Chapter 4: Security Architecture and Design Continue

• The Importance of Control Unit:
• Basically, the control unit controls when instructions are executed and enables applications to process data, but the control unit does not actually process the data.
• While different applications' code and operating system instructions are being executed, the control unit manages and synchronizes the system.
• The control unit gets the code, interprets the code, and oversees execution of the different instruction sets. 
• The control unit is responsible for determining what application instructions get processed and in what priority and what time. 
• CUP's time is sliced into individual units which is assigned to processes. 
• The operating system can carry out several different function at one time, this is called multitasking, but the CPU actually has to execute instructions in a serial fashion (one at a time). 
• Types of registers:
1. General register: 
1. Hold variables and temporary results as the ALU (arithmetic logic unit) works through its execution steps.
2. Like a scratch pad which you use while working.
2. Special registers:
1. Dedicated registers
2. Hold information such as program counter, stack pointer, and program status word (PSW).
1. PSW :
1. holds different conditions bits, and one bit indicates whether the CUP should be working in user mode.
3. Program counter:
1. Contains memory address of the next instruction to be fetched.

• How do operating systems protect themselves:
• Operating systems need to protect themselves from applications, software utilities, and user activities if they are going to provide a stable and safe environment. 
• One protection mechanisms is implemented through different execution mode uses. 
• CPU carries out instructions and it works in a user mode of a lower privilege level. Many of the CPU's instructions and functions are not available to the requesting application. This is because developers of the operating system and CPU do not know who developed the application or how it is going to react.

• How do processes communicate to other processes and the CPU:
• Processes communicate to other processes and the CPU through the process of stacking and the CPU needs to keep track of where it is in each stack. This is called stack pointer. 
• Stack is a data structure in memory that the process can read from and write to in a last in, first out (LIFO) fashion (Harris, 309).
• Each stack is a level of information that tells the processes something, weather it is an introduction, how to respond, containing data etc. Once the first item on the stack is executed, then the stack pointer moves down to tell the CPU where the next pice of data is located.