1. Chapter 2: Computer-System Structures
The traditional/general computer-system architecture consists of one or more CPUs and a number of device controllers connected via a common bus that provides access to shared memory
The CPU(s) and device controllers execute concurrently competing for memory cycles (access synchronized by the memory controller)
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2. Computer-System Operation
I/O devices and the CPU can execute concurrently
Each device controller has a local buffer
I/O is from the device to local buffer of controller
OS provides mechanism for issuing commands to device controllers
Device driver - portion of OS which converts OS requests into controller register loads/reads
OS provides main memory “buffer” for I/O
CPU issues commands by setting device control registers
CPU moves data from/to main memory to/from local buffers
w/o DMA, CPU must transfer each word (bus size)
w/DMA, CPU is invoked once per transaction (block)
Device controller informs CPU that it requires attention by causing an interrupt
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3. Common Functions of Interrupts
When powered-up, a computer needs an initial program to run
Bootstrap program - initializes the machine
Loads the OS
The OS kernel starts the first process, after which it only processes events.
Hardware Interrupt - generally from I/O devices
Trap (Exception) - software-generated interrupt caused either by an error or a user request
An operating system is event (or interrupt) driven
An interrupt is a request to transfers control of the CPU from the current process to the OS via the interrupt service routine
Some devices do not support the interrupt structure and simply set flags in one of their local registers to request service
The OS must periodically poll such devices
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4. CEG 433/633 - Operating Systems I
Interrupt Handling
An interrupt is handled by:
A hardware check against the interrupt mask to see if the interrupt is enabled
Incoming interrupts of lower priority are disabled to prevent a lost interrupt
Storing the current state of the process
The operating system preserves the state of the CPU by storing registers and the program counter
Calling the interrupt handling routine which determines the type of interrupt and determines the OS response
Re-enable interrupts and restore normal operation
Separate segments of code determine what action should be taken for each type of interrupt
The OS response is dependent upon the interrupt vector, which contains the addresses of all the service routines
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5. Interrupt Time Line For a Single Process Doing Output
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6. CEG 433/633 - Operating Systems I
I/O Structure
Synchronous I/O Operation
After I/O request, OS returns control to user program only upon I/O completion
CPU is idle (wait loop) in OS until the next interrupt.
The next interrupt will be from the specified device since at most one I/O request is outstanding (No vectoring)
Simple, but not efficient
No overlapping of computation or additional I/O with I/O
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7. CEG 433/633 - Operating Systems I
I/O Structure
Asynchronous I/O Operation
After I/O request, control returns without waiting for I/O to complete
Blocking I/O library function: includes a system call to to allow the user to wait for I/O completion
CPU idles or control is passed to another process
Non-Blocking I/O: The user program continues
Requires more complex user code to handle the completion of the I/O event
Pro: The OS may have multiple outstanding I/O requests
One process may make several concurrent requests
Multiple processes may make requests on multi-programmed systems
Con: The OS must maintain a device-status table to record current and requested transactions for each I/O device
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8. CEG 433/633 - Operating Systems I
Storage Structure
Main memory – only large storage media that the CPU can access directly
Secondary storage – extension of main memory that provides large nonvolatile storage capacity (requires I/O operation)
Storage systems organized in hierarchy
Speed, cost, volatility
Memory Hierarchy
Speed Vs. Cost
. . . registers, cache, main memory, disk, CD, tape . . .
Caching – copying information into faster storage system; main memory can be viewed as a last cache for secondary storage
Cache management is generally handled below the OS level by dedicated hardware
requires an policy for replacement, coherency, etc.
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9. Moving-Head Disk Mechanism
Magnetic disks – rigid metal or glass platters covered with magnetic recording material
Disk surface is logically divided into tracks, which are subdivided into sectors
The disk controller determines the logical interaction between the device and the computer
Transfer rate - drive to memory (MB/s)
Access time - seek time (arm movement) + rotational delay (platter movement) (ms)
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10. CEG 433/633 - Operating Systems I
Hardware Protection
How do we keep erroneous/malicious processes from interfering with other processes on a timesharing system?
Dual-Mode Operation
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 supervisor mode or system mode) – execution done on behalf of operating system
When an interrupt or fault occurs hardware switches to monitor mode
monitor
user
Interrupt/fault
set user mode
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11. CEG 433/633 - Operating Systems I
Dual-Mode Operation
Mode bit added to computer hardware to indicate the current mode: monitor (0) or user (1)
The OS is called (via trap) to handle hardware-generated exceptions (ex: seg. fault) in monitor mode
Privileged instructions can be issued only in monitor mode
All I/O instructions are privileged instructions (I/O Protection)
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)
Load-timer is a privileged instruction (CPU Protection)
Timer – interrupts computer after specified period to ensure operating system maintains control
When timer reaches the value 0, an interrupt occurs
Timer commonly used to implement time sharing
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12. CEG 433/633 - Operating Systems I
Memory Protection
Problem: We must protect the OS and other user’s routines from a deranged or malicious process without compromising the speed of memory access
Must provide memory protection at least for the interrupt vector and the interrupt service routines
In order to have memory protection, add two registers that determine the range of legal addresses a program may access:
base register – holds the smallest legal physical memory address.
Limit register – contains the size of the range
Memory outside the defined range is protected.
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13. A Base And A limit Register Define A Logical Address Space
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14. CEG 433/633 - Operating Systems I
Protection Hardware
When executing in monitor mode, the operating system has unrestricted access to both monitor and user’s memory.
The load instructions for the base and limit registers are privileged instructions.
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15. General-System Architecture
Time-sharing led directly to hardware modifications to allow the OS to maintain control of the system
Problem: How do users programs access necessary privileged instructions (such as I/O)?
The user must ask the OS to perform the operation on the user’s behalf
System call – the method used by a process to request action by the operating system
Usually takes the form of a trap to a specific location in the interrupt vector
Control passes through the interrupt vector to a service routine in the OS, and the mode bit is set to monitor mode
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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16. Use of A System Call to Perform I/O
System calls are complex
Information must be passed via registers or ordered in the stack
Library functions exist which act as wrappers for the trap
These library functions are the system functions provided in each language’s system library
Example: Memory Management
The OS only provides sbrk() which increases or decreases the address space of the process by some number of bytes
C provides the language specific malloc() and free() routines to manage the heap
Consider java
CEG 433/633 - Operating Systems I

17. System Calls
An application written to a standard set of supported system calls (such as POSIX) is dependent upon the OS, not the machine!
User Interface
System Programs
Programmer Interface
Application Code
C function library
System Interface
System Calls
Kernel
Hardware Abstraction Layer
Hardware
CEG 433/633 - Operating Systems I