Lecture 003

CPU State

%esi, %edi: ???

Processor status register(s): EFLAGS

• Currently running: user code / kernel code?

• Interrupts: on / off

• Virtual memory: on / off

• Memory model: small, medium, large, "unreal" (x86-64)

Floating point number registers: (still user register)

• Sometimes another “special” set of registers

• Some machines don't have floating point

• Some processes don't use floating point

Execution environment: status registers and %eip

System Calls and Interrupts

getpid(): example system call

• execute TRAP (in intel, TRAP is called INT, IT IS NOT INTERRUPT)

• then as result, some register dump to memory, some load from memory

• the process enter kernel mode: more hardware device become visible

Kernel runtime language (CPU don't care which you use):

• ML: may not use "stack"

• C: must use "stack"

Trap Handler: job is to build kernel runtime environment

• switch to correct stack

• save registers (EFLAGS, CS/EIP, error code (when faults) of current process) to kernel stack

• load registers (new %eip, and status registers) according to table

• other stuff

  • turns on virtual memory (certain CPU turn off virtual memory when page fault)
  • flush caches

• interrupt handler use kernel stack ...

• empty stack to original state

• invoke IRET (interrupt return), register loaded from kernel stack, back to user mode

Trap: switching request from user code

Interrupt: switching request from hardware

There is a look up table involved for interrupt source storing CPU state - table base pointer established in OS startup - table entry size defined by hardware

Running: currently executing code Block: waiting for device (e.g. disk interrupt), can't run Runable: CPU is not scheduling for this code to run, but it can run

Solution: (1) temporary suspend/mask/defer device interrupt while checking and enqueue (2) lock-free data structure. But don't block all interrupt or for too long time.

Countdown Timer

Timer:

• Behavior

  • periodic interrupt to count: CPU cycles, bus cycles, microseconds...
  • it has no "request" or "result"

• Reason

  • avoid CPU hogs while True: continue, force process switch
  • maintain precise time