Commit b67ac83

@@ -619,3 +619,98 @@ and between the user and kernel layers).
 
 DTrace is composed of a compiler, a framework, `providers` of `probes` written within
 that framework, and `consumers` of those probes.
+
+## Operating-System Generation
+
+Operating systems are designed to runon any of a class of machines at a variety of sites with a 
+variety of peripheral configurations. The system must then be configured or generated
+for each specific computer site, a process sometimes known as system generation `SYSGEN`.
+
+Operating systems are usually distributed as an ISO image, which if a file in the format
+of a CD-ROM or DVD-ROM. To generate a system we use a special program.
+The SYSGEN program reads from a given file, or asks the operator of the system for info
+concerning the specific configuration of the hardware system, or probes the hardware
+directly to determine what components are there.
+
+The following kinds of info mut be determined:
+
+* What CPU is to be used?
+* What options are installed? e.g. extended instruction sets, floating point arithmetic
+* How will the boot disk be formatted?
+* How many sections or partitions will it be separated into?
+* What will go in each partition?
+* How much memory is available?
+* What devices are available?
+* What operating-system options are desired or what parameter values are to be used?
+
+Once this information is determined, it can be used in several ways.
+
+1. A sysadmin can modify a copy of the src to compile a custom copy of the OS.
+1. A less tailored version can lead to the creation of tables and the selection of modules from a precompiled library.
+
+The major differences are the size and generality of the generated system and the easy of
+modifying it as the hardware configuration changes.
+
+## System Boot
+
+How does the hardware know where the kernel is or how to load the kernel?
+The procedure of starting a computer by loading the kernel is known as booting the system.
+
+On most computer systems, a small piece of code known as the bootstrap program or bootstrap loader
+locates the kernel, loads it into main memory, and starts its execution.
+
+Some computer systems use a two-step process in which a simple bootstrap loader fetches a more
+complex boot program from disk, which in turn loads the kernel.
+
+When a CPU receives a reset event the instruction register is loaded with a predefined
+memory location, and execution starts there. At that location is the initial bootstrap
+program. This program is in the form of read-only memory (ROM) because the RAM is in an unknown
+state at system startup. ROM is convenient because it needs no initialization and cannot
+easily be infected by a computer virus.
+
+The bootstrap program can run diagnostics to determine the state of the machine.
+If the diagnostics pass, the program can continue with the booting steps.
+It can also initialize all aspects of the system, from CPU registers to device controllers
+and the contents of main memory.
+
+Some systems store the entire operating systems in ROM.
+Storing the OS in ROM is suitable for small operating systems,
+simple supporting hardware, and rugged operation.
+
+A problem with this approach is that changing the bootstrap code requires changing the ROM
+hardware chips. Some systems use erasable programable read-only memory (EPROM), which
+is read-only except when explicitly given a command to become writable.
+
+All forms of ROM are also known as firmware, since their characteristics fall somewhere
+between those of hardware and those of software. Executing code on firmware is slower
+than executing code in RAM.
+
+Some systems store the OS in firmware and copy it to RAM for fast execution.
+
+For large operating systems the bootstrap loader is stored in firmware and the operating
+system is on disk. In this case, the bootstrap runs diagnostics and has a bit of code
+that can read a single block at a fixed location from disk into memory and execute the
+code from that boot block.
+
+The program stored in the boot block may be sophisticated enough to load the entire OS
+into memory and begin its execution.
+
+GRUB is an example of an open-source bootstrap program for Linux systems. All of the
+disk-bound bootstrap, and the operating system itself, can be easily changed by writing
+new versions to disk.
+A disk that has a boot partition is called a boot disk or system disk.
+
+Once the bootstrap program has been loaded, it can traverse the file system
+to find the operating system kernel, load it into memory, and start its execution.
+At this point the system is running.
+
+## Summary
+
+Operating systems provide a # of services. At the lowest level, system calls allow a running
+program to make requests from the operating system directly. At a higher level, the command
+interpreter or shell provides a mechanism for a user to issue a request without writing a
+program.
+
+Commands may come from files during batch-mode execution or directly from a terminal
+or desktop GUI when in an interactive or time-shared mode. System programs are provided
+to satisfy many common user requests.