Explanation
Each classic operating system corresponds to its primary architectural structure as follows:
A. MS-DOS → 2 (Monolithic structure without clear modules): MS-DOS was originally written to provide the most functionality in the least space. It was not divided into well-defined modules, allowing user programs to directly access basic I/O routines.
B. Mach / QNX → 4 (Commercial Microkernel implementation): Mach and QNX are classic, prominent examples of microkernel systems, where core services like process management and IPC remain in the kernel, while other services run in user space.
C. Early THE OS → 1 (Purely Layered Architecture): Designed by Edsger Dijkstra, THE operating system was one of the earliest systems built using a strict 6-layer architecture (Layer 0 to Layer 5).
D. Windows NT → 3 (Modern Hybrid Kernel architecture): Windows NT (and its modern derivatives like Windows 10/11) uses a hybrid kernel architecture, combining microkernel modularity with monolithic performance features.
Therefore, the correct mapping is A-2, B-4, C-1, D-3.
