# Mainframe Systems
* One of the earliest computers
* Large in size
* Slow I/O devices: card reader/printer, tape drivers
* Evolution
* Batch -> Multi-programming -> Time-shared

* 時至今日，如果某些專門用來大量處理某件工作的機器，我們仍會稱之為mainframe
* For critical application with better reliability and security
* bulk data processing
* Widely used in hospitals, banks, etc.

---
## Batch System

* 批次，也就是一次執行一個任務
* Processing steps:
* User submit jobs(program, data, control card)
* Operator sort jobs with similar requirement
* **OS just simply transfer control from one job to the next** -> The only thing computer does
* Memory Layout:
* Two layer (user program area | operating system)

* Drawbacks:
1. One job at a time
2. No interaction between users and jobs (執行過程中，user無法更改任何事情)
3. CPU is often idle
* Why: I/O speed <<<<< CPU speed (at least 1:1000), so CPU is often waiting for I/O
* (Although this problem still exists nowadays)
* Solution:
* For drawback 1 and 3: Process as many programs as possible
* For drawback 2: Use Time-sharing concepts(a fast-switching program)

* Keypoints: OS in Mainframe/Batch Systems does not need to make any decision

---
## Multi-programming System
* Solution:
* Overlaps I/O and computation (keep CPU and I/O devices working at higher rates)

* How? **Spooling (Simultaneous Peripheral Operation On-Line)**
* I/O is done with no CPU intervention
* CPU just needs to be notified when I/O is done
* Can be done by a mechanism called "interupt"

* Several jobs are kept in the main memory (CPU can only execute jobs that are in the memory), and the CPU is multiplexed among them
* 將磁碟機視為一個大 buffer，資料讀取與寫入時，不直接讀取，而由磁碟機中讀取。 例如有很多筆資料要交給印表機輸出時，可將待印資料的清單放在一個表格中，印表機印完一筆資料後，可直接至表格讀取下一筆待印資料，而 CPU 把待印資料的清單放在表格中後，可以去做別的計算，I/O也可繼續執行。

* 以前記憶體還沒那麼大的時候，就會利用job scheduling去決定哪些job要載入OS裡面，但現今已不會這麼做，現在大部分的系統都會直接全部載進去

* OS tasks in multi-programming systems
* Memory mangement - allocate memory to jobs
* CPU scheduling - choose job to run
* I/O system - I/O routine, allocation of devices

* Jobs are still running in batch, so there might have delays between running different jobs

---
## Time-sharing System (Multi-tasking System)
* An interactive system provides direct communication between the users and the system
* CPU switches among jobs frequently (frequently detecting I/O)
* Users can see results immediately (<1s)
* Usually, keyboard/screen are used
* Multiple users can share the computer simulaneously
* In time-sharing system, jobs are switched when
* finished
* waiting for I/O
* **a short period of time**

* 簡單來說，time-sharing就會把CPU的執行分割成一小塊一小塊的timeslots，每一個job執行幾毫秒，所有在系統內的程式就輪流去使用這些timeslots
* 現代電腦就是使用time-sharing system在進行多工

* OS tasks in time-sharing systems
* Virtual memory (consider disk as memory) - jobs are swapped in and out of memory to get reasonable response time
* 為什麼要擴大記憶體容量？因為我們希望系統能載入更多程式
* File system and disk management
* Process synchronization and deadlock - concurrent execution
---
## Compare Each System