1. Bootstrap program

A "bootstrap" most commonly refers to the simple program itself that actually begins the initialization of the computer's operating system, like GRUB, LILO or NTLDR. Modern personal computers have the ability of using their network interface card (NIC) for bootstrapping; on IA-32 (x86) and IA-64 (Itanium) this method is implemented by PXE and Etherboot.

The computer is regarded as starting in a "blank slate" condition - either its main memory is blank, or else its content is suspect due to a prior crash. Although magnetic core memory retains its state with the power off, there would still exist the problem of loading in to it the very first program. A special start program could be very large, and given the modern affordability of read-only memory chips, could constitute the entire program to be run (as in embedded systems) but such an arrangement is inflexible. The bootstrap part would be a short simple piece of code that loads the main code.

2. Difference of interrupt and trap and their use.

The difference between interrupt and trap is that in computing and operating systems, a trap is a type of synchronous interrupt typically caused by an exceptional condition (e.g. division by zero or invalid memory access) in a user process. A trap usually results in a switch to kernel mode, wherein the operating system performs some action before returning control to the originating process. In some usages, the term trap refers specifically to an interrupt intended to initiate a context switch to a monitor program or debugger.[1]In SNMP, a trap is a type of PDU used to report an alert or other asynchronous event about a managed subsystem.

While interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution. A hardware interrupt causes the processor to save its state of execution via a context switch, and begin execution of an interrupt handler.Software interrupts are usually implemented as instructions in the instruction set, which cause a context switch to an interrupt handler similar to a hardware interrupt.Interrupts are a commonly used technique for computer multitasking, especially in real-time computing. Such a system is said to be interrupt-driven.[1]An act of interrupting is referred to as an interrupt request (IRQ).

3. Monitor mode

Monitor mode, or RFMON (Radio Frequency Monitor) mode, allows a computer with a wireless network interface card (NIC) to monitor all traffic received from the wireless network. Unlike promiscuous mode, which is also used for packet sniffing, monitor mode allows packets to be captured without having to associate with an access point or ad-hoc network first. Monitor mode only applies to wireless networks, while promiscuous mode can be used on both wired and wireless networks. Monitor mode is one of the six modes that 802.11 wireless cards can operate in: Master (acting as an access point), Managed (client, also known as station), Ad-hoc, Mesh, Repeater, and Monitor mode.

4. User mode. Contains the userhelper program, which can be used to allow configured programs to be run with superuser privileges by ordinary users, and several graphical tools for users:

• userinfo allows users to change their finger information.


• usermount lets users mount, unmount, and format filesystems.


• userpasswd allows users to change their passwords.

5. Device Status Table



6. Direct memory access (DMA) IO

• requires DMA enhanced controller
  
• controller given command: (read or write, file manager buffer memory address, byte count)
  
• controller copies block of count bytes from its internal buffer to memory address in file manager for a read
  
• controller copies block of count bytes from memory address in file manager to its internal buffer for a write
• the controller then generates interrupt, now that it has copied all requested bytes from or to the file manager buffer
  
• the only downside is the device controller and the CPU must share the system bus to RAM while the controller copies bytes to of from the file manager buffer in RAM and the CPU reads and writes RAM.

7. Difference of RAM and DRAM

RAM - Random Access memory is a second memory section. This is where the computer stores information that you enter through thr keyboard. The amount or capacity of memory depends on the selected system. RAM is referred to as the workspace. You can write in it or erase it with just a few keystroke.

By flashing letters and numbers on the screen, the computer communicates with the operator- telling him, for example, that it is ready to receive the next program. The operator uses the keyboard to give the computer working directions or, in some cases, program material or data.

DRAM- Dynamic random access memory.Type of random access memory that stores each bit of data in a separate capacitor within an integrated circuit. Since real capacitors leak charge, the information eventually fades unless the capacitor charge is refreshed periodically. Because of this refresh requirement, it is a dynamic memory as opposed to SRAM and other static memory.
The advantage of DRAM is its structural simplicity: only one transistor and a capacitor are required per bit, compared to four transistors in SRAM. This allows DRAM to reach very high density. Unlike Flash memory, it is volatile memory (cf. non-volatile memory), since it loses its data when the power supply is removed.

8. Storage Structure

• Main memory. All program execution and data processing takes place in memory, often called "main memory" to differentiate it from memory chips on other circuit boards in the machine. The program's instructions are copied into memory from disk, tape or from the network and then extracted from memory into the control unit circuit for analysis and execution. The instructions direct the computer to input data into memory from a keyboard, disk, tape, modem or network. As data are entered into memory, the previous contents of that space are lost. Once the data are in memory, they can be processed (calculated, compared and copied). The results are sent to a screen, printer, disk, tape, modem or network. Memory is like an electronic checkerboard, with each square holding one byte of data or instruction. Each square has a separate address like a post office box and can be manipulated independently. As a result, the computer can break apart programs into instructions for execution and data records into fields for processing. See early memories and RAM. Memory is an important resource that cannot be wasted. It must be allocated by the operating system as well as by applications and then released when not needed. Errant programs can grab memory and not let go of it, which results in less and less memory available as you load and use more programs. Restarting the computer gives memory a clean slate, which is why rebooting the computer clears up so many problems with applications.
  In addition, if the operating system has bugs, a malfunctioning application can write into the same memory used by another program, causing all kinds of unspecified behavior. You discover it when the system freezes or something weird happens all of a sudden. If you were to be able to look into memory and watch how fast data and instructions are written into and out of it in the course of even 10 minutes, you would know it is truly a miracle that it works at all.
  Other terms for the computer's main memory are RAM, primary storage and read/write memory. Earlier terms were core and core storage. See dynamic RAM, static RAM and memory module.

• Magnetic Disk. Provides direct-access storage.Data stored as a magnetic spots on the disk that make up the pack. As thepack spins, a comblike constantly moves in and out of the spaces between the individual disks, "reading" data they contain or adding new data. A memory device, such as a floppy disk, a hard disk, or a removable cartridge, that is covered with a magnetic coating on which digital information is stored in the form of microscopically small, magnetized needles.

Moving Head Disk Mechanism -

• Magnetic Tape - Early secondary-storage medium of choice.

Persistent, inexpensive, and has large data capacity .Very slow access due to sequential nature .Used for backup and for storing infrequently-used data .Kept on spools .Transfer rates comparable to disk if read write head is positioned to the data . 20-200GB are typical storage capacities.

9. Storage Hierarchy

The range of memory and storage devices within the computer system. The following list starts with the slowest devices and ends with the fastest.

• Caching - is a collection of data duplicating original values stored elsewhere or computed earlier, where the original data is expensive to fetch (owing to longer access time) or to compute, compared to the cost of reading the cache. In other words, a cache is a temporary storage area where frequently accessed data can be stored for rapid access. Once the data is stored in the cache, it can be used in the future by accessing the cached copy rather than re-fetching or recomputing the original data.


• Coherency and Consistency (Coherency) the integrity of data stored in local caches of a shared resource. Cache coherence is a special case of memory coherence.
  When clients in a system maintain caches of a common memory resource, problems may arise with inconsistent data. This is particularly true of CPUs in a multiprocessing system. Referring to the "Multiple Caches of Shared Resource" figure, if the top client has a copy of a memory block from a previous read and the bottom client changes that memory block, the top client could be left with an invalid cache of memory without any notification of the change. Cache coherence is intended to manage such conflicts and maintain consistency between cache and memory.(Consistency) A set of rules governing how the memory systems will process memory operations from multiple processors. Contract between the programmer and system. Determines what optimizations can be performed for correct programs.

10. Hardware Protection

• Dual-mode Operation - The two modes of the NXT software can also be used together, in order to allow the robot to both move (control its motors) and gather data as part of the same task. For example, in the Motion lesson, the robot must run forward while “looking” backward and recording how far it is from the wall behind it at different points in time. The use of both software modes in a combined task is called Dual-Mode Operation.


• I/O Protection - To prevent illegal I/O, or simultaneousI/O requests from multiple processes,perform all I/O via privileged instructions. User programs must make a system callto the OS to perform I/O. When user process makes a system call: A trap (software-generated interrupt)occurs, which causes: The appropriate trap handler to be invokedusing the trap vectorn. Kernel mode to be set. Trap handler: Saves state. Performs requested I/O (if appropriate). Restores state, sets user mode, andreturns to calling program.


• Memory Protection - A technique that prohibits one program from accidentally clobbering another active program. Using various different techniques, a protective boundary is created around the program, and instructions within the program are prohibited from referencing data outside of that boundary.
  When a program does go outside of its boundary, DOS, Windows 3.x, Windows 95/98 and earlier personal computer operating systems simply lock up (crash, bomb, abend, etc.). Operating systems such as Unix, OS/2 and Windows NT, 2000 and XP are more robust and generally allow the errant program to be closed without affecting the remaining active programs.


• CPU Protection - to prevent a user programs gets stuck in infinite loop and never returning back to the os.