Computer Organization &
Computer Organization &
Assembly Languages Assembly Languages
Pu-Jen Cheng
Procedure
Adapted from the slides prepared by Kip Irvine for the book, Assembly Language for Intel-Based Computers, 5th Ed.
Chapter Overview
Linking to an External Library
The Book's Link Library
Stack Operations
Defining and Using Procedures
The Book's Link Library
Link Library Overview
Calling a Library Procedure
Linking to a Library
Library Procedures – Overview
Si E l
Six Examples
Link Library Overview
A file containing procedures that have been compiled into machine code
¾ constructed from one or more OBJ files
To build a library, . . .
¾ start with one or more ASM source files
¾ start with one or more ASM source files
¾ assemble each into an OBJ file
¾ create an empty library file (extension .LIB)
¾ add the OBJ file(s) to the library file, using the Microsoft LIB utility
Calling a Library Procedure
• Call a library 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 ; input argument call WriteHex ; show hex number
call Crlf ; end of line
Linking to a Library
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
¾ the latter is part of the Microsoft Win32 Software Development Kit (SDK)
Your program
kernel32.lib
kernel32.dll Irvine32.lib links
executes to
links to can link to
What's Next
Linking to an External Library
The Book's Link Library
Stack Operations
Defining and Using Procedures
Library Procedures - Overview
CloseFile – Closes an open disk file
Clrscr - Clears console, locates cursor at upper left corner
CreateOutputFile - 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 Delay Pauses program execution for n millisecond interval 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 GetMaxXY - Gets number of cols, rows in console window buffer GetMseconds - Returns milliseconds elapsed since midnight
Library Procedures - Overview
(cont.)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
O I tFil O i ti fil f i t
OpenInputFile – Opens existing file for input
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
RandomRange - Generates a pseudorandom integer within a specified range
ReadChar - Reads a single character from standard input
Library Procedures - Overview
(cont.)ReadFromFile – Reads input disk file into buffer
ReadDec - Reads 32-bit unsigned decimal integer from keyboard ReadHex - Reads 32-bit hexadecimal integer from keyboard
ReadInt - Reads 32-bit signed decimal integer from keyboard ReadKey – Reads character from keyboard input buffer
ReadString - Reads string from standard input, terminated by [Enter]
SetTextColor - Sets foreground and background colors of all subsequent console text output
StrLength – Returns length of a string
WaitMsg - Displays message, waits for Enter key to be pressed WriteBin - Writes unsigned 32-bit integer in ASCII binary format.
WriteBinB – Writes binary integer in byte, word, or doubleword format WriteChar - Writes a single character to standard output
Library Procedures - Overview
(cont.)WriteDec - Writes unsigned 32-bit integer in decimal format WriteHex - Writes an unsigned 32-bit integer in hexadecimal format
WriteHexB – Writes byte, word, or doubleword in hexadecimal format
WriteInt - Writes signed 32-bit integer in decimal formatg g
WriteString - Writes null-terminated string to console window WriteToFile - Writes buffer to output file
WriteWindowsMsg - Displays most recent error message generated by MS-Windows
Example 1
.code
call Clrscr mov eax,500 call Delay
Clear the screen, delay the program for 500 milliseconds, and dump the registers and flags.
call Delay call DumpRegs
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
Sample output:
Example 2
.data
str1 BYTE "Assembly language is easy!",0
Display a null-terminated string and move the cursor to the beginning of the next screen line.
.code
mov edx,OFFSET str1 call WriteString
call Crlf
Example 3
IntVal = 35 .code
mov eax,IntVal
call WriteBin ; display binary
Display an unsigned integer in binary, decimal, and hexadecimal, each on a separate line.
call Crlf
call WriteDec ; display decimal call Crlf
call WriteHex ; display hexadecimal call Crlf
0000 0000 0000 0000 0000 0000 0010 0011 35
23
Sample output:
Example 4
.data
fileName BYTE 80 DUP(0)
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.
.code
mov edx,OFFSET fileName
mov ecx,SIZEOF fileName – 1 call ReadString
A null byte is automatically appended to the string.
Example 5
.code
mov ecx,10 ; loop counter
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.
, ; p
L1: mov eax,100 ; ceiling value
call RandomRange ; generate random int call WriteInt ; display signed int
call Crlf ; goto next display line
loop L1 ; repeat loop
Example 6
.data
str1 BYTE "Color output is easy!",0 .code
Display a null-terminated string with yellow characters on a blue background.
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.
What's Next
Linking to an External Library
The Book's Link Library
Stack Operations
Defining and Using Procedures
Stack Operations
Runtime Stack
PUSH Operation
POP Operation
PUSH and POP Instructions
Using PUSH and POP
Example: Reversing a String
Related Instructions
Runtime Stack
Imagine a stack of plates . . .
¾ plates are only added to the top
¾ plates are only removed from the top
¾ LIFO (Last-In, First-Out) structure
¾ Push & pop operations
1 2 3 4 5 6 7 8 9
10 top
bottom
Runtime Stack
Managed by the CPU, using two registers
¾ SS (stack segment)
¾ ESP (stack pointer) *
00000006 00001000
Offset SS
* SP in Real-address mode
00000006 ESP
00001000
00000FF8 00000FF4 00000FF0 00000FFC
PUSH Operation
A 32-bit push operation decrements the stack pointer by 4 and copies a value into the location pointed to by the stack pointer.
00000006 ESP 00001000 00000006 00001000
BEFORE AFTER
00000006 00000006
ESP 00001000
00000FFC 00000FF8 00000FF4 00000FF0
000000A5 ESP
00001000 00000FFC 00000FF8 00000FF4 00000FF0
PUSH 0A5h
PUSH Operation (cont.)
Same stack after pushing two more integers:
00000006 00001000
Offset
00000FFC 000000A5
ESP 00000FF8
00000FF4 00000FF0
00000001 00000002
The stack grows downward. The area below ESP is always available (unless the stack has overflowed).
POP Operation
Copies value at stack[ESP] into a register or variable.
Adds n to ESP, where n is either 2 or 4.
¾ value of n depends on the attribute of the operand receiving the data
BEFORE AFTER
00000006 000000A5 00000001
00000002 ESP
00000006 000000A5
00000001 ESP 00001000
00000FFC 00000FF8 00000FF4 00000FF0
00001000 00000FFC 00000FF8 00000FF4 00000FF0
Pop EAX
EAX = 00000002
PUSH and POP Instructions
PUSH syntax:
¾ PUSH r/m16
¾ PUSH r/m32
¾ PUSH imm32
POP syntax:
POP syntax:
¾ POP r/m16
¾ POP r/m32
When to Use Sacks
To save and restore registers
To save return address of a procedure
To pass arguments
To support local variables
Example: Using PUSH and POP
push esi ; push registers
push ecx push ebx
Save and restore registers when they contain important values. PUSH and POP instructions occur in the opposite order.
push ebx
mov esi,OFFSET dwordVal ; display some memory mov ecx,LENGTHOF dwordVal
mov ebx,TYPE dwordVal call DumpMem
pop ebx ; restore registers
pop ecx ; opposite order pop esi
Example: Nested Loop
mov ecx,100 ; set outer loop count
L1: ; begin the outer loop
push ecx ; save outer loop count
When creating a nested loop, push the outer loop counter before entering the inner loop:
mov ecx,20 ; set inner loop count
L2: ; begin the inner loop
;
;
loop L2 ; repeat the inner loop pop ecx ; restore outer loop count loop L1 ; repeat the outer loop
Related Instructions
PUSHFD and POPFD
¾ push and pop the EFLAGS register
PUSHAD pushes the 32-bit general-purpose registers on the stack
d EAX ECX EDX EBX ESP EBP ESI EDI
¾ order: EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI
POPAD pops the same registers off the stack in reverse order
¾ PUSHA and POPA do the same for 16-bit registers
Example: Reversing String
.data
aName BYTE "Abraham Lincoln",0 nameSize = ($ - aName) – 1
.code
i PROC 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
Example: Reversing String (cont.)
; 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 pop eax ; get character mov aName[esi],al ; store in string inc esi
Loop L2 exit
main ENDP END main
What's Next
Linking to an External Library
The Book's Link Library
Stack Operations
Defining and Using Procedures
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
Creating Procedures
Procedure
¾ A named block of statements that ends in a return statement
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
Documenting Procedures
A description of all tasks accomplished by the procedure.
Receives: A list of input parameters; state their usage and requirements.
Suggested documentation for each procedure:
q
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.
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
CALL and RET Instructions
The CALL instruction calls a procedure
¾ pushes offset of next instruction on the stack
¾ copies the address of the called procedure into EIP ESP = ESP - 4 ; push return address
SS:ESP = EIP ; onto the stack SS:ESP EIP ; onto the stack
EIP = EIP + relative offset (or displacement)
; update EIP to point to procedure
The RET instruction returns from a procedure
¾ pops top of stack into EIP
EIP = SS:ESP ; pop return address ESP = ESP + 4 ; from the stack
CALL-RET Example
main PROC
00000020 call MySub 00000025 mov eax,ebx .
.
main ENDP 0000025 is the offset of
the instruction
immediately following the CALL instruction
MySub PROC
00000040 mov eax,edx .
. ret
MySub ENDP 00000040 is the offset
of the first instruction inside MySub
CALL-RET Example (cont.)
00000025 ESP
EIP 00000040
The CALL
instruction pushes 00000025 onto the stack, and loads 00000040 into EIP
00000025 ESP
EIP 00000025
The RET
instruction pops 00000025 from the stack into EIP
(stack shown before RET executes)
Nested Procedure Calls
main PROC .
.
call Sub1 exit
main ENDP Sub1 PROC .
.
call Sub2 (ret to main)
By the time Sub3 is called, the stack contains all three return addresses:
ret Sub1 ENDP Sub2 PROC .
.
call Sub3 ret
Sub2 ENDP Sub3 PROC .
. ret Sub3 ENDP
( )
(ret to Sub1)
(ret to Sub2) ESP
How Is Program Control Transferred?
Offset(hex) machine code(hex)
main:
. . . . 00000002 E816000000 call sum
00000007 89C3 mov EBX,EAX
. . . .
; end of main procedure sum:
sum:
0000001D 55 push EBP
. . . .
; end of sum procedure avg:
. . . . 00000028 E8F0FFFFFF call sum
0000002D 89D8 mov EAX,EBX
. . . .
; end of avg procedure
Local and Global Labels
main PROC
jmp L2 ; error
L1:: ; global label
A local label is visible only to statements inside the same procedure. A global label is visible everywhere.
g exit
main ENDP sub2 PROC
L2: ; local label
jmp L1 ; ok
ret sub2 ENDP
Procedure Parameters
• A good procedure might be usable in many different programs
¾ but not if it refers to specific variable names
• Parameters help to make procedures flexible because parameter values can change at runtime
parameter values can change at runtime
Parameter Passing Mechanisms
Call-by-value
¾ Receives only values
¾ Similar to mathematical functions
C ll b f
Call-by-reference
¾ Receives pointers
¾ Directly manipulates parameter storage
Parameter Passing
Parameter passing is different and complicated than in a high-level language
In assembly language
¾ You should first place all required parameters in a mutually accessible storage area
¾ Then call the procedure
Types of storage area used
¾ Registers (general-purpose registers are used)
¾ Memory (stack is used)
Two common methods of parameter passing:
¾ Register method
¾ Stack method
Parameter Passing: Register Method
ArraySum PROC
mov esi,0 ; array index
mov eax,0 ; set the sum to zero mov ecx,LENGTHOF myarray ; set number of elements
The ArraySum procedure calculates the sum of an array. It makes two references to specific variable names:
Call-by-reference
, y y ;
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 ; store the sum ret
ArraySum ENDP
What if you wanted to calculate the sum of two or three arrays within the same program?
Procedure Parameters (cont.)
ArraySum PROC
; Receives: ESI points to an array of doublewords,
; ECX = number of array elements.
This version of ArraySum returns the sum of any
doubleword array whose address is in ESI. The sum is returned in EAX:
; Returns: EAX = sum
;--- 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
ret
ArraySum ENDP
Calling ArraySum
.data
array DWORD 10000h, 20000h, 30000h, 40000h theSum DWORD ?
.code
main PROC main PROC
mov esi, OFFSET array mov ecx, LENGTHOF array call ArraySum
mov theSum, eax
USES Operator
Lists the registers that will be preserved
ArraySum PROC USES esi ecx
mov eax,0 ; set the sum to zero
etc.
MASM generates the code shown in blue:
MASM generates the code shown in blue:
ArraySum PROC push esi push ecx .
.
pop ecx pop esi ret
ArraySum ENDP
When Not to Push a Register
SumOf PROC ; sum of three integers
push eax ; 1
add eax,ebx ; 2
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):
add eax,ecx ; 3
pop eax ; 4
ret
SumOf ENDP
SumOf PROC ; sum of three integers
add eax,ebx ; 2
add eax,ecx ; 3
ret
SumOf ENDP
Pros and Cons of the Register Method
Advantages
¾ Convenient and easier
¾ Faster
Disadvantages g
¾ Only a few parameters can be passed using the register method
Only a small number of registers are available
¾ Often these registers are not free
freeing them by pushing their values onto the stack negates the second advantage
Parameter Passing: Stack Method
All parameter values are pushed onto the stack before calling the procedure
Example:
push number1
h b 2
push number2
call sum
Accessing Parameters on the Stack
Parameter values are buried inside the stack
We can use the following to read number2 mov EBX,[ESP+4]
Problem: The ESP value changes with push and i
pop operations
Relative offset depends of the stack operations performed
Is there a better alternative?
¾ Use EBP to access parameters on the stack
Using BP Register to Access Parameters
Preferred method of accessing parameters on the stack is
mov EBP,ESP
mov EAX,[EBP+4]
to access number2 in the previous example
Problem: BP contents are lost!
¾ We have to preserve the contents of BP
¾ Use the stack (caution: offset value changes)
push EBP
mov EBP,ESP
Clearing the Stack Parameters
Stack state after saving EBP
Stack state after pop EBP
Stack state after executing ret
Clearing the Stack Parameters (cont.)
Two ways of clearing the unwanted parameters on the stack:
¾ Use the optional-integer in the ret instruction
in the previous example, you can use ret 4
EIP = SS:ESP
ESP = ESP + 4 + optional-integer
¾ Add the constant to ESP in calling procedure (C uses this method)
push number1 push number2 call sum
add ESP,4
Housekeeping Issues
Who should clean up the stack of unwanted parameters?
¾ Calling procedure
Need to update ESP with every procedure call
Not really needed if procedures use fixed number of
Not really needed if procedures use fixed number of parameters
C uses this method because C allows variable number of parameters
¾ Called procedure
Code becomes modular (parameter clearing is done in only one place)
Cannot be used with variable number of parameters
Housekeeping Issues (cont.)
Need to preserve the state (contents of the registers) of the calling procedure across a procedure call.
Stack is used for this purpose
Which registers should be saved?
Which registers should be saved?
¾ Save those registers that are used by the calling procedure but are modified by the called procedure
Might cause problems as the set of registers used by the calling and called procedures changes over time
¾ Save all registers (brute force method) by using pusha
Increased overhead (pusha takes 5 clocks as opposed 1 to save a register)
Housekeeping Issues (cont.)
Who should preserve the state of the calling procedure?
¾ Calling procedure
Need to know the registers used by the called procedure
Need to include instructions to save and restore registers with every procedure call
Causes program maintenance problems
¾ Called procedure
Preferred method as the code becomes modular (state preservation is done only once and in one place)
Avoids the program maintenance problems mentioned
Housekeeping Issues (cont.)
Stack state after pusha
