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TCP internals on XNU: the state machine, congestion control, and the receive path
The 4.4 BSD TCP stack as it actually runs on macOS — state transitions, send/receive locking, Apple's congestion-control variants, and the kqueue path back to your app.
bsdnetworkingtcpkernel
The XNU scheduler: bands, QoS, and how Mach decides who runs
Real-time, fixed-priority, timeshare, idle — four scheduling classes, 128 priorities, and a QoS layer on top. Here's how XNU picks a thread to put on a core.
machschedulerqoskernel
Inside APFS: copy-on-write, snapshots, and the sealed system volume
Apple File System, the format under every modern Mac: how it lays out blocks, how it gets snapshots almost for free, and why your /System is read-only at the cryptographic level.
apfsfilesystemkernelxnu
IOKit and the driver model: how a Mac talks to its hardware
Embedded C++, an object tree, and matching dictionaries — IOKit is how every driver on macOS gets loaded, paired with hardware, and called.
iokitdriverskernelxnu
Mach ports: how every macOS process actually talks to another
Tasks, ports, messages, and rights — the IPC primitive that quietly carries every IPC on your Mac, from XPC to drag-and-drop.
machipckernelxnu
The BSD personality: how XNU pretends to be Unix
Processes, file descriptors, signals, sockets — the FreeBSD-derived layer that sits on top of Mach and makes macOS pass POSIX.
bsdposixkernelxnu
Virtual memory in XNU: pmap, the VM map, and the compressor
Every macOS process gets a private address space it can't possibly afford. Here's how XNU gives it one anyway — pmap, vm_map, the compressor, and jetsam.
virtual-memorykernelxnucompressor