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School of Computer Science & Engineering
University of New South Wales
 Advanced Operating Systems 
 COMP9242 2002/S2 

L4 device driver

• Runs at user level.
• Has mappings for device registers (MIPS) and physical memory.
• Typically consists of top half and bottom half.

Top half handler

processes device interrupts:

• receive L4 interrupt IPC,
• check success (status register),
• make data available (copying or mapping),
• initiate next request (if one),
• notify/reply to user (IPC).

Bottom half handler

processes user requests:

• initiate I/O (set up parameter buffer, write device registers)
  or queue request if device busy,
• reply to user (if asynchronous)