Short note: Mach

Mach is a microkernel-based operating system developed at Carnegie Mellon University (from 1985 onwards). It supports distributed and parallel computing, and is the basis of systems such as OSF/1, NeXTSTEP and macOS (XNU).

Main features

• Microkernel: only the essential services are in the kernel (processes/threads, memory management, IPC); other services, such as file systems, run as user-level servers.
• Tasks and threads: a task is an address space and resource holder; threads are the units of execution inside a task and are scheduled by the kernel.
• Ports and messages (IPC): a port is a protected kernel message queue, and tasks communicate by sending messages to ports. Access to a port is controlled by port rights (send, receive). Port rights can be passed between tasks in messages, and ports work transparently across the network through a network message server.
• Virtual memory: supports memory objects, copy-on-write, and external pagers, so user-level programs can manage the backing store; memory can be shared between tasks.
• Multiprocessor and distributed support: threads run on several processors; network message servers give location transparency.

 User tasks, servers (file, network)
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 Mach microkernel: tasks, threads,
     ports/IPC, virtual memory
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 Hardware

Because of its modular design, Mach is portable and flexible, but its heavy use of message passing makes it slower than monolithic kernels.