hardware support to differentiate between at
least two modes of operations 1. User mode – execution done on behalf of a user 2. Monitor mode (also kernel mode or system mode) – execution done on behalf of operating system
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Dual-Mode Operation (Cont.) n Mode bit added to computer hardware to indicate the
current mode: monitor (0) or user (1) n When an interrupt or fault occurs hardware switches to
monitor mode Interrupt/fault
monitor
user set user mode
Privileged instructions can be issued only in monitor mode
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I/O Protection n All I/O instructions are privileged instructions n Must ensure that a user program could never gain control of the
computer in monitor mode (I.e., a user program that, as part of its execution, stores a new address in the interrupt vector)
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Use of A System Call to Perform I/O
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Memory Protection n Must provide memory protection at least for the interrupt vector
and the interrupt service routines n In order to have memory protection, at a minimum add two
registers that determine the range of legal addresses a program may access: l Base register – holds the smallest legal physical memory address l Limit register – contains the size of the range
n Memory outside the defined range is protected
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Use of A Base and Limit Register
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Hardware Address Protection
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Hardware Protection n When executing in monitor mode, the operating system has
unrestricted access to both monitor and user’s memory n The load instructions for the base and limit registers are
privileged instructions
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CPU Protection n Timer – interrupts computer after specified period to ensure
operating system maintains control l Timer is decremented every clock tick l When timer reaches the value 0, an interrupt occurs
n Timer commonly used to implement time sharing n Time also used to compute the current time n Load-timer is a privileged instruction
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General-System Architecture n Given the I/O instructions are privileged, how does the user
program perform I/O? n System call – the method used by a process to request action by
the operating system l Usually takes the form of a trap to a specific location in the interrupt
vector l Control passes through the interrupt vector to a service routine in
the OS, and the mode bit is set to monitor mode l The monitor verifies that the parameters are correct and legal,
executes the request, and returns control to the instruction following the system call
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Network Structure n Local Area Networks (LAN) n Wide Area Networks (WAN)
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Local Area Network Structure
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Wide Area Network Structure
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