CS454 Lecture Notes

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CS454 Lecture Notes

• Process Management In Xinu

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Process Management (Ch 4)

• Refer to notes for the x86 register definitions.
• Context switch Method of saving the state
  (context) of the machine during execution of one
  process (registers), and restoring the machine
  state of another process and continue execution.
• Context switches can occur in the following
  situations
• Current process time quantum expires
• Waiting for I/O request
• Semaphore wait

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Context Switching During Subroutine Execution

• Caller
• Push parameters on stack. (1st parameter is on
  TOP of the stack)
• Call subroutine (JSR)
• Callee
• Create a current frame ptr (push BP, which points
  to the previous record).
• Reserve space for locals save registers used
  for locals.
• Execute Parameters and locals are accessed from
  stack using BP as the base pointer.
• Release space for locals restore registers used
  for locals.
• Restore the previous frame pointer (pop BP).
• Return from subroutine (RTS)
• Caller
• Remove the parameters from the stack.

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General C Stack Diagram

• C Stack

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C Conventions Function Invocation

• 1. Type conversion takes place when arguments are
  pass to functions
• char, short --gt int (long) (4 bytes)
• float --gt double (8 bytes)
• 2. Return value
• char, short, int, float returned through d0 as 4
  bytes
• double returned through d0, d1 as 8 bytes
• 3. Before any procedure (function) call
• Push parameters in the reverse order on the stack
  (callers responsibility)
• 4. Jump to subroutine instruction
• return address is pushed on the stack
• control goes to the subroutine

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C Conventions Function Invocation cont.

• 5. When any procedure (function) is compiled
• the first instruction is always link, a6, -d
• d is the number of bytes for local variables
• link is a powerful instruction, its effect is as
  follows
• push a6
• sp -gt a6
• sp - d -gt sp
• 6. The register a6 is used as the Frame Pointer
  (FP) for the activation stack record (frame) for
  the procedure call. The local variables and
  parameters are accessed relative to a6. The
  activation stack record is released when the
  procedure returns.

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C Conventions Function Invocation cont.

• 7. In the compiled code for a procedure, the
  instruction before the return instruction (rts or
  rte) is always unlk a6
• The effect of the unlk instruction is as
  follows
• a6 -gt sp (release local variable area)
• pop a6 (recover old FP pop the contents and
  store in a6)
• As a result of pop sp points to the location
  where the return address is saved. The register
  a6 points to the activation stack record before
  the call.
• 8. Return instruction
• To return from a call, the called routine
  executes an rts (return from subroutine) or an
  rte (return from exception).
• rts expects the top of the stack to contain the
  return address
• rts
• load PC from top of stack and increment SP (pop
  stack)
• rte expects the top of the stack to contain the
  new value of SR (process status word) and below
  that in the stack there should be a return
  address
• rte
• load SR from top of stack and then pop stack
• load PC from top of stack and pop stack

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C Conventions Function Invocation cont.

• 9. The jump to subroutine instruction is always
  followed by an instruction addq e, sp. The
  purpose of the instruction is to release the
  parameter area. Note here the convention is that
  the caller removes the parameters from the stack
  once the call is returned.
• 10. As a result of the conventions for a
  procedure call the first argument (for the
  procedure) is always supposed to be located at 8
  relative to FP (also known as a6).

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Diagrams of Subroutine Context Switches

• Caller before JSR

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Diagram cont.

• Caller JSR

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Diagram cont.

• Callee
• Link a6, -d
• Push a6
• SP -gt a6
• SP - d -gt SP

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Diagram cont.

• Callee
• Unlk
• a6 -gt SP
• pop a6

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Diagram cont.

• Callee
• RTS
• pop (PC)

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Diagram cont.

• Caller
• Releases the arguments