Procedures and Addressing Modes

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• Procedures and Addressing Modes

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Procedures

• Place parameters in a place where the called
  procedure can access them.
• Transfer control to the procedure (keep track of
  where to continue when it returns).
• Acquire the storage resources needed by the
  procedure.
• Perform the desired task.
• Place the result value in a place where the
  calling program can access it.
• Return control to point of origin.

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A Leaf Procedure
Remember that MIPS conventions mandate that the
callee preserve s0-s7.
leaf addi sp,sp,-12 sw t1,8(sp) sw
t0,4(sp) sw s0,0(sp) add t0,a0,a1 add
t1,a2,a3 sub s0,t0,t1 add v0,s0,zero lw
s0,0(sp) lw t0,4(sp) lw t1,8(sp) addi
sp,sp,12 jr ra
caller
int x leaf(1,2,3,4)


callee

• int leaf(int g,int h,
• int i, int j)
• int f
• f (gh)-(ij)
• return f

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The Stack in a Leaf Procedure
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Nested Procedures
caller
int x p1(1,2,3,4)


callee caller
int p1(int a, int b, int c, int d)
int r p2(a,b,c,d)

callee

• int p2(int g,int h,
• int i, int j)
• int f
• f (gh)-(ij)
• return f

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Nested Procedures

• Question What are the added complications in
  generating assembly code when we deal with
  procedures that call other procedures?

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Nested Procedures

• int fact(int n)
• if (n lt 1)
• return n
• else
• return (n fact(n-1))

fact addi sp,sp,-8 sw ra,4(sp) sw a0,0(sp
) slti t0,a0,1 beq t0,zero,L1 addi v0,zero,1
addi sp,sp,8 L1 addi a0,0(sp) jal fact lw
a0,0(sp) lw ra,4(sp) addi sp,sp,8 mul v0,
a0,v0 jr rah
Question How can a procedure access global
variables?
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Allocation on the Stack
procedure frame or activation record
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Allocation on the Heap
gp Pointer to static variables. fp Pointer to
the first word of the frame of a procedure. It is
used as the base for access to saved registers
and local variables. It remains fixed throughout
the procedure. sp Pointer to the top of the
stack.
Questions What is a memory leak? Does it happen
in Java? What is garbage collection?
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MIPS Addressing 32-bit immediates

• In immediate addressing, the operand is a
  constant within the instruction itself.
• I-format instructions allocate space for 16-bit
  immediates.

Question How can MIPS programs work with
immediates that are longer than 16 bits, say,
when they are 32-bits long?
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MIPS Register Addressing

• In register addressing, the operand is a
  register.
• R-format instructions use register addressing.

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MIPS Base Addressing

• In base or displacement addressing, the operand
  is the memory location whose address is the sum
  of a register and a constant in the instruction.

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MIPS Addressing in Jumps(Pseudodirect addressing)
J-format
6 bits
26 bits
The address in these instructions is a long
address. The program could be jumping to any
location in memory. Remember, however, that the
pc is 32-bits long.
Question Where do the remaining 4 bits come from?
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MIPS Addressing in Branches(PC-relative
addressing)
I-format
op
rs
rt
address
6 bits
5 bits
5 bits
16 bits
The address in these instructions is a short
address. The program can branch to a location in
memory somewhat near that of the current pc.
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MIPS Addressing