RU Register Values After Stack Operations Questions

Assembly Language for x86 ProcessorsEighth Edition
Chapter 5
Procedures
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5-1
Chapter Overview
• Stack Operations
• Defining and Using Procedures
• Linking to an External Library
• The Irvine32 Library
• 64-Bit Assembly Programming
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5-2
Stack Operations
• Runtime Stack
• PUSH Operation
• POP Operation
• PUSH and POP Instructions
• Using PUSH and POP
• Example: Reversing a String
• Related Instructions
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5-3
Runtime Stack (1 of 2)
• Imagine a stack of plates . . .
– plates are only added to the top
– plates are only removed from the top
– LIFO structure
10
9
8
7
6
5
4
3
2
1
top
bottom
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5-4
Runtime Stack (2 of 2)
• Managed by the CPU, using two registers
– SS (stack segment)
– ESP (stack pointer) *
 4096
 4092
 4088
 …
Offset
00001000
00000006
ESP
00000FFC
00000FF8
00000FF4
00000FF0
* SP in Real-address mode
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5-5
PUSH Operation (1 of 2)
• A 32-bit push operation decrements the stack pointer by
4 and copies a value into the location pointed to by the
stack pointer.
BEFORE
AFTER
00001000
00000006
00000FFC
00000FFC
000000A5
00000FF8
00000FF8
00000FF4
00000FF4
00000FF0
00000FF0
00001000
00000006
ESP
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ESP
5-6
PUSH Operation (2 of 2)
• Same stack after pushing two more integers:
Offset
00001000
00000006
00000FFC
000000A5
00000FF8
00000001
00000FF4
00000002
ESP
00000FF0
The stack grows downward. The area below ESP is
always available (unless the stack has overflowed).
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5-7
POP Operation
• POP – Copies value at stack[ESP] into a register or variable.
• Adds n to ESP, where n is either 2 or 4.
– value of n depends on the attribute of the operand receiving the
data
BEFORE
AFTER
00001000
00000006
00001000
00000006
00000FFC
000000A5
00000FFC
000000A5
00000FF8
00000001
00000FF8
00000001
00000FF4
00000002
00000FF0
ESP
ESP
00000FF4
00000FF0
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5-8
PUSH and POP Instructions
• PUSH syntax:
– PUSH r/m16
– PUSH r/m32
– PUSH imm32
• POP syntax:
– POP r/m16
– POP r/m32
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Using PUSH and POP
Save and restore registers when they contain important
values. PUSH and POP instructions occur in the opposite
order.
push esi
push ecx
push ebx
; push registers
mov esi,OFFSET dwordVal
mov ecx,LENGTHOF dwordVal
mov ebx,TYPE dwordVal
call DumpMem
; display some memory
pop ebx
pop ecx
pop esi
; restore registers
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Example: Nested Loop
When creating a nested loop, push the outer loop counter
before entering the inner loop:
mov ecx,100
L1:
push ecx
mov ecx,20
L2:
; set outer loop count
; begin the outer loop
; save outer loop count
; set inner loop count
; begin the inner loop
;
;
loop L2
; repeat the inner loop
pop ecx
loop L1
; restore outer loop count
; repeat the outer loop
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Irvine, Kip R. Assembly Language
for x86 Processors 7/e, 2015.
Example: Nested Loop
How many times does the outer loop iterate?
The inner loop?
mov ecx,100
L1:
push ecx
mov ecx,20
L2:
; set outer loop count
; begin the outer loop
; save outer loop count
; set inner loop count
; begin the inner loop
;
;
loop L2
; repeat the inner loop
pop ecx
loop L1
; restore outer loop count
; repeat the outer loop
12
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5 – 12
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 back into the string
• 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.
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Related Instructions
• PUSHFD and POPFD
– push and pop the EFLAGS register
• PUSHAD pushes the 32-bit general-purpose registers on
the stack
– 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
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5 – 15
What’s Next
• Stack Operations
• Defining and Using Procedures
• Linking to an External Library
• The Irvine32 Library
• 64-Bit Assembly Programming
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5 – 17
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
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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
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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.
If a procedure is called without its preconditions satisfied, it will
probably not produce the expected output.
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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
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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
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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
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CALL-RET Example (2 of 2)
The CALL instruction
pushes 00000025 onto the
stack, and loads 00000040
into EIP
The RET instruction pops
00000025 from the stack
into EIP
00000025
ESP
00000040
EIP
00000025
ESP
00000025
EIP
(stack shown before RET executes)
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Nested Procedure Calls
main PROC
.
.
call Sub1
exit
main ENDP
Sub1 PROC
.
.
call Sub2
ret
Sub1 ENDP
Sub2 PROC
.
.
call Sub3
ret
Sub2 ENDP
By the time Sub3 is called, the stack
contains all three return addresses:
(ret to main)
(ret to Sub1)
(ret to Sub2)
ESP
Sub3 PROC
.
.
ret
Sub3 ENDP
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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
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Procedure Parameters (1 of 3)
• A good procedure might be usable in many different
programs
– but not if it refers to specific variable names
• Parameters help to make procedures flexible because
parameter values can change at runtime
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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
; array index
mov eax,0
; set the sum to zero
mov ecx,LENGTHOF myarray ; set number of elements
L1:
add eax,myArray[esi]
add esi,4
loop L1
mov theSum,eax
ret
ArraySum ENDP
; add each integer to sum
; point to next integer
; repeat for array size
; store the sum
What if you wanted to calculate the sum of two or three arrays within the
same program?
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Procedure Parameters (3 of 3)
This version of ArraySum returns the sum of any doubleword
array whose address is in ESI. The sum is returned in EAX:
ArraySum PROC
; Receives: ESI points to an array of doublewords,
; ECX = number of array elements.
; Returns: EAX = sum
;—————————————————————————————-mov eax,0
; set the sum to zero
L1: add eax,[esi]
add esi,4
loop L1
; add each integer to sum
; point to next integer
; repeat for array size
ret
ArraySum ENDP
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USES Operator
• Lists the registers that will be preserved
ArraySum PROC USES esi ecx
mov eax,0
etc.
; set the sum to zero
MASM generates the code shown in cyan:
ArraySum PROC
push esi
push ecx
.
.
pop ecx
pop esi
ret
ArraySum ENDP
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What’s Next (1 of 3)
• Stack Operations
• Defining and Using Procedures
• Linking to an External Library
• The Irvine32 Library
• 64-Bit Assembly Programming
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5 – 32
Linking to an External Library
• What is a Link Library?
• How the Linker Works
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What is a Link Library?
• A file containing procedures that have been compiled into
machine code
– constructed from one or more OBJ files
• To build a library, . . .
– start with one or more ASM source files
– assemble each into an OBJ file
– create an empty library file (extension .LIB)
– add the OBJ file(s) to the library file, using the Microsoft LIB utility
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How The Linker Works
• Your programs link to Irvine32.lib using the linker
command inside a batch file named make32.bat.
• Notice the two LIB files: Irvine32.lib, and kernel32.lib
– the latter is part of the Microsoft Win32 Software Development Kit
(SDK)
Your program
links
to
Irvine32.lib
links to
can link to
kernel32.lib
executes
kernel32.dll
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What’s Next (2 of 3)
• Stack Operations
• Defining and Using Procedures
• Linking to an External Library
• The Irvine32 Library
• 64-Bit Assembly Programming
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5 – 36
Calling Irvine32 Library Procedures
• Call each procedure using the CALL instruction. Some
procedures require input arguments. The INCLUDE
directive copies in the procedure prototypes (declarations).
• The following example displays “1234” on the console:
INCLUDE Irvine32.inc
.code
mov eax,1234h
call WriteHex
call Crlf
; input argument
; show hex number
; end of line
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Library Procedures – Overview (1 of 12)
CloseFile – Closes an open disk file
Clrscr – Clears console, locates cursor at upper left corner
Create OutputFile – Creates new disk file for writing in output
mode
Crlf – Writes end of line sequence to standard output
Delay – Pauses program execution for n millisecond interval
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Library Procedures – Overview (2 of 12)
DumpMem – Writes block of memory to standard output in
hex
DumpRegs – Displays general-purpose registers and flags
(hex)
GetCommandtail – Copies command-line args into array of
bytes
GetDateTime – Gets the current date and time from the
system
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Library Procedures – Overview (3 of 12)
GetMaxXY – Gets number of cols, rows in console window
buffer
GetMseconds – Returns milliseconds elapsed since midnight
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Library Procedures – Overview (4 of 12)
GetTextColor – Returns active foreground and background
text colors in the console window
Gotoxy – Locates cursor at row and column on the console
IsDigit – Sets Zero flag if AL contains ASCII code for decimal
digit (0–9)
MsgBox, MsgBoxAsk – Display popup message boxes
OpenInputFile – Opens existing file for input
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Library Procedures – Overview (5 of 12)
ParseDecimal32 – Converts unsigned integer string to binary
ParseInteger32 – Converts signed integer string to binary
Random32 – Generates 32-bit pseudorandom integer in the
range 0 to FFFFFFFFh
Randomize – Seeds the random number generator
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Library Procedures – Overview (6 of 12)
RandomRange – Generates a pseudorandom integer within
a specified range
ReadChar – Reads a single character from standard input
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Library Procedures – Overview (7 of 12)
ReadDec – Reads 32-bit unsigned decimal integer from
keyboard
ReadFromFile – Reads input disk file into buffer
ReadHex – Reads 32-bit hexadecimal integer from keyboard
ReadInt – Reads 32-bit signed decimal integer from
keyboard
ReadKey – Reads character from keyboard input buffer
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Library Procedures – Overview (8 of 12)
ReadString – Reads string from stdin, terminated by [Enter]
SetTextColor – Sets foreground/background colors of all
subsequent text output to the console
Str_compare – Compares two strings
Str_copy – Copies a source string to a destination string
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Library Procedures – Overview (9 of 12)
Str_length – Returns the length of a string in EAX
Str_trim – Removes unwanted characters from a string.
Str_ucase – Converts a string to uppercase letters.
WaitMsg – Displays message, waits for Enter key to be
pressed
WriteBin – Writes unsigned 32-bit integer in ASCII binary
format.
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Library Procedures – Overview (10 of 12)
WriteBinB – Writes binary integer in byte, word, or
doubleword format
WriteChar – Writes a single character to standard output
WriteDec – Writes unsigned 32-bit integer in decimal format
WriteHex – Writes an unsigned 32-bit integer in hexadecimal
format
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Library Procedures – Overview (11 of 12)
WriteHexB – Writes byte, word, or doubleword in
hexadecimal format
WriteInt – Writes signed 32-bit integer in decimal format
WriteStackFrame – Writes the current procedure’s stack
frame to the console.
WriteStackFrameName – Writes the current procedure’s
name and stack frame to the console.
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Library Procedures – Overview (12 of 12)
WriteString – Writes null-terminated string to console window
WriteToFile – Writes buffer to output file
WriteWindowsMsg – Displays most recent error message
generated by MS-Windows
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Example 1
Clear the screen, delay the program for 500 milliseconds,
and dump the registers and flags.
.code
call Clrscr
mov eax,500
call Delay
call DumpRegs
Sample output:
EAX=00000613 EBX=00000000 ECX=000000FF EDX=00000000
ESI=00000000 EDI=00000100 EBP=0000091E ESP=000000F6
EIP=00401026 EFL=00000286 CF=0 SF=1 ZF=0 OF=0
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Example 2
Display a null-terminated string and move the cursor to the
beginning of the next screen line.
.data
str1 BYTE “Assembly language is easy!”,0
.code
mov edx,OFFSET str1
call WriteString
call Crlf
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Example 2a
Display a null-terminated string and move the cursor to the
beginning of the next screen line (use embedded CR/LF)
.data
str1 BYTE “Assembly language is easy!”,0Dh,0Ah,0
.code
mov edx,OFFSET str1
call WriteString
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Example 3
• Display an unsigned integer in binary, decimal, and
hexadecimal, each on a separate line.
IntVal = 35
.code
mov eax,IntVal
call WriteBin
call Crlf
call WriteDec
call Crlf
call WriteHex
call Crlf
; display binary
; display decimal
; display hexadecimal
• Sample output:
0000 0000 0000 0000 0000 0000 0010 0011
35
23
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Example 4
Input a string from the user. EDX points to the string and
ECX specifies the maximum number of characters the user
is permitted to enter.
.data
fileName BYTE 80 DUP(0)
.code
mov edx,OFFSET fileName
mov ecx,SIZEOF fileName – 1
call ReadString
A null byte is automatically appended to the string.
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Example 5
Generate and display ten pseudorandom signed integers in
the range 0 – 99. Pass each integer to WriteInt in EAX and
display it on a separate line.
.code
mov ecx,10
; loop counter
L1: mov eax,100
call RandomRange
call WriteInt
call Crlf
loop L1
; ceiling value
; generate random int
; display signed int
; goto next display line
; repeat loop
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Example 6
Display a null-terminated string with yellow characters on a
blue background.
.data
str1 BYTE “Color output is easy!”,0
.code
mov eax,yellow + (blue * 16)
call SetTextColor
mov edx,OFFSET str1
call WriteString
call Crlf
The background color is multiplied by 16 before being added
to the foreground color.
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Summary
• Procedure – named block of executable code
• Runtime stack – LIFO structure
– holds return addresses, parameters, local variables
– PUSH – add value to stack
– POP – remove value from stack
• Use the Irvine32 library for all standard I/O and
data conversion
– Want to learn more? Study the library source code in
the c:\Irvine\Examples\Lib32 folder
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Extra
55 64 67 61 6E 67 65 6E
Udgangen
End (Danish)
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Copyright
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