Computer Science 210 Computer Organization

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Computer Science 210Computer Organization

• Introduction to MIPS Assembly Language
• Subroutines

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MIPS Memory Allocation

• Stack grows down
• Register sp points to top of stack
• Push decreases sp
• Pop increases sp
• pc (program counter) is register that keeps up
  with current location in the program
• Note The stack is in memory so stack operations
  are slow.

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Subroutines

• Subroutines in assembly language provide the low
  level mechanism for high level functions,
  procedures, subroutines, methods, etc.

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Important Assumption

• Unless stated otherwise, we assume that our
  subroutines are completely independent of the
  calling program or subroutine
• The subroutine knows nothing about labeled data
  of the caller only receives data via
  appropriate registers or system stack.
• Subroutine can have no labeled data (doesnt know
  labels used by caller)

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Issues with Subroutines

• We must learn
• How to call a subroutine
• How do we know the return location
• How to pass parameters to subroutine
• How to deal with local variables of subroutine
• How to protect register values of caller
• How to return values to caller
• How to protect callers stack values
• How to return to caller

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1) Calling the Subroutine2) Where to return

• To call a subroutine, we use the jal (jump and
  link) instruction.
• This is an unconditional branch instruction jal
  labelThe label is that of the subroutine
• This instruction
• Puts 4(pc) into register ra (return address)
  so well know where to return.
• Puts the address of the label in pc.

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3) Passing parameters to subroutine

• To pass up to 4 parameters by value, the caller
  uses registers a0 - a3 to hold the values.
• To pass up to 4 parameters by reference, use
  these same registers for the addresses.
• If we need to pass more parameters, say 6, we
  could use these registers for 4 parameters and
  put the other two on the stack. Of course, the
  specifics must be agreed upon between callers and
  the callee.
• Arrays are usually passed by reference pass the
  base address and length of array.

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4) Local variables

• The basic way to handle the need for local
  variables is to
• Use registers if possible
• Use the stack otherwise

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5) Saved registers One MIPS convention

• It is the callees responsibility to preserve the
  values in s0 - s7. So if the subroutine uses
  these registers, their original values should be
  placed on stack and then restored before
  returning.
• It is the callers responsibility to preserve the
  values in t0 - t7. So if there are needed
  values in these registers, the caller should put
  these on stack before calling the subroutine and
  then restore them after the return.

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5) Saved registers Other conventions

• Caller Saves Convention It is the callers
  responsibility to preserve the values in s0 -
  s7 and t0 - t7 before calling and then restore
  after the call.
• Callee Saves Convention It is the callees
  responsibility to preserve the values in s0 -
  s7 and t0 - t7 for any of the registers that
  it uses. So if there are needed values in these
  registers, the caller should put these on stack
  before calling the subroutine and then restore
  them after the return.
• Unless stated otherwise,we will use Callee Saves.

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6) Returning values

• Registers v0 and v1 are used for returning
  values from a subroutine.
• If more values need to be returned, these could
  be on the stack. Of course, the caller would need
  to get them from the stack.

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7) Preserving stack

• Often the caller will have valuable data on the
  stack. Therefore, the callee must be sure to
  return to the caller with the stack as before the
  call, possibly with return values on top of the
  stack.
• The special register fp (frame pointer) is used
  to hold the value of the stack pointer when the
  subroutine first gets control move fp, sp
• Before returning, move sp, fp
• We refer to the section of stack memory used by
  the subroutine as a frame.
• If the frame is of fixed size, we may not need
  fp.

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8) Returning

• After doing the appropriate housekeeping, jr
  ra
• since ra holds the appropriate return address.

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What if subroutine calls a subroutine

• Frames will get stacked on frames thats ok.
• One problem is that when a subroutine1 calls a
  subroutine2, ra gets the return address back
  into subroutine1. The return address that
  subroutine1 will need when it returns will be
  lost.
• So, subroutine1 must keep it in a saved register
  or on the stack.

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Subroutine Example
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Subroutine Example
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Subroutine Example