Operating System
<Static vs Dynamic type checking>
Static type checking: The type of variable is known at compile time, instead of runtime
Dynamic type checking: The process of verifying the type safety of a program at runtime. Type checking at runtime increases overhead.
<프로세스의 상태변화>
The OS uses preemptive scheduling!
Dispatch latency의 정의
The time to stop one process and start running another one.
The amount of time it takes for a system to respond to a request for a process to begin operation.
Dispatcher is used to transit the process from ready to running sate.
<Virtual Memory>
Logical MEM (generated by CPU) -- Page Table --> Physical MEM (RAM) or HDD/SSD (Virtual Memory)
Can be larger than the amount of physical memory (RAM)
Supplementing the RAM with secondary storage (HDD/SSD) like this is called Virtual Memory
Each application (process) has its own view of memory, known as logical memory
Pages in the secondary memory are back into MEM again when the pages are needed again by the process.
With segmented memory, the large process may not get access to the memory very often.
Paged memory makes better use of free space
<Page fault>
If the demand page is NOT present in the main memory (MEM) then we call it a 'page fault'
It occurs when the CPU generates a logical address for a page that is not in physical memory (MEM)
Steps to handle 'Page Fault' (The number below and the number in the figure are unrelated)
OS looks at PCB (Process Control Block) to decide
1-1. Invalid reference then abort
1-2. Just not in the primary memory then load the page from the secondary memory
Get empty frame
Swap the page into the frame via disk operation
Reset the page table to indicate the required page is in the page table.
Restart the instruction that caused the page fault.
<Internal fragmentation (내부단편화 / paging) vs External fragmentation (외부단편화 / segmentation)>
Internal fragmentation occurs when the memory is distributed into fixed-sized blocks. It is defined as the difference between the memory allocated and the memory space required by a process.
프로세스가 필요한 양보다 더 많은 메모리 공간이 할당된 상태여서 메모리가 낭비되는 상황
External fragmentation: External fragmentation exists when there is enough total memory space to satisfy a request but the available spaces are not contiguous: storage is fragmented into a large number of small holes.
External fragmentation: 총 공간을 계산해봤을 때 요청을 만족할만한 충분한 메모리가 있음에도, 가능한 공간들이 연속적이지 않을 때 (즉 저장공간이 많은 작은 hole들로 조각조각 나있을 때) External fragmentation은 발생한다)
How to circumvent external fragmentation? We use compaction. Compaction이란 비어있는 공간을 연속적인 공간으로 만들고 움직이는 작업이다.
<User (bit=1) / Kernel (bit=0) mode>
User mode: 시스템 데이터에 제한된 접근만이 허용되며, 하드웨어를 직접 접근 할 수 없다. 어플리케이션 프로그램이 수행되는 모드는 유저모드이다.
Kernel mode: CPU 명령어이며 특정한 권한이 있어야만 수행되는 명령어이다.
<Software pipelining>
Software pipelining is a type of out-of-order execution, except that the reordering is done by a compiler instead of the processor.