# OS Ch1: Introduction
###### tags: `OS`

## Bootstrap program
- Bootstrap program is loaded at power-up or reboot
- Bootstrap program is stored in ROM or EPROM
- Called Firmware (韌體)
- Bootstrap program initializes all aspects of system
- Bootstrap program loads operating system kernel and starts execution

## Interrupt
> https://mropengate.blogspot.com/2015/01/operating-system-ch2-os-structures.html

**Device controller** informs CPU that it has finished its operation by causing an **interrupt**.

- **Interrupt vector**:
- Interrupt transfers control to the interrupt service routine through the interrupt vector.
- Interrupt vector contains the addresses of all the service routines.
- Incoming interrupts are disabled if another interrupt is being processed to prevent a **lost interrupt**.
- **Trap** (Exception): a **software-generated** interrupt caused either by an error or a user request.
- An operating system is **interrupt driven**.
- 現在的作業系統是中斷驅動式 (interrupt driven) 。如果沒有 process 要執行，沒有 I/O 裝置要服務和沒有使用者需要回應，則作業系統將安靜的進入等待事件的發生。

> A **trap** is an **exception in a user process**. It's caused by **division by zero** or **invalid memory access**. It's also the usual way to invoke a kernel routine (a system call) because those run with a higher priority than user code. Handling is synchronous (so the user code is suspended and continues afterwards). In a sense they are "active" - most of the time, the code expects the trap to happen and relies on this fact.
>
> An **interrupt** is something **generated by the hardware** (devices like the hard disk, graphics card, I/O ports, etc). These are asynchronous (i.e. they don't happen at predictable places in the user code) or "passive" since the interrupt handler has to wait for them to happen eventually.
>
> = https://stackoverflow.com/questions/3149175/what-is-the-difference-between-trap-and-interrupt

### Interrupt Handling
- The operating system preserves the state of the CPU by storing registers and the program counter.
- **Polled interrupt**: is a specific type of I/O interrupt that notifies the part of the computer containing the I/O interface that a device is ready to be read or otherwise handled, **but does not indicate which device.** The interrupt controller must poll (send a signal out to) each device to determine which one made the request.
- **Vectored interrupt**: In a computer, a vectored interrupt is an I/O interrupt that tells the part of the computer that handles I/O interrupts at the hardware level that a request for attention from an I/O device has been received and and also **identifies the device that sent the request**.

中斷發生時的處理步驟。
1. 令 CPU 暫停目前 process 的執行，同時保存其當時執行的狀態。
2. 根據 interrupt ID 去查詢 interrupt vector，以便可以找到相對應ISR之起始位址。
3. Jump to 相對應的 ISR (Interrupt Service Routine) 之起始位址，然後執行 ISR 。ISR 執行完畢後，將控制權交回給系統。
4. （原則上）resume 原先中斷前的 process 之執行。

## I/O
**Synchronized I/O**
- After I/O starts, **control returns to user program only when I/O completion**.
- At most one I/O request is outstanding at a time, **no simultaneous I/O processing**.

**Asynchronized I/O**
- After I/O starts, **control returns to user program without waiting for I/O completion**.
- OS indexes into I/O device table to determine device status and to modify table entry to include interrupt. 必須有一個表以記錄各種裝置 I/O 的位址和使用狀況。
- Users send a request to the OS via system call to allow to wait for I/O completion.
- Device-status table contains entry for each I/O device indicating its type, address, and state.

### System call
System call is a programming interface to the services provided by the OS.
- System call is mostly accessed by programs via a high-level API rather than direct system call use because of portability and simplicity issue.
- Win32 API for Windows.
- POSIX API for POSIX-based systems (including virtually all versions of UNIX, Linux, and Mac OS X).
- Java API for JVM.

## Storage Structure
> 硬體儲存裝置的速度 Register > Cache > Main memory > SSD > HDD。

### Main Memory
- Main memory is the only large storage media the CPU can access directly.
- Main memory is random-accessed.
- Main memory is **volatiled** (斷電後資料會遺失).

### Secondary Storage
Secondary storage is an extension of main memory that provides large **non-volatile** storage capacity.

### Hard disks
Disks are rigid metal or glass platters covered with magnetic recording material.
- A disk can be divided into tracks. A track can be divided into sectors.
- The disk controller determines the logical interaction between the device and the computer

### Device driver (驅動程式)
- Device driver is for each device controller to manage I/O.
- Device driver provides uniform interface between controller and kernel.

### Direct Memory Access Structure (DMA)
- Used for high-speed I/O devices able to transmit information **at close to memory speeds**.
- Device controller transfers blocks of data from buffer storage **directly to main memory** without CPU intervention.

## Multiprocessors Systems
> aka Parallel Systems, aka Tightly-coupled Systems
- **Asymmetric Multiprocessing**: each processor is assigned a task.
- **Symmetric Multiprocessing**: each processor performs all tasks.

## Clustered Systems
Like multiprocessor systems, but **multiple systems working together**. Usually sharing storage via a **storage-area network (SAN)**.
- **Asymmetric clustering** has one machine in hot-standby mode.
- 有一台機器處於 Hot-standby ，用於監督其他正在執行應用程式的機器，若某台機器壞了，則此監督者可以取得壞掉機器的儲存體所有權，並重新執行其正執行的應用程式。
- **Symmetric clustering** has multiple nodes running applications, monitoring each other.

## Operating System Structure
Multiprogramming (Batch system)
- Needed for efficiency. Multiprogramming organizes jobs (code and data) so CPU always has one to execute. When it has to wait (for I/O for example), OS switches to another job

Timesharing (multitasking)
- logical extension in which CPU switches jobs so frequently.

## Memory Management
- All or part of the instructions must be in memory.
- All or part of the data that is needed by the program must be in memory.
-