Procedure
Computer Organization and Assembly Languages
Yung-Yu Chuang
2007/12/24
with slides by Kip Irvine
Overview
•
•
•
•
•
Stack Operations
Defining and Using Procedures
Stack frames, parameters and local variables
Recursion
Related directives
2
Stack operations
Stacks
• LIFO (Last-In, First-Out) data structure.
• push/pop operations
• You probably have had experiences on
implementing it in high-level languages.
• Here, we concentrate on runtime stack,
directly supported by hardware in the CPU.
It is essential for calling and returning from
procedures.
4
Runtime stack
• Managed by the CPU, using two registers
– SS (stack segment)
– ESP (stack pointer) * : point to the top of the stack
usually modified by CALL, RET, PUSH and POP
SS
stack
segment
ESP
memory
* SP in Real-address mode
5
PUSH and POP instructions
• PUSH syntax:
– PUSH r/m16
– PUSH r/m32
– PUSH imm32
• POP syntax:
– POP r/m16
– POP r/m32
6
PUSH operation (1 of 2)
• A push operation decrements the stack pointer by 2 or
4 (depending on operands) and copies a value into the
location pointed to by the stack pointer.
0FEC
0FEC
0FF0
0FF0
0FF4
PUSH 0A5h
0FF8
0FF8
ESP
0FFC
ESP
1000
0FF4
00000006
0FFC
1000
000000A5
00000006
7
PUSH operation (2 of 2)
• The same stack after pushing two more integers:
0FEC
0FEC
0FF0
0FF0
ESP
0FF4
ESP
0FF8
0FFC
1000
00000001
000000A5
00000006
PUSH 01h
0FF4
0FF8
0FFC
1000
00000002
00000001
000000A5
00000006
PUSH 02h
8
POP operation
• Copies value at stack[ESP] into a register or variable.
• Adds n to ESP, where n is either 2 or 4, depending on
the attribute of the operand receiving the data
ESP
0FEC
0FEC
0FF0
0FF0
0FF4
0FF4
0FF8
0FFC
1000
00000002
00000001
ESP
0FFC
000000A5
00000006
0FF8
1000
POP EAX
00000001
000000A5
00000006
EAX=00000002
9
When to use stacks
•
•
•
•
•
Temporary save area for registers
To save return address for CALL
To pass arguments
Local variables
Applications which have LIFO nature, such as
reversing a string
10
Example of using stacks
Save and restore registers when they contain important
values. Note that the PUSH and POP instructions are in
the opposite order:
push esi
push ecx
push ebx
; push registers
mov esi,OFFSET dwordVal ; starting OFFSET
mov ecx,LENGTHOF dwordVal; number of units
mov ebx,TYPE dwordVal ;size of a doubleword
call DumpMem
; display memory
pop ebx
pop ecx
pop esi
; opposite order
11
Example: Nested Loop
When creating a nested loop, push the outer loop counter
before entering the inner loop:
mov ecx,100
L1:
push ecx
; set outer loop count
; begin the outer loop
; save outer loop count
mov ecx,20
L2:
;
;
loop L2
; set inner loop count
; begin the inner loop
pop ecx
loop L1
; repeat the inner loop
; restore outer loop count
; repeat the outer loop
12
Example: reversing a string
.data
aName BYTE "Abraham Lincoln",0
nameSize = ($ - aName) – 1
.code
main PROC
; Push the name on the stack.
mov ecx,nameSize
mov esi,0
L1:
movzx eax,aName[esi]
; get character
push eax
; push on stack
inc esi
Loop L1
13
Example: reversing a string
; Pop the name from the stack, in reverse,
; and store in the aName array.
mov ecx,nameSize
mov esi,0
L2:
pop eax
; get character
mov aName[esi],al ; store in string
inc esi
Loop L2
exit
main ENDP
END main
14
Related instructions
• PUSHFD and POPFD
– push and pop the EFLAGS register
– LAHF, SAHF are other ways to save flags
• PUSHAD pushes the 32-bit general-purpose
registers on the stack in the following order
– EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI
• POPAD pops the same registers off the stack in
reverse order
– PUSHA and POPA do the same for 16-bit registers
15
Example
MySub PROC
pushad
...
; modify some register
...
popad
Do not use this if your procedure uses
ret
registers for return values
MySub ENDP
16
Defining and using procedures
Creating Procedures
• Large problems can be divided into smaller
tasks to make them more manageable
• A procedure is the ASM equivalent of a Java or
C++ function
• Following is an assembly language procedure
named sample:
sample PROC
.
.
ret
sample ENDP
A named block of statements that ends with a return.
18
Documenting procedures
Suggested documentation for each procedure:
• A description of all tasks accomplished by the
procedure.
• Receives: A list of input parameters; state their
usage and requirements.
• Returns: A description of values returned by the
procedure.
• Requires: Optional list of requirements called
preconditions that must be satisfied before the
procedure is called.
For example, a procedure of drawing lines could assume
that display adapter is already in graphics mode.
19
Example: SumOf procedure
;----------------------------------------------SumOf PROC
;
; Calculates and returns the sum of three 32-bit
;
integers.
; Receives: EAX, EBX, ECX, the three integers.
;
May be signed or unsigned.
; Returns: EAX = sum, and the status flags
;
(Carry, Overflow, etc.) are changed.
; Requires: nothing
;----------------------------------------------add eax,ebx
add eax,ecx
ret
SumOf ENDP
20
CALL and RET instructions
• The CALL instruction calls a procedure
– pushes offset of next instruction on the stack
– copies the address of the called procedure into EIP
• The RET instruction returns from a procedure
– pops top of stack into EIP
• We used jl and jr in our toy computer for
CALL and RET, BL and MOV PC, LR in ARM.
21
CALL-RET example (1 of 2)
0000025 is the offset
of the instruction
immediately following
the CALL instruction
00000040 is the offset
of the first instruction
inside MySub
main PROC
00000020 call MySub
00000025 mov eax,ebx
.
.
main ENDP
MySub PROC
00000040 mov eax,edx
.
.
ret
MySub ENDP
22
CALL-RET example (2 of 2)
The CALL instruction
pushes 00000025 onto
the stack, and loads
00000040 into EIP
ESP
The RET instruction
pops 00000025 from
the stack into EIP
00000040
00000025
EIP
00000025
ESP
00000025
EIP
23
Nested procedure calls
0050
0100
0150
0200
0250
0300
main PROC
.
.
call Sub1
exit
main ENDP
Sub1 PROC
.
.
call Sub2
ret
Sub1 ENDP
EIP
Sub2 PROC
.
.
call Sub3
ret
Sub2 ENDP
Sub3 PROC
.
.
ret
Sub3 ENDP
Stack
24
Local and global labels
A local label is visible only to statements inside the same
procedure. A global label is visible everywhere.
main PROC
jmp L2
L1::
exit
main ENDP
sub2 PROC
L2:
jmp L1
ret
sub2 ENDP
; error!
; global label
; local label
; ok
25
Procedure parameters (1 of 3)
• A good procedure might be usable in many
different programs
• Parameters help to make procedures flexible
because parameter values can change at
runtime
• General registers can be used to pass
parameters
26
Procedure parameters (2 of 3)
The ArraySum procedure calculates the sum of an array.
It makes two references to specific variable names:
ArraySum PROC
mov esi,0
mov eax,0
; array index
; set the sum to zero
L1:
add eax,myArray[esi] ; add each integer to sum
add esi,4
; point to next integer
loop L1
; repeat for array size
mov theSum,eax
ret
ArraySum ENDP
; store the sum
27
Procedure parameters (3 of 3)
This version returns the sum of any doubleword array
whose address is in ESI. The sum is returned in EAX:
ArraySum PROC
; Recevies: ESI points to an array of doublewords,
;
ECX = number of array elements.
; Returns: EAX = sum
;-----------------------------------------------push esi
push ecx
mov eax,0
; set the sum to zero
L1: add eax,[esi]
; add each integer to sum
add esi,4
; point to next integer
loop L1
; repeat for array size
pop ecx
pop esi
ret
ArraySum ENDP
28
Calling ArraySum
.data
array DWORD 10000h, 20000h, 30000h, 40000h
theSum DWORD ?
.code
main PROC
mov
esi, OFFSET array
mov
ecx, LENGTHOF array
call
ArraySum
mov
theSum, eax
29
USES operator
• Lists the registers that will be saved (to avoid
side effects) (return register shouldn’t be saved)
ArraySum PROC USES esi ecx
mov eax,0 ; set the sum to zero
...
MASM generates the following code:
ArraySum PROC
push esi
push ecx
.
.
pop ecx
pop esi
ret
ArraySum ENDP
30
Stack frames, parameters and
local variables
Stack frame
• Also known as an activation record
• Area of the stack set aside for a procedure's
return address, passed parameters, saved
registers, and local variables
• Created by the following steps:
– Calling procedure pushes arguments on the stack
and calls the procedure.
– The subroutine is called, causing the return
address to be pushed on the stack.
– The called procedure pushes EBP on the stack, and
sets EBP to ESP.
– If local variables are needed, a constant is
subtracted from ESP to make room on the stack.
– The registers needed to be saved are pushed.
32
Stack frame
ESP
saved
registers
[EBP-4]
EBP
[EBP+4]
ESP
EBP
ebp
local
variables
ebp
ebp
ret addr
[EBP+8]
parameters
ebp
33
Explicit access to stack parameters
• A procedure can explicitly access stack
parameters using constant offsets from EBP.
– Example: [ebp + 8]
• EBP is often called the base pointer or frame
pointer because it holds the base address of the
stack frame.
• EBP does not change value during the
procedure.
• EBP must be restored to its original value when
a procedure returns.
34
Parameters
• Two types: register parameters and stack
parameters.
• Stack parameters are more convenient than
register parameters.
pushad
mov esi,OFFSET array
mov ecx,LENGTHOF array
mov ebx,TYPE array
call DumpMem
popad
register parameters
push
push
push
call
TYPE array
LENGTHOF array
OFFSET array
DumpMem
stack parameters
35
Parameters
call by value
call by reference
int sum=AddTwo(a, b); int sum=AddTwo(&a, &b);
.date
a DWORD
b DWORD
push b
push a
call AddTwo
5
6
5
6
push OFFSET b
push OFFSET a
call AddTwo
ESP
offset(a)
ESP
offset(b)
36
Stack frame example
.data
sum DWORD ?
.code
push 6
push 5
call AddTwo
mov sum,eax
;
;
;
;
AddTwo PROC
push ebp
mov ebp,esp
.
.
second argument
first argument
EAX = sum
save the sum
ebp
ESP
EBP
ret addr
[EBP+4]
5
[EBP+8]
6
[EBP+12]
37
Stack frame example
AddTwo PROC
push ebp
mov ebp,esp
; base of stack frame
mov eax,[ebp + 12] ; second argument (6)
add eax,[ebp + 8] ; first argument (5)
pop ebp
ret 8
; clean up the stack
AddTwo ENDP
; EAX contains the sum
Who should be responsible to
remove arguments? It depends
on the language model.
ebp
EBP
ret addr
[EBP+4]
5
[EBP+8]
6
[EBP+12]
38
RET Instruction
• Return from subroutine
• Pops stack into the instruction pointer (EIP or
IP). Control transfers to the target address.
• Syntax:
– RET
– RET n
• Optional operand n causes n bytes to be added
to the stack pointer after EIP (or IP) is assigned
a value.
39
Passing arguments by reference
• The ArrayFill procedure fills an array with
16-bit random integers
• The calling program passes the address of the
array, along with a count of the number of
array elements:
.data
count = 100
array WORD count DUP(?)
.code
push OFFSET array
push COUNT
call ArrayFill
40
Passing arguments by reference
ArrayFill can reference an array without
knowing the array's name:
ArrayFill PROC
push ebp
mov ebp,esp
pushad
mov esi,[ebp+12]
mov ecx,[ebp+8]
.
.
ebp
EBP
ret addr
[EBP+4]
count
[EBP+8]
offset(array)
[EBP+12]
41
Passing 8-bit and 16-bit arguments
• When passing stack argements, it is best to
push 32-bit operands to keep ESP aligned on a
doubleword boundary.
Uppercase PROC
push ebp
mov ebp, esp
mov al, [ebp+8]
cmp al, ‘a’
jb
L1
cmp al, ‘z’
ja
L1
sub al, 32
L1: pop ebp
ret 4
Uppercase ENDP
push
Call
‘x’ ; error
Uppercase
.data
charVal BYTE ‘x’
.code
movzx eax, charVal
push eax
Call Uppercase
42
Saving and restoring registers
• When using stack parameters, avoid USES.
MySub2
push
mov
mov
pop
ret
MySub2
PROC USES ecx, edx
ebp
ebp, esp
eax, [ebp+8]
ebp
4
ENDP
ESP,EBP
ebp
edx
ecx
MySub2
push
push
push
mov
mov
pop
pop
pop
ret
MySub2
PROC
ecx
edx
ebp
ebp, esp
eax, [ebp+8]
ebp
edx
ecx
4
ENDP
[EBP+8]
ret addr
parameter
[EBP+16]
43
Local variables
• The variables defined in the data segment can
be taken as static global variables.
visibility=the whole program
lifetime=program duration
• A local variable is created, used, and destroyed
within a single procedure (block)
• Advantages of local variables:
–
–
–
–
Restricted access: easy to debug, less error prone
Efficient memory usage
Same names can be used in two different procedures
Essential for recursion
44
Creating local variables
• Local variables are created on the runtime
stack, usually above EBP.
• To explicitly create local variables, subtract
their total size from ESP.
MySub PROC
push ebp
mov ebp,esp
sub esp,8
mov [ebp-4],123456h
mov
[ebp-8],0
.
[EBP-4]
ESP
EBP
[EBP+4]
[EBP+8]
ebp
ret addr
…
.
[EBP-8]
45
Local variables
• They can’t be initialized at assembly time but
can be assigned to default values at runtime.
MySub PROC
push ebp
void MySub() mov ebp, esp
sub esp, 8
{
mov DWORD PTR [ebp-4], 10
int X=10;
mov DWORD PTR [ebp-8], 20
int Y=20;
...
...
mov esp, ebp
}
pop ebp
ret
MySub ENDP
20
10
EBP
return
address
stack
ESP
EBP
46
Local variables
X_local EQU DWORD PTR [ebp-4]
Y_local EQU DWORD PTR [ebp-8]
MySub PROC
push ebp
mov ebp, esp
sub esp, 8
X_local,
mov DWORD
PTR 10
[ebp-4], 10
Y_local,
mov DWORD
PTR 20
[ebp-8], 20
...
mov esp, ebp
pop ebp
ret
MySub ENDP
47
LEA instruction (load effective address)
• The LEA instruction returns offsets of both
direct and indirect operands at run time.
– OFFSET only returns constant offsets (assemble time).
• LEA is required when obtaining the offset of a
stack parameter or local variable. For example:
CopyString PROC,
count:DWORD
LOCAL temp[20]:BYTE
mov
mov
lea
lea
edi,OFFSET count; invalid operand
esi,OFFSET temp ; invalid operand
edi,count
; ok
esi,temp
; ok
48
LEA example
void makeArray()
makeArray PROC
{
push ebp
char myString[30];
mov ebp, esp
for (int i=0; i<30; i++)
sub esp, 32
myString[i]=‘*’;
lea esi, [ebp-30]
}
mov ecx, 30
L1: mov BYTE PTR [esi], ‘*’
inc esi
loop L1
add esp 32
pop ebp
ret
makeArray ENDP
49
ENTER and LEAVE
• ENTER instruction creates stack frame for a
called procedure
– pushes EBP on the stack
push ebp
– set EBP to the base of stack frame mov ebp, esp
– reserves space for local variables sub esp, n
• ENTER nbytes, nestinglevel
– nbytes (for local variables) is rounded up to a
multiple of 4 to keep ESP on a doubleword boundary
– nestinglevel: 0 for now
MySub PROC
enter 8,0
MySub PROC
push ebp
mov ebp,esp
sub esp,8
50
ENTER and LEAVE
• LEAVE reverses the action of a previous ENTER
instruction.
MySub PROC
enter 8, 0
.
.
.
.
leave
ret
MySub ENDP
MySub PROC
push ebp
mov ebp, esp
sub esp, 8
.
.
mov esp, ebp
pop ebp
ret
MySub ENDP
51
LOCAL directive
• The LOCAL directive declares a list of local
variables
– immediately follows the PROC directive
– each variable is assigned a type
• Syntax:
LOCAL varlist
Example:
MySub PROC
LOCAL var1:BYTE, var2:WORD, var3:SDWORD
52
MASM-generated code
BubbleSort PROC
LOCAL temp:DWORD, SwapFlag:BYTE
. . .
ret
BubbleSort ENDP
MASM generates the following code:
BubbleSort PROC
push ebp
mov ebp,esp
add esp,0FFFFFFF8h ; add -8 to ESP
. . .
mov esp,ebp
pop ebp
ret
BubbleSort ENDP
53
Non-Doubleword Local Variables
• Local variables can be different sizes
• How are they created in the stack by LOCAL
directive:
– 8-bit: assigned to next available byte
– 16-bit: assigned to next even (word) boundary
– 32-bit: assigned to next doubleword boundary
54
MASM-generated code
[EBP-8]
ESP
SwapFlag
temp
[EBP-4]
EBP
ebp
mov
mov
eax, temp
bl, SwapFlag
mov
mov
eax, [ebp-4]
bl, [ebp-5]
55
Reserving stack space
• .STACK 4096
• Sub1 calls Sub2, Sub2 calls Sub3, how many
bytes will you need in the stack?
Sub1 PROC
LOCAL array1[50]:DWORD ; 200 bytes
Sub2 PROC
LOCAL array2[80]:WORD
; 160 bytes
Sub3 PROC
LOCAL array3[300]:WORD ; 300 bytes
660+8(ret addr)+saved registers…
56
Recursion
Recursion
• The process created when . . .
– A procedure calls itself
– Procedure A calls procedure B, which in turn
calls procedure A
• Using a graph in which each node is a
procedure and each edge is a procedure call,
recursion forms a cycle:
A
E
B
D
C
58
Calculating a factorial
This function calculates the factorial of integer n.
A new value of n is saved in each stack frame:
int factorial(int n)
{
if (n == 0)
return 1;
else
return n*factorial(n-1);
}
recursive calls
backing up
5! = 5 * 4!
5 * 24 = 120
4! = 4 * 3!
4 * 6 = 24
3! = 3 * 2!
3*2=6
2! = 2 * 1!
2*1=2
1! = 1 * 0!
1*1=1
0! = 1
1=1
factorial(5);
(base case)
59
Calculating a factorial
Factorial PROC
push ebp
mov ebp,esp
mov eax,[ebp+8]
cmp eax,0
ja
L1
mov eax,1
jmp L2
L1:dec eax
push eax
call Factorial
ReturnFact:
mov ebx,[ebp+8]
mul ebx
L2:pop ebp
ret 4
Factorial ENDP
;
;
;
;
get n
n > 0?
yes: continue
no: return 1
; Factorial(n-1)
; get n
; edx:eax=eax*ebx
; return EAX
; clean up stack
60
Calculating a factorial
ReturnFact:
mov ebx,[ebp+8]
mul ebx
L2:pop ebp
ret 4
Factorial ENDP
ebp
ret Factorial
0
…
Factorial PROC
push ebp
mov ebp,esp
mov eax,[ebp+8]
cmp eax,0
ja
L1
mov eax,1
jmp L2
L1:dec eax
push eax
call Factorial
push 12
call Factorial
ebp
ret Factorial
11
ebp
ret main
12
61
Related directives
.MODEL directive
• .MODEL directive specifies a program's memory
model and model options (language-specifier).
• Syntax:
.MODEL memorymodel [,modeloptions]
• memorymodel can be one of the following:
– tiny, small, medium, compact, large, huge, or flat
• modeloptions includes the language specifier:
– procedure naming scheme
– parameter passing conventions
• .MODEL flat, STDCALL
63
Memory models
• A program's memory model determines the
number and sizes of code and data segments.
• Real-address mode supports tiny, small,
medium, compact, large, and huge models.
• Protected mode supports only the flat model.
Small model: code < 64 KB, data (including stack) < 64 KB.
All offsets are 16 bits.
Flat model: single segment for code and data, up to 4 GB.
All offsets are 32 bits.
64
Language specifiers
• STDCALL (used when calling Windows functions)
– procedure arguments pushed on stack in reverse
order (right to left)
– called procedure cleans up the stack
– _name@nn (for example, _AddTwo@8)
• C
– procedure arguments pushed on stack in reverse
order (right to left)
– calling program cleans up the stack (variable number
of parameters such as printf)
– _name (for example, _AddTwo)
• PASCAL
– arguments pushed in forward order (left to right)
– called procedure cleans up the stack
• BASIC, FORTRAN, SYSCALL
65
INVOKE directive
• The INVOKE directive is a powerful replacement
for Intel’s CALL instruction that lets you pass
multiple arguments
• Syntax:
INVOKE procedureName [, argumentList]
• ArgumentList is an optional comma-delimited
list of procedure arguments
• Arguments can be:
–
–
–
–
immediate values and integer expressions
variable names
address and ADDR expressions
register names
66
INVOKE examples
.data
byteVal BYTE 10
wordVal WORD 1000h
.code
; direct operands:
INVOKE Sub1,byteVal,wordVal
; address of variable:
INVOKE Sub2,ADDR byteVal
; register name, integer expression:
INVOKE Sub3,eax,(10 * 20)
; address expression (indirect operand):
INVOKE Sub4,[ebx]
67
INVOKE example
.data
val1 DWORD 12345h
val2 DWORD 23456h
.code
INVOKE AddTwo, val1, val2
push val1
push val2
call AddTwo
68
ADDR operator
• Returns a near or far pointer to a variable,
depending on which memory model your
program uses:
• Small model: returns 16-bit offset
• Large model: returns 32-bit segment/offset
• Flat model: returns 32-bit offset
• Simple example:
.data
myWord WORD ?
.code
INVOKE mySub,ADDR myWord
69
ADDR example
.data
Array DWORD 20 DUP(?)
.code
...
INVOKE Swap, ADDR Array, ADDR [Array+4]
push OFFSET Array+4
push OFFSET Array
Call Swap
70
PROC directive
• The PROC directive declares a procedure with
an optional list of named parameters.
• Syntax:
label PROC [attributes] [USES] paramList
•paramList is a list of parameters separated by
commas. Each parameter has the following
syntax:
paramName:type
type must either be one of the standard ASM types (BYTE,
SBYTE, WORD, etc.), or it can be a pointer to one of
these types.
• Example: foo PROC C USES eax, param1:DWORD
71
PROC example
• The AddTwo procedure receives two integers and
returns their sum in EAX.
• C++ programs typically return 32-bit integers from
functions in EAX.
AddTwo PROC,
val1:DWORD,
val2:DWORD
mov eax,val1
add eax,val2
ret
AddTwo ENDP
AddTwo PROC,
push ebp
mov ebp, esp
mov eax, dword ptr [ebp+8]
add eax, dword ptr [ebp+0Ch]
leave
ret 8
AddTwo ENDP
72
PROC example
Read_File PROC USES eax, ebx,
pBuffer:PTR BYTE
LOCAL fileHandle:DWORD
mov esi, pBuffer
mov fileHandle, eax
.
.
ret
Read_File ENDP
Read_File PROC
push ebp
mov ebp, esp
add esp, 0FFFFFFFCh
push eax
push ebx
mov esi, dword ptr [ebp+8]
mov dword ptr [ebp-4], eax
.
.
pop ebx
pop eax
ret
Read_File ENDP
73
PROTO directive
• Creates a procedure prototype
• Syntax:
– label
PROTO
paramList
• Every procedure called by the INVOKE directive
must have a prototype
• A complete procedure definition can also serve
as its own prototype
74
PROTO directive
• Standard configuration: PROTO appears at top of the
program listing, INVOKE appears in the code segment,
and the procedure implementation occurs later in
the program:
MySub PROTO
; procedure prototype
.code
INVOKE MySub
; procedure call
MySub PROC
.
.
MySub ENDP
; procedure implementation
75
PROTO example
• Prototype for the ArraySum procedure,
showing its parameter list:
ArraySum PROTO,
ptrArray:PTR DWORD, ; points to the array
szArray:DWORD
; array size
ArraySum PROC USES esi, ecx,
ptrArray:PTR DWORD, ; points to the array
szArray:DWORD
; array size
76
Multimodule programs
Multimodule programs
• A multimodule program is a program whose
source code has been divided up into separate
ASM files.
• Each ASM file (module) is assembled into a
separate OBJ file.
• All OBJ files belonging to the same program
are linked using the link utility into a single
EXE file.
– This process is called static linking
78
Advantages
• Large programs are easier to write, maintain,
and debug when divided into separate source
code modules.
• When changing a line of code, only its
enclosing module needs to be assembled
again. Linking assembled modules requires
little time.
• A module can be a container for logically
related code and data
• encapsulation: procedures and variables are
automatically hidden in a module unless you
declare them public
79
Creating a multimodule program
• Here are some basic steps to follow when
creating a multimodule program:
– Create the main module
– Create a separate source code module for each
procedure or set of related procedures
– Create an include file that contains procedure
prototypes for external procedures (ones that are
called between modules)
– Use the INCLUDE directive to make your procedure
prototypes available to each module
80
Multimodule programs
• MySub PROC PRIVATE
sub1 PROC PUBLIC
• EXTERN sub1@0:PROC
• PUBLIC count, SYM1
SYM1=10
.data
count DWORD 0
• EXTERN name:type
81
INCLUDE file
The sum.inc file contains prototypes for external
functions that are not in the Irvine32 library:
INCLUDE Irvine32.inc
PromptForIntegers PROTO,
ptrPrompt:PTR BYTE,
ptrArray:PTR DWORD,
arraySize:DWORD
; prompt string
; points to the array
; size of the array
ArraySum PROTO,
ptrArray:PTR DWORD,
count:DWORD
; points to the array
; size of the array
DisplaySum PROTO,
ptrPrompt:PTR BYTE,
theSum:DWORD
; prompt string
; sum of the array
82
Main.asm
TITLE Integer Summation Program
INCLUDE sum.inc
.code
main PROC
call Clrscr
INVOKE PromptForIntegers,
ADDR prompt1,
ADDR array,
Count
...
call Crlf
INVOKE ExitProcess,0
main ENDP
END main
83
Example: ArraySum Program
• Let's review the ArraySum program from
Chapter 5.
Summation
Program (main)
Clrscr
PromptForIntegers
WriteString
ReadInt
ArraySum
DisplaySum
WriteString
WriteInt
Each of the four white rectangles will become a module.
85
AddTwo Procedure
(1 of 2)
• Recall the AddTwo Procedure
AddTwo PROC,
val1:DWORD, val2:DWORD
mov eax,val1
add eax,val2
ret
AddTwo ENDP
86
AddTwo Procedure
(2 of 2)
• MASM generates the following code when we assemble
AddTwo (from the previous panel):
AddTwo PROC,
val1:DWORD, val2:DWORD
push ebp
mov ebp, esp
mov
add
The LEAVE instruction is
shorthand for:
mov
pop
esp,ebp
ebp
eax,val1
eax,val2
leave
ret 8
AddTwo ENDP
p.229
87
Passing by value
• When a procedure argument is passed by
value, a copy of a 16-bit or 32-bit integer is
pushed on the stack. Example:
.data
myData WORD 1000h
.code
main PROC
INVOKE Sub1, myData
MASM generates the following code:
push myData
call Sub1
88
Passing by reference
• When an argument is passed by reference, its
address is pushed on the stack. Example:
.data
myData WORD 1000h
.code
main PROC
INVOKE Sub1, ADDR myData
MASM generates the following code:
push OFFSET myData
call Sub1
89
Statistics for midterm
• Average=52.82, Std. dev.=18.01
90
91
Stack Parameters
•
•
•
•
•
•
•
Register vs. Stack Parameters
INVOKE Directive
PROC Directive
PROTO Directive
Passing by Value or by Reference
Parameter Classifications
Example: Exchanging Two
Integers
• Trouble-Shooting Tips
92
Recursion
• What is recursion?
• Recursively Calculating a Sum
• Calculating a Factorial
93
Recursively Calculating a Sum
The CalcSum procedure recursively calculates the sum of an
array of integers. Receives: ECX = count. Returns: EAX = sum
CalcSum PROC
cmp ecx,0
jz L2
add eax,ecx
dec ecx
call CalcSum
L2: ret
CalcSum ENDP
Stack frame:
;
;
;
;
;
check counter value
quit if zero
otherwise, add to sum
decrement counter
recursive call
View the complete
program
94
Inspect Individual Modules
•
•
•
•
Main
PromptForIntegers
ArraySum
DisplaySum
Custom batch file for assembling and linking.
95
Sample Program output
Enter a signed integer: -25
Enter a signed integer: 36
Enter a signed integer: 42
The sum of the integers is: +53
96
Stack Frames
•
•
•
•
•
Memory Models
Language Specifiers
Explicit Access to Stack Parameters
Passing Arguments by Reference
Creating Local Variables
97
Your turn . . .
• Create a procedure named Difference that
subtracts the first argument from the second
one. Following is a sample call:
push 14
push 30
call Difference
Difference PROC
push ebp
mov ebp,esp
mov eax,[ebp + 8]
sub eax,[ebp + 12]
pop ebp
ret 8
Difference ENDP
; first argument
; second argument
; EAX = 16
; second argument
; first argument
98
PROC examples
Swap PROC,
pValX:PTR DWORD,
pValY:PTR DWORD
. . .
Swap ENDP
ReadFile PROC,
pBuffer:PTR BYTE
LOCAL fileHandle:DWORD
. . .
ReadFile ENDP
99
Trouble-Shooting Tips
• Save and restore registers when they are modified
by a procedure.
– Except a register that returns a function result
• When using INVOKE, be careful to pass a pointer to the correct
data type.
• For example, MASM cannot distinguish between a DWORD
argument and a PTR BYTE argument.
• Do not pass an immediate value to a procedure that expects a
reference parameter.
• Dereferencing its address will likely cause a generalprotection fault.
100
Recursively calculating a sum
The CalcSum procedure recursively calculates the sum of
1 to n. Receives: ECX = count. Returns: EAX = sum
mov ecx, 5
mov eax, 0
call CalcSum
CalcSum PROC
cmp ecx,0
jz L2
add eax,ecx
dec ecx
call CalcSum
L2: ret
CalcSum ENDP
;
;
;
;
;
check counter value
quit if zero
otherwise, add to sum
decrement counter
recursive call
Stack frame:
101
PROC examples
FillArray receives a pointer to an array of bytes, a
single byte fill value that will be copied to each element
of the array, and the size of the array.
FillArray PROC,
pArray:PTR BYTE, fillVal:BYTE
arraySize:DWORD
mov ecx,arraySize
mov esi,pArray
mov al,fillVal
L1:mov [esi],al
inc esi
loop L1
ret
FillArray ENDP
102
LOCAL directive
Examples:
LOCAL flagVals[20]:BYTE
; array of bytes
LOCAL pArray:PTR WORD ; pointer to an array
myProc PROC,
p1:PTR WORD
LOCAL t1:BYTE,
t2:WORD,
t3:DWORD,
t4:PTR DWORD
; procedure
; parameter
; local variables
103
Program design using procedures
• Top-Down Design (functional decomposition)
involves the following:
– design your program before starting to code
– break large tasks into smaller ones
– use a hierarchical structure based on procedure
calls
– test individual procedures separately
104
Integer summation program (1 of 4)
Spec.: Write a program that prompts the user for multiple
32-bit integers, stores them in an array, calculates the
sum of the array, and displays the sum on the screen.
Main steps:
• Prompt user for multiple integers
• Calculate the sum of the array
• Display the sum
105
Procedure design (2 of 4)
Main
Clrscr
PromptForIntegers
WriteString
ReadInt
ArraySum
DisplaySum
WriteString
WriteInt
; clear screen
; display string
; input integer
; sum the integers
; display string
; display integer
106
Structure chart (3 of 4)
Summation
Program (main)
Clrscr
PromptForIntegers
WriteString
ReadInt
ArraySum
DisplaySum
WriteString
WriteInt
107
PromptForIntegers
;--------------------------------------------------PromptForIntegers PROC
;
; Prompts the user for an array of integers, and
; fills the array with the user's input.
; Receives: ESI points to the array,
;
ECX = array size
; Returns: nothing
;-------------------------------------------------pushad
; save all registers
mov
cmp
jle
edx,OFFSET prompt1 ; address of the prompt
ecx,0
; array size <= 0?
L2
; yes: quit
108
PromptForIntegers
L1:
call
call
call
mov
add
loop
WriteString
ReadInt
Crlf
[esi],eax
esi,4
L1
;
;
;
;
;
display string
read integer into EAX
go to next output line
store in array
next integer
L2:
popad
; restore all registers
ret
PromptForIntegers ENDP
109
PromptForIntegers
;--------------------------------------------------DisplaySum PROC
; Displays the sum on the screen
; Receives: EAX = the sum
; Returns: nothing
;--------------------------------------------------push edx
mov edx,OFFSET prompt2 ; display message
call WriteString
call WriteInt
; display EAX
call Crlf
pop edx
ret
DisplaySum ENDP
110
Code fragment
IntegerCount = 3
; array size
.data
prompt1 BYTE "Enter a signed integer: ",0
prompt2 BYTE "The sum of the integers is: ",0
array
DWORD IntegerCount DUP(?)
.code
main PROC
call Clrscr
mov esi,OFFSET array
mov ecx,IntegerCount
call PromptForIntegers
call ArraySum
call DisplaySum
exit
main ENDP
111
Sample output (4 of 4)
Enter a signed integer: 550
Enter a signed integer: -23
Enter a signed integer: -96
The sum of the integers is: +431
112
113
Offset
00001000
00000006
ESP
00000FFC
00000FF8
00000FF4
00000FF0
114
Example: reversing a string
• Use a loop with indexed addressing
• Push each character on the stack
• Start at the beginning of the string, pop the stack in
reverse order, insert each character into the string
• Source code
• Q: Why must each character be put in EAX before it is
pushed?
Because only word (16-bit) or doubleword (32-bit) values
can be pushed on the stack.
115
Your turn . . .
• Using the String Reverse program as a starting
point,
• #1: Modify the program so the user can input a string
of up to 50 characters.
• #2: Modify the program so it inputs a list of 32-bit
integers from the user, and then displays the integers
in reverse order.
116
Your Turn . . .
• Write a program that does the following:
– Assigns integer values to EAX, EBX, ECX, EDX, ESI,
and EDI
– Uses PUSHAD to push the general-purpose
registers on the stack
– Using a loop, the program pops each integer from
the stack and displays it on the screen
117
Defining and using procedures
•
•
•
•
•
•
•
•
•
Creating Procedures
Documenting Procedures
Example: SumOf Procedure
CALL and RET Instructions
Nested Procedure Calls
Local and Global Labels
Procedure Parameters
Flowchart Symbols
USES Operator
118
Your turn . . .
Draw a flowchart that expresses the
following pseudocode:
input exam grade from the user
if( grade > 70 )
display "Pass"
else
display "Fail"
endif
119
. . . (Solution)
begin
input exam grade
yes
no
grade > 70?
display "Pass"
display "Fail"
end
120
Your turn . . .
• Modify the flowchart in the previous slide to
allow the user to continue to input exam scores
until a value of –1 is entered
121
When not to push a register
The sum of the three registers is stored in EAX on line (3), but
the POP instruction replaces it with the starting value of EAX on
line (4):
SumOf PROC
; sum of three integers
push eax
; line 1
add eax,ebx
; line 2
add eax,ecx
; line 3
pop eax
; line 4
ret
SumOf ENDP
122
Flowchart symbols
• The following symbols are the basic building
blocks of flowcharts:
begin / end
manual input
process (task)
procedure
call
display
decision
yes
no
123
ArraySum Procedure
begin
Flowchart for
the ArraySum
Procedure
push esi, ecx
eax = 0
push esi
push ecx
mov eax,0
add eax,[esi]
add esi, 4
cx = cx - 1
AS1:
add eax,[esi]
add esi,4
loop AS1
pop
pop
yes
ecx
esi
CX > 0?
no
pop ecx, esi
end
124
Overview
• Stack Frames (a communication protocol
between high-level-language procedures)
• Stack Parameters (passing by value, passing by
reference, memory model and language
specifiers)
• Local Variables (creating and initializing on the
stack, scope and lifetime, LOCAL)
• Recursion
• Related directives: INVOKE, PROC, PROTO
• Creating Multimodule Programs
125
WriteStackFrame Procedure
• Displays contents of current stack frame
WriteStackFrame PROTO,
numParam:DWORD, ; # of passed parameters
numLocalVal: DWORD, ; # of DWordLocal
; variables
numSavedReg: DWORD ; # of saved registers
126
WriteStackFrame Example
main PROC
mov eax, 0EAEAEAEAh
mov ebx, 0EBEBEBEBh
INVOKE aProc, 1111h, 2222h
exit
main ENDP
aProc PROC USES eax ebx,
x: DWORD, y: DWORD
LOCAL a:DWORD, b:DWORD
PARAMS = 2
LOCALS = 2
SAVED_REGS = 2
mov a,0AAAAh
mov b,0BBBBh
INVOKE WriteStackFrame, PARAMS, LOCALS, SAVED_REGS
127
WriteStackFrame Example
Stack Frame
00002222 ebp+12 (parameters)
00001111 ebp+8 (parameters)
00401083 ebp+4 (return address)
0012FFF0 ebp+0 (saved ebp)  ebp
0000AAAA ebp-4 (local variable)
0000BBBB ebp-8 (local variable)
EAEAEAEA ebp-12 (saved register)
EBEBEBEB ebp-16 (saved register)  esp
128
Parameter classifications
• An input parameter is data passed by a calling program
to a procedure.
– The called procedure is not expected to modify the
corresponding parameter variable, and even if it does,
the modification is confined to the procedure itself.
• An output parameter is created by passing a pointer to a
variable when a procedure is called.
• The procedure does not use any existing data from the
variable, but it fills in a new value before it returns.
• An input-output parameter represents a value passed as
input to a procedure, which the procedure may modify.
• The same parameter is then able to return the changed
data to the calling program.
129
Example: exchanging two integers
The Swap procedure exchanges the values of two 32-bit
integers. pValX and pValY do not change values, but the
integers they point to are modified.
Swap PROC USES eax esi edi,
pValX:PTR DWORD, ; pointer to first integer
pValY:PTR DWORD ; pointer to second integer
mov esi,pValX
; get pointers
mov edi,pValY
mov eax,[esi]
; get first integer
xchg eax,[edi]
; exchange with second
mov [esi],eax
; replace first integer
ret ; MASM changes it to ret 8 due to PROC
Swap ENDP
130