Operating Systems and Software
This domain covers what an operating system actually does - process, memory, file, device and user management - plus how software is installed, licensed and kept current.
Core rules
- Kernel vs user mode - a fault in user mode kills one process; a kernel fault can crash the whole system.
- Process vs thread - a process has its own address space; threads inside it share memory, so they are cheaper but can corrupt each other's data.
- Virtual memory - pages move to a page file or swap partition when RAM is exhausted. Constant paging (thrashing) shows as heavy disk use and sluggish response.
- File systems - NTFS uses ACLs and journaling; ext4 is the Linux default; APFS is macOS; FAT32 caps a single file at 4 GB.
- Permissions - POSIX uses read/write/execute for owner, group and other; Windows ACLs inherit from parent folders and an explicit deny normally wins.
- Licensing - OEM licences stay with the first machine, retail licences transfer, subscriptions expire.
Worked example
A user cannot install a 64-bit application. The 64-bit CPU is fine, but the installed OS is 32-bit. A 32-bit OS cannot load 64-bit binaries, and 32-bit processes address only about 4 GB. Reasoning: no patch fixes this - the OS must be reinstalled as 64-bit, with data backed up first.
Traps that cost marks
- "Fix permissions with chmod 777" - it grants everyone full access and hides the real cause.
- Deleting the page file to "speed up" a machine; it removes the safety net for memory spikes.
- Assuming a file extension proves its content or safety.
- Believing registry cleaners are routine maintenance.