Storage
File blocks on SSD/HDD
deviceDisk controller
Issues disk DMA
devicePage cache
Kernel-resident file pages
kernelUser buffer
Application-visible payload
userSocket queue
Payload bytes or page references
kernelNIC
Network transmission DMA
device0context switches
0CPU payload copies
0DMA payload moves
0descriptor ops
Quiet routes: thin lines are possible connections; the selected lesson exposes only its causal route. Teal = DMA. Gold = CPU payload copy.