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@@ -155,3 +155,207 @@ Events are almost always signaled by the occurrence of an interrupt or a trap.
 A trap, or an exception, is a software generated interrupt caused either by
 an error or by a specific request from a user program that an operating-system
 service be performed.
+
+Terms:
+
+* CPU scheduling
+* job scheduling
+* logical memory
+* physical memory
+* swapping
+* virtual memory
+
+
+For each type of interrupt, separate segments of code in the operating system
+determine what action should be taken. An interrupt service routing
+is provided that is responsible for dealing with the interrupt.
+
+## Dual Mode Operation
+
+Must distinguish between operating system code and user-defined code.
+
+* user mode
+* kernel mode (supervisor mode, system mode, priviliged mode)
+
+The `mode bit` is added to hardware to indicate the current mode: kernel (0), user (1)
+The hardware allows privileged instructions to be executed only in kernel mode.
+
+System calls provide the means for a user program to ask the operating system to perform
+tasks reserved for hte operating system on the user program's behalf.
+
+A system call usually takes the form of a trap to a specific location in the interrupt vector.
+This trap can be executed by a generic trap instruction, although some systems have a specific
+syscall instruction.
+
+### Timer
+
+We must ensure that the operating system maintains control over the CPU.
+We cannot allow a user program to get stuck in an infinite loop or to fail
+to call system services and never return control to the operating system.
+
+To accomplish this, we can use a `timer`. A timer can be set to interrupt
+the computer after a specified period.
+
+## Process Management
+
+A programs instructions are executed by a CPU.
+A program in execuation is a `process`.
+A time-shared user program is a `process`.
+
+A process needs:
+
+* CPU time
+* memory
+* files
+* I/O devices
+
+A single-threaded process has one `program counter`
+specifying the next instruction to execute.
+The execution of such a process must be sequential.
+
+Although more than one process may be associated with
+the same program, they are considered two separate execution
+sequences.
+
+The operating system is responsible for:
+
+* scheduling processes and threads on the CPU
+* creating and deleting both user and system processes
+* suspending and resuming processes
+* providing mechanisms for process synchronization
+* providing mechanisms for process communication
+
+## Memory Management
+
+Main memory is a large array of words or bytes, ranging
+in size from hundreds of thousands to billions.
+
+Each word or byte has it's own address.
+Main memory is a pool of quickly accessible data shared by
+the CPU and I/O devices.
+
+On a von Neuman architecture, the central processor reads
+instructions from main memory during the instruction-fetch
+cycle and both reads and writes data from main memory during the
+data-fetch cycle.
+
+The main memory is generally the only large storage device
+that the CPU is able to address and access directly.
+
+For example, for the CPU to process data from disk, those data
+must first be transferred to main memory by CPU-generated I/O calls.
+
+For a program to be executed, it must be mapped to absolute
+addresses and loaded into memory. As the program executes, it
+accesses program instructions and data from memory by generating
+these absolute addresses.
+
+The operating system is responsible for the following:
+
+* keeping track of which parts of memory are currently being used and by whom
+* deciding which processes (or parts thereof) and data to move into and out of memory
+* allocating and deallocating memory space as needed.
+
+## Storage Management
+
+The OS provides uniform, logical view of information storage.
+The OS abstracts from the physical properties of its storage
+devices to define a logical storage unit, the `file`.
+The OS maps files onto physical media and accesses these files
+via the storage devices.
+
+### File-System Management
+
+File management is one of the most visible components of an operating system.
+Computers can store information on several different types of physical media.
+Magnetic disk, optical disk, and magnetic tape are th emost common.
+Each of these media has its own characteristics and physical organization.
+Each medium is controlled by a device, such as a disk drive or tape drive,
+that also has its own unique characteristics. These properties include
+access speed, capacity, data-transfer rate, and access method (sequential or random).
+
+
+A `file` is a collection of related information defined by it's creator.
+Files represent programs and data. Data files may be numeric, alphabetic,
+alphanumeric or binary. File may be free-from or they may be formatted rigidly.
+
+The OS is responsible for:
+
+* creating and deleting files
+* creating and deleting directories to organize files
+* supporting primitives for manipulating files and directories
+* mapping files onto secondary storage
+* backing up files on stable (nonvolatile) storage media
+
+### Mass-Storage Managment
+
+OS is responsible for
+
+* free space management
+* storage allocation
+* disk scheduling
+
+### Caching
+
+| Level | 1 | 2 | 3 | 4 |
+| ----- | - | - | - | - |
+| Name | registers | cache | main memory | disk storage |
+| Size | < 1 KB | < 16 MB | < 64 GB | > 100 GB |
+| Access time (ns) | 0.25 - 0.5 | 0.5 - 25 | 80 - 250 | 5,000.000 |
+| Bandwith (MB/sec) | 20,000 - 100,000 | 5,000 - 10,000 | 1,000 - 5,000 | 20 - 150 |
+| Managed by | compiler | hardware | operating system | operating system |
+| Backed by | cache | main memory | disk | CD or tape |
+
+### I/O Systems
+
+OS's hide the pecularities of specific hardware devices from the user.
+The I/O subsystem consists of:
+
+* memory management component that includes buffering, caching, and spooling
+* general device-driver interface
+* drivers for specific hardware devices
+
+Only the device driver knows the peculiarities of the specific device to
+which it is assigned.
+
+## Protection and Security
+
+Memory-addressing hardware ensures that a process can execute only within its own address space.
+The timer ensures that no process can gain control of the CPU without eventually relinquishing control.
+Device-control registers are not accessible to users, so the integrity of the various peripheral devices is protected.
+
+`Protection` is any mechanism for controlling the access of processes or users to the resources defined by a computer system.
+It is the job of `security` to defend a system from external and internal attacks.
+
+Operating systems maintain a list of user names and associated user identifiers (user IDs aka SIDs).
+
+The GNU General Public License (GPL) codifies copylefting and is a common license under
+which free software is released. Fundamentally, GPL requires that the source code be
+distributed with any binaries and that any changes made to the source code be released
+under the same GPL license.
+
+## BSD Unix
+
+BSD Unix has a longer and more complicated history than Linux.
+It started in 1978 as a derivative of AT&T's UNIX.
+
+Releases from the University of California at Berkeley came in source and binary
+form, but they were not open-source because a license form AT&T was required.
+BSD UNIX's development was slowed by a lawsuit by AT&T, but eventually a fully
+functional open-source version, 4.4BSD-lite, was released in 1994.
+
+There are many distributions of UNIX, including FreeBSD, NetBSD, OpenBSD and DragonflyBSD.
+The source code for FreeBSD can be found in `/usr/src/`. Ther kernel source code
+is in `/usr/src/sys`.
+
+Darwin, the core kernel component of macOS is based on BSD UNIX and is open-sourced as well.
+
+## Summary
+
+An operating system is software that manages the computer hardware, as well as providing
+an environment for application programs to run.
+
+For a computer to do its job of executing programs, the programs must be in main memory.
+Main memory is the only large storage area that the processor can access directly.
+It is an array of words or bytes, ranging in size from millions to billions. Each
+word in memory has its own address.