1. In this lecture Global variables Local variables System stack
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2. Global variables Exist for duration of program
Accessible from any part of program
registers too volatile
Use memory
data segment
# ASSEMBLY PROGRAM
# Global variables
# follow.
.data
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3. .word allocates 4-byte value
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
value to store
.word allocates 4-byte value
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4. .space allocates empty space
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
number of bytes
.space allocates empty space
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5. contents (some uninitialized)
Memory diagrams
/* global variables */
int n = 42;
int i;
int *iptr = &i;
start of data segment
contents (some uninitialized) 0x 0x 0x
lower addresses
higher addresses
variable names 42 n
??? i 0x
iptr
when variables contain addresses of other variables, helpful to draw arrow (pointer)
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6. 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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7. 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 */ }
void B() {
int b;
/* create b */
C();
/* destroy b */ }
void C() {
int c;
/* create c */
...
/* destroy c */ }
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8. 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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9. 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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10. 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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11. System stack lower addresses
this is free space for the stack to grow into
0x7FFFB308
0x7FFFB30C
0x7FFFB310
$sp
0x7FFFB310
0x7FFFB314
all of these words are part of the stack
0x7FFFB318
0x7FFFB31C
higher addresses
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12. to push another word (this one) onto the stack ...
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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13. to push another word (this one) onto the stack ...
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
higher addresses
then store a value here
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14. System stack: popping lower addresses
to pop a word (this one) off the stack ...
0x7FFFB308
... fetch this word into a register ...
0x7FFFB30C
0x7FFFB310
$sp
0x7FFFB30C
0x7FFFB314
0x7FFFB318
... then add 4 to $sp
0x7FFFB31C
higher addresses
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15. System stack: popping lower addresses
to pop a word (this one) off the stack ...
0x7FFFB308
... fetch this word into a register ...
0x7FFFB30C
0x7FFFB310
$sp
0x7FFFB310
0x7FFFB314
0x7FFFB318
... then add 4 to $sp
0x7FFFB31C
higher addresses
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16. Examples programs
stack1.a
stack6.a
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