Download Chapter 5 (Part 1)

Procedures and the Stack
Chapter 5
S. Dandamudi
Outline
• What is stack?
• Pentium implementation
of stack
• Stack instructions
• Uses of stack
• Procedures
 Pentium instructions
• Parameter passing
 Register method
 Stack method
2005
• Examples
 Call-by-value
 Call-by-reference
 Bubble sort
• Procedures with variable
number of parameters
• Local variables
• Multiple source program
modules
• Performance: Procedure
overheads
 S. Dandamudi
Chapter 5: Page 2
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
What is a Stack?
• Stack is a last-in-first-out (LIFO) data structure
• If we view the stack as a linear array of elements,
both insertion and deletion operations are
restricted to one end of the array
• Only the element at the top-of-stack (TOS) is
directly accessible
• Two basic stack operations:
 push (insertion)
 pop (deletion)
2005
 S. Dandamudi
Chapter 5: Page 3
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
What is a Stack? (cont’d)
• Example
 Insertion of data items into the stack
» Arrow points to the top-of-stack
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 S. Dandamudi
Chapter 5: Page 4
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
What is a Stack? (cont’d)
• Example
 Deletion of data items from the stack
» Arrow points to the top-of-stack
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 S. Dandamudi
Chapter 5: Page 5
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Pentium Implementation of the Stack
• Stack segment is used to implement the stack
 Registers SS and ESP (32-bit) or SP (16-bit) are used
 SS:ESP or SS:SP represents the top-of-stack
• Pentium stack implementation characteristics are:
 Only words (i.e., 16-bit data) or doublewords (i.e., 32bit data) are saved on the stack, never a single byte
 Stack grows toward lower memory addresses (i.e.,
stack grows “downward”)
 Top-of-stack (TOS) always points to the last data item
placed on the stack
2005
 S. Dandamudi
Chapter 5: Page 6
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Pentium Stack Instructions
• Pentium provides two basic instructions:
push
pop
source
destination
• source and destination can be a
 16- or 32-bit general register
 a segment register
 a word or doubleword in memory
• source of push can also be an immediate
operand of size 8, 16, or 32 bits
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 S. Dandamudi
Chapter 5: Page 7
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Pentium Stack Example - 1
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 S. Dandamudi
Chapter 5: Page 8
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Pentium Stack Instructions: Examples
• On an empty stack, the following sequence of
push instructions
push
push
21ABH
7FBD329AH
results in the stack state shown in (c) in the last figure
• On this stack, executing
pop
EBX
results in the stack state shown in (b) in the next figure
and the register EBX gets the value 7FBD329AH
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 S. Dandamudi
Chapter 5: Page 9
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Pentium Stack Example - 2
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 S. Dandamudi
Chapter 5: Page 10
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
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 S. Dandamudi
Chapter 5: Page 11
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition,
Additional Pentium Stack Instructions
Stack Operations on Flags
• push and pop instructions cannot be used with
the Flags register
• Two special instructions for this purpose are
pushfd (push 32-bit flags)
popfd (pop 32-bit flags)
• No operands are required
• Use pushfw and popfw for 16-bit flags
(FLAGS)
2005
 S. Dandamudi
Chapter 5: Page 12
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Additional Pentium Stack Instructions (cont’d)
Stack Operations on All General-Purpose Registers
• pushad and popad instructions can be used to save and
restore the eight general-purpose registers
EAX, ECX, EDX, EBX, ESP, EBP, ESI, and EDI
• Pushad pushes these eight registers in the above order
(EAX first and EDI last)
• popad restores these registers except that ESP value is not
loaded into the ESP register
• Use pushaw and popaw for saving and restoring 16-bit
registers
2005
 S. Dandamudi
Chapter 5: Page 13
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Uses of the Stack
• Three main uses
» Temporary storage of data
» Transfer of control
» Parameter passing
Temporary Storage of Data
Example: Exchanging value1 and value2 can be
done by using the stack to temporarily hold data
push
push
pop
pop
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value1
value2
value1
value2
 S. Dandamudi
Chapter 5: Page 14
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Uses of the Stack (cont’d)
• Often used to free a set of registers
;save AX & BX registers on the stack
push
AX
push
BX
; AX and BX registers can now be used
mov
AX,value1
mov
BX,value2
mov
value1,BX
mov
value2,AX
;restore AX & BX registers from the stack
pop
BX
pop
AX
. . .
2005
 S. Dandamudi
Chapter 5: Page 15
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Assembly Example1
; Assembly language that reads two characters
; and swap them using stack
.586
option segment:use16
.model small
.stack 200h
print_msg MACRO msg
mov dx,offset msg
mov ah,9
int 21h
ENDM
2005
 S. Dandamudi
Chapter 5: Page 16
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition,
Assembly Example1
read_char MACRO
mov ah,1
int 21h
sub ah,ah
ENDM
print_char MACRO char
mov dl,char
mov ah,2
int 21h
ENDM
2005
 S. Dandamudi
Chapter 5: Page 17
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition,
Assembly Example1
.data
msg1 db 'Enter 1st character:$'
msg2 db 0ah,0dh,'Enter 2nd character:$'
resmsg1 db 0ah,0dh,'1st character after swapping:$'
resmsg2 db 0ah,0dh,'2nd character after swapping:$'
.code
.startup
print_msg msg1
read_char
push ax
print_msg msg2
2005
 S. Dandamudi
Chapter 5: Page 18
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition,
Assembly Example1
read_char
push ax
print_msg resmsg1
pop ax
print_char al
print_msg resmsg2
pop ax
print_char al
.exit
end
2005
 S. Dandamudi
Chapter 5: Page 19
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition,
Assembly Example2
; Assembly language that reads n characters
; and print them in reverse order using stack
.586
option segment:use16
.model small
.stack 200h
print_msg MACRO msg
mov dx,offset msg
mov ah,9
int 21h
ENDM
2005
 S. Dandamudi
Chapter 5: Page 20
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition,
Assembly Example2
read_char MACRO
mov ah,1
int 21h
sub ah,ah
push ax
ENDM
print_char MACRO
pop dx
mov ah,2
int 21h
ENDM
2005
 S. Dandamudi
Chapter 5: Page 21
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition,
Assembly Example2
.data
msg1 db 'How many characters do you want to enter?$'
msg2 db 0ah,0dh,'Enter characters:',0ah,0dh,'$'
resmsg db 0ah,0dh,'Characters you entered in reverse
order:',0ah,0dh,'$'
errormsg db 0ah,0dh,'Error,You should enter a number$'
counter dw ?
.code
.startup
print_msg msg1
read_char
2005
 S. Dandamudi
Chapter 5: Page 22
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition,
Assembly Example2
pop counter
sub counter,30h
cmp counter,1
jl error
cmp counter,9
jg error
mov cx,counter
print_msg msg2
read_loop:
read_char
loop read_loop
print_msg resmsg
mov cx,counter
2005
 S. Dandamudi
Chapter 5: Page 23
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition,
Assembly Example2
print_loop:
print_char
loop print_loop
exit:.exit
error:print_msg errormsg
jmp exit
end
2005
 S. Dandamudi
Chapter 5: Page 24
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition,
Assembly Example3
; Assembly language that reads n numbers
; and then prints all even numbers in reverse
; order using stack
print_msg MACRO msg
mov dx,offset msg
mov ah,9
int 21h
ENDM
2005
 S. Dandamudi
Chapter 5: Page 25
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition,
Assembly Example3
read_char MACRO
mov ah,1
int 21h
mov bl,al
sub bl,30h
sub bh,bh
ENDM
newline MACRO
mov dl,0ah
mov ah,2
int 21h
mov dl,0dh
int 21h
ENDM
2005
 S. Dandamudi
Chapter 5: Page 26
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition,
Assembly Example3
print_char MACRO
pop dx
add dl,30h
mov ah,2
int 21h
newline
ENDM
.586
option segment:use16
.model small
.stack 200h
2005
 S. Dandamudi
Chapter 5: Page 27
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition,
Assembly Example3
.data
msg1 db 'How many numbers do you want to enter?$'
msg2 db 0ah,0dh,'Enter numbers:',0ah,0dh,'$'
resmsg db 0ah,0dh,'Even numbers you entered in reverse
order:',0ah,0dh,'$'
errormsg db 0ah,0dh,'Error,You should enter a number$'
ecounter dw 0
.code
.startup
print_msg msg1
read_char
cmp bl,1
jl error
2005
 S. Dandamudi
Chapter 5: Page 28
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition,
Assembly Example3
cmp bl,9
jg error
mov cl,bl
sub ch,ch
print_msg msg2
read_loop:
read_char
test bl,01h
jnz next
push bx
inc ecounter
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 S. Dandamudi
Chapter 5: Page 29
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition,
Assembly Example3
next: newline
loop read_loop
print_msg resmsg
mov cx,ecounter
print_loop:
print_char
loop print_loop
exit:.exit
error:print_msg errormsg
jmp exit
end
2005
 S. Dandamudi
Chapter 5: Page 30
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition,
Uses of the Stack (cont’d)
Transfer of Control
• In procedure calls and interrupts, the return
address is stored on the stack
• Our discussion on procedure calls clarifies this
particular use of the stack
Parameter Passing
• Stack is extensively used for parameter passing
• Our discussion later on parameter passing
describes how the stack is used for this purpose
2005
 S. Dandamudi
Chapter 5: Page 31
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Procedures
• Two types
 Call-by-value
» Receives only values
» Similar to mathematical functions
 Call-by-reference
» Receives pointers
» Directly manipulates parameter storage
2005
 S. Dandamudi
Chapter 5: Page 32
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Pentium Instructions for Procedures
• Pentium provides two instructions: call and ret
• call instruction is used to invoke a procedure
• The format is
call
proc-name
proc-name is the procedure name
• Actions taken during a near procedure call:
ESP = ESP - 4
; push return address
SS:ESP = EIP
; onto the stack
EIP = EIP + relative displacement
; update EIP to point to the procedure
2005
 S. Dandamudi
Chapter 5: Page 33
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Pentium Instructions for Procedures (cont’d)
• ret instruction is used to transfer control back to
the calling procedure
• How will the processor know where to return?
 Uses the return address pushed onto the stack as part of
executing the call instruction
 Important that TOS points to this return address when
ret instruction is executed
• Actions taken during the execution of ret are:
EIP = SS:ESP
ESP = ESP + 4
2005
; pop return address
; from the stack
 S. Dandamudi
Chapter 5: Page 34
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Pentium Instructions for Procedures (cont’d)
• We can specify an optional integer in the ret
instruction
 The format is
ret
 Example:
optional-integer
ret 8
• Actions taken on ret with optional-integer are:
EIP = SS:ESP
ESP = ESP + 4 + optional-integer
2005
 S. Dandamudi
Chapter 5: Page 35
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
How Is Program Control Transferred?
Offset(hex)
00000002
00000007
0000001D
machine code(hex)
main:
. . . .
E816000000 call
sum
89C3
mov
EBX,EAX
. . . .
; end of main procedure
sum:
55
push
EBP
. . . .
; end of sum procedure
avg:
00000028
0000002D
2005
. . . .
E8F0FFFFFF call
sum
89D8
mov
EAX,EBX
. . . .
; end of avg procedure
 S. Dandamudi
Chapter 5: Page 36
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Procedure Declaration in MASM
• The syntax of procedure declaration is the
following:
name PROC
; here goes the code
; of the procedure ...
RET
name ENDP
2005
 S. Dandamudi
Chapter 5: Page 37
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Procedure Declaration in MASM
• Procedure is a part of code that can be called from
your program in order to make some specific task.
• Procedures make program more structural and
easier to understand.
• Generally procedure returns to the same point from
where it was called.
2005
 S. Dandamudi
Chapter 5: Page 38
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Parameter Passing
• Parameter passing is different and complicated
than in a high-level language
• In assembly language
» You should first place all required parameters in a mutually
accessible storage area
» Then call the procedure
• Type of storage area used
» Registers (general-purpose registers are used)
» Memory (stack is used)
• Two common methods of parameter passing:
» Register method
» Stack method
2005
 S. Dandamudi
Chapter 5: Page 39
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Parameter Passing: Register Method
• Calling procedure places the necessary parameters
in the general-purpose registers before invoking
the procedure through the call instruction
• Examples:
 Example1:
» call-by-value using the register method
» a simple sum procedure
 Example2:
» call-by-reference using the register method
» string length procedure
2005
 S. Dandamudi
Chapter 5: Page 40
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Example1 : Register Method
; Assembly language that adds two numbers using
;call by value procedure
.586
option segment:use16
.model small
print_msg Macro msg
mov dx,offset msg
mov ah,9
int 21h
ENDM
2005
 S. Dandamudi
Chapter 5: Page 41
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Example1 : Register Method
newline Macro
mov dl,0ah
mov ah,2
int 21h
mov dl,0dh
int 21h
ENDM
2005
 S. Dandamudi
Chapter 5: Page 42
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Example1 : Register Method
read_num Macro
mov ah,1
int 21h
sub al,30h
ENDM
.DATA
msg1 db 'Please input the first number:$'
msg2 db 'Please input the second number:$'
sum_msg db 'The sum is: $'
2005
 S. Dandamudi
Chapter 5: Page 43
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Example1 : Register Method
.code
.startup
print_msg msg1
read_num
mov cl,al
newline
print_msg msg2
read_num
mov ch,al
2005
 S. Dandamudi
Chapter 5: Page 44
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Example1 : Register Method
newline
call sum
print_msg sum_msg
call print_res
.exit
; return to operating system.
sum PROC
add ch,cl
RET
; return to caller.
sum ENDP
2005
 S. Dandamudi
Chapter 5: Page 45
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Example1 : Register Method
print_res PROC
mov dl,ch
add dl,30h
mov ah,2
int 21h
RET
; return to caller.
print_res ENDP
end
2005
 S. Dandamudi
Chapter 5: Page 46
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Example2 : Register Method
; Assembly language that prints the length of text
; using call by reference procedure
.586
option segment:use16
.model small
print_msg Macro msg
mov dx,offset msg
mov ah,9
int 21h
ENDM
2005
 S. Dandamudi
Chapter 5: Page 47
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Example2 : Register Method
.DATA
text db 'Assembly$'
txt db 'The length of text <$'
res db '> is $'
2005
 S. Dandamudi
Chapter 5: Page 48
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Example2 : Register Method
.code
.startup
mov bx,offset text
print_msg txt
print_msg text
print_msg res
call txt_len
add dl,'0'
mov ah,2
int 21h
.exit
2005
 S. Dandamudi
Chapter 5: Page 49
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Example2 : Register Method
txt_len PROC
sub dl,dl
loops:
mov dh,[bx]
cmp dh,'$'
je str_len_done
inc bx
inc dl
jmp loops
2005
 S. Dandamudi
Chapter 5: Page 50
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Example2 : Register Method
str_len_done:
RET
; return to caller.
txt_len ENDP
end
2005
 S. Dandamudi
Chapter 5: Page 51
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.
Pros and Cons of the Register Method
• Advantages
 Convenient and easier
 Faster
• Disadvantages
 Only a few parameters can be passed using the register
method
– Only a small number of registers are available
 Often these registers are not free
– freeing them by pushing their values onto the stack
negates the second advantage
2005
 S. Dandamudi
Chapter 5: Page 52
To be used with S. Dandamudi, “Introduction to Assembly Language Programming,” Second Edition, Springer, 2005.