Procedures (Cont.)

Report
COMPUTER ARCHITECTURE &
OPERATIONS I
Instructor: Yaohang Li
Review
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Last Class
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Procedure Call
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Leaf Procedure
This Class
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Quiz
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Non-leaf Procedure
Next Class
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Characters
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Starting a Program
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Linking
Non-Leaf Procedures
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Procedures that call other procedures
For nested call, caller needs to save on the
stack:
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Its return address
Any arguments and temporaries needed after
the call
Restore from the stack after the call
Non-Leaf Procedure Example
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C code:
int fact (int n)
{
if (n < 1) return 1;
else return n * fact(n - 1);
}
 Argument n in $a0
 Result in $v0
Non-Leaf Procedure Example
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MIPS code:
fact:
addi
sw
sw
slti
beq
addi
addi
jr
L1: addi
jal
lw
lw
addi
mul
jr
$sp,
$ra,
$a0,
$t0,
$t0,
$v0,
$sp,
$ra
$a0,
fact
$a0,
$ra,
$sp,
$v0,
$ra
$sp, -8
4($sp)
0($sp)
$a0, 1
$zero, L1
$zero, 1
$sp, 8
$a0, -1
0($sp)
4($sp)
$sp, 8
$a0, $v0
#
#
#
#
adjust stack for 2 items
save return address
save argument
test for n < 1
#
#
#
#
#
#
#
#
#
#
if so, result is 1
pop 2 items from stack
and return
else decrement n = n - 1
recursive call
restore original n
and return address
pop 2 items from stack
multiply to get result
and return
What is preserved and what is not?
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Data and registers preserved and not
preserved across a procedure call
Global Pointer
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Two kinds of C/C++ variables
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automatic
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static
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Local to a procedure
Discarded when the procedure exits
Global to a procedure
Still exist after procedure exits
Can be revisited
Global Pointer
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$gp
Point to static area
Procedure Frame
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Revisiting Stack
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Stack not only stores the saved registers
but also local variables that do not fit in
registers
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local arrays or structures
Procedure Frame (activation record)
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Segment of the stack containing a procedure’s
saved registers and local variables
Frame pointer
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Point to the location of the saved registers and
local variables for a given procedure
Local Data on the Stack
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Local data allocated by callee
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e.g., C automatic variables
Procedure frame (activation record)
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Used by some compilers to manage stack storage
MIPS Memory Layout
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32-bit address space
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0x80000000 ~ 0xFFFFFFFF
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0x00000000~0x003FFFFF
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Reserved
0x00400000~0x0FFFFFFF
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Not available for user program
For OS and ROM
Text: Machine language of the user program
0x10000000~0x7FFFFFFF
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Data
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Static
 Static variables
 Constants
Dynamic
 Malloc() in C, New in java
Stack
Memory Layout
Register Summary
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Register 1: $at
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reserved for the assembler
Register 26-27: $k0-$k1
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reserved for the OS
Summary
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Procedure Call
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Registers used
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Stack
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jal and jr
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leaf and no-leaf procedure
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Allocating space for new data on the heap
What I want you to do
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Review Chapter 2
Work on your assignment 5

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