Transcript The MIPS architecture - University of Alberta

In this lecture
 Global variables
 Local variables
 System stack
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Global variables
 Exist for duration of program
 Accessible from any part of program
 registers too volatile
 Use memory
 data segment
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# ASSEMBLY PROGRAM
# Global variables
# follow.
.data
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Global variables
 Initialized global variables are
declared with initial value
 int (32 bits) with .word and value
 other types according to size
/* C PROGRAM */
/* declare var = -42 */
int var = -42;
# ASSEMBLY PROGRAM
# Global variables
# follow.
.data
var:
.word -42
.word allocates 4-byte value
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value to
store
3
Global variables
 Uninitialized global variables are
allocated by specifying size only
 use .space directive with bytes
/* C PROGRAM */
/* declare var = -42 */
int var = -42;
/* empty has no
initial value. */
int empty;
# ASSEMBLY PROGRAM
# Global variables
# follow.
.data
var:
.word -42
empty: .space 4
.space allocates empty space
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number
of bytes
4
Memory diagrams
contents (some
uninitialized)
/* global variables */
int n = 42;
int i;
int *iptr = &i;
start of
data
segment
lower
addresses
variable
names
n
42
0x10000000
i
???
0x10000004
0x10000004
0x10000008
iptr
when variables contain
addresses of other variables,
helpful to draw arrow (pointer)
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higher
addresses
5
Local variables
 Properties of local variables
 accessible only within function
 may have more than one variable with same
name (in different functions)
 may have more than one version of the same
function’s variables existing (recursion)
 Properties of data segment
 accessible from all of program
 all labels must be different
 each location can hold only one discrete value
 Data segment is not suited to storing
local variables
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Local variables
 Properties of local variables
 must be allocated at function entry
 must be destroyed at function exit
 other functions may be called in
between, with same rules
void A()
{
int a;
/* create a */
B();
/* destroy a */
}
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void B()
{
int b;
/* create b */
C();
/* destroy b */
}
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void C()
{
int c;
/* create c */
...
/* destroy c */
}
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Local variables
 Properties of local variables
 allocation/destruction obeys LIFO (last in,
first out) principle
• like stack data structure
 stack is ideal data structure for storing local
variables
• allocate a variable by pushing it on the stack
• destroy a variable by popping it off the stack
 stack also helpful for storing other function
information, for same reason
• saving registers
• storing return address
• passing function arguments
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System stack
 Function call/return mechanism is very
widespread among computers
 Most computers provide a stack in
memory for programs to use
 called system stack or runtime stack or stack
 in MIPS, stack resides in its own segment of
memory
• stack segment, to address 0x7FFFFFFF
 stack is initialized by operating system
 user programs push/pop stack as needed
• instruction set provides operations for doing this
• Which ?
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System stack
 Register $sp (stack pointer)
indicates top of stack
 contains address of word of memory at
top of stack (with lowest address)
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System stack
lower
addresses
this is free
space for the
stack to grow
into
0x7FFFB308
0x7FFFB30C
0x7FFFB310
$sp
0x7FFFB310
0x7FFFB314
0x7FFFB318
all of these
words are part
of the stack
0x7FFFB31C
higher
addresses
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System stack: pushing
lower
addresses
to push
another word
(this one) onto
the stack ...
0x7FFFB308
0x7FFFB30C
0x7FFFB310
$sp
0x7FFFB310
0x7FFFB314
0x7FFFB318
subtract 4
from $sp ...
0x7FFFB31C
higher
addresses
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System stack: pushing
lower
addresses
to push
another word
(this one) onto
the stack ...
0x7FFFB308
0x7FFFB30C
0x7FFFB310
$sp
0x7FFFB30C
0x7FFFB314
0x7FFFB318
subtract 4
from $sp ...
0x7FFFB31C
then store a
value here
higher
addresses
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System stack: popping
lower
addresses
to pop a word
(this one) off
the stack ...
0x7FFFB308
... fetch this word
into a register ...
$sp
0x7FFFB30C
0x7FFFB310
0x7FFFB30C
0x7FFFB314
0x7FFFB318
... then add
4 to $sp
0x7FFFB31C
higher
addresses
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System stack: popping
lower
addresses
to pop a word
(this one) off
the stack ...
0x7FFFB308
... fetch this word
into a register ...
$sp
0x7FFFB30C
0x7FFFB310
0x7FFFB310
0x7FFFB314
0x7FFFB318
... then add
4 to $sp
0x7FFFB31C
higher
addresses
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 Examples programs
 stack1.a
 stack6.a
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