How to store in computer?

Two’s Complement

0000 0000 is 0

0000 0001 is 1

0000 0010 is 2

0111 1110 is 126

1000 0000 is -128

1000 0001 is -127

1000 0010 is -126

1111 1110 is -2

1111 1111 is -1

-x equals reverse all bits then +1.

Floating Point

In IEEE 754,

32 bit:

1-bit s for ±

8-bit E for power

23-bit M for number after dot

+/−0.1M×2E+/-0.1M\times2^E

Character Set

ASCII 1 byte

UTF-8 1~4 byte

Unicode usually 2 byte.

Instruction Set

CISC RISC

Exception Control

Reasons

  • internal exception (page fault, privilege, divided by zero…)
  • external interruption (ctrl+c, printer lack of paper…)
  • context switch
  • signal passed from a process to another

Context Switch

  • The state of the currently executing process must be saved so it can be restored when rescheduled for execution.
  • The process state includes all the registers that the process may be using, especially the program counter, plus any other operating system specific data that may be necessary. Usually stored in PCB or switchframe
  • OS suspends execution of one process, switches context by choosing a process from the ready queue and restoring its PCB.

Exception and Interruption

Internal

  • Hard error: power shutoff, circuit error
  • Soft error: divide 0, illegal instruction, stack overflow, out of time, page fault…

External

  • external clock, device ready…

When exception or interruption occurs, OS get into kernel to handle.

current instruction → OS get into Kernel to response to exception/interruption → return to current/next instruction or abort.