Converting a Negative Decimal Number to the 2’s Complement Representation :

Converting a Negative Decimal Number to the 2’s Complement Representation:

• Represent (-18)10 as a signed 8-bit binary number

• Solution:• Abs(-18)10 : 0001 0010

• 1’s Complement: 1110 1101 • Add 1: + 0000 0001

• Result: 1110 1110

• Therefore, (-18)10 = (1110 1110) 2

1

Converting a Negative 2’s Complement Number to Decimal Representation:

• Example: Represent (1001 1000)2 as a signed decimal number• Since MSB = 1 then the number is negative• Use signed conversion process

• Solution:The number: 1001

1000

1’s Complement: 0110 0111

Add 1: + 0000 0001

Result: 0110 1000

Decimal 104

-ve sign -104

• Therefore (1001 1000)2 = (-104)10

Addition of 8-bit (positive and negative) signed number:

• Perform the addition: 8 + (-17)

• Find the 2’s complement representation

(8)10 = 0000 1000(-17)10 = ???

(17)2 0001 0001 1’s complement: 1110 1110Add 1 + 0000 0001

Result 1110 1111

• Therefore (-17)10 = (1110 1111)2

• Now

(8)10 0000 1000(-17)10 + 1110 1111

Result (1111 0111)2 = (?) 10

• Convert the result to decimal representation:

1111 0111

take 1’s complement 0000 1000Add 1 + 0000 0001

Result (0000 1001)2 = (+9)10

Therefore, (1111 0111)2 is -9 in decimal

• There are eight general-purpose registers, six segment registers, a

register that holds processor status flags (EFLAGS), and an instruction pointer (EIP).

• EAX ( Accumulator) is automatically used by multiplication and division instructions. It is often called the extended accumulator register for accumulating operands and results .

• EBP ( Base) is used by high-level languages to reference function parameters and local variables on the stack (data on the stack). It should not be used for ordinary arithmetic or data transfer except at an advanced level of programming. It is often called the extended frame pointer register.

• ECX ( Count) used as counter for string and loop operations. • EDX (Data) register that can be used for I/O pointer. • ESP stack pointer register.

• ESI and EDI are used by high-speed memory transfer instructions. They are sometimes called the extended source index and extended destination index registers.

Status Flags

• The Status flags reflect the outcomes of arithmetic and logical operations performed by the CPU.

• They are the Overflow, Sign, Zero, Auxiliary Carry, Parity, and Carry flags. Their abbreviations are shown immediately after their names.

• The Carry flag (CF) is set when the result of an unsigned arithmetic operation is too large to fit into the destination.

• The Overflow flag (OF) is set when the result of a signed arithmetic operation is too wide (too many bits) to fit into the destination.

• The Sign flag (SF) is set when the result of an arithmetic or logical operation generates a negative result.

• The Zero flag (ZF) is set when the result of an arithmetic or logical operation generates a result of zero.

• The Auxiliary Carry flag is set when an arithmetic operation causes a carry from bit 3 to bit 4 in an 8-bit operand.

• The Parity flag sums the number of bits that are set in a number, and indicates whether the sum is odd or even.

Reserved Words

• Assembly language has a list of words called reserved words. These have special meaning and can be used in their correct context. Reserved words can be any of the following:

• Instruction mnemonics, such as MOV, ADD, or MUL, which correspond to built-in operations preformed by Intel processor,

• Directives, which tell MAZM how to assemble programs.

• Attributes, which provide size and usage information for variables and operands. Examples are BYTE and WORD.

• Operators, used in constant expressions.

• A complete list of MAZM reserved words will be found in Appendix D.

Identifiers

• An identifier is a programmer chosen name. it might identify a variable, a constant, a procedure, or a code label. Keep the following in mind when creating identifiers:

• They may contain between 1 and 247 characters.• They are not case sensitive.• The first character must be either a letter (A…Z, a…z ),

underscore(_), $. Subsequent character may also be digits.• An identifier can not be the same as an assembler reserved word.

• Example of identifiers

• Var1 count $first Max open

Directive

• A directive is a command that is recognized and acted upon by the assembler as the program's source cod is being assembled.

• Directives are being used for defining logical segments, choosing a model, defining variables, creating procedures, and so on.

• Different capitalization of the same directive are assumed to be equivalent. For example the assembler does not recognize any

difference between .data, .DATA, and .Data.

• Examples of directives are:

• .Data ‘identifies the area of a program that contains variables’• .code ‘ identifies the area of a program that contains instructions’• A-name proc ‘identifies the beginning of procedures

• It would take a very long time to learn all the directives in MAZM, so we concentrate on the few that are most essential.

Comments

• Comments as you probably know, are an important way for the writer of a program to communicate information about how the program works to the person reading the source code.

• Comments can be specified by semicolon.

• Example:

• ;This line is coment;

• For Block comments you can use COMMENT directive and a user specified symbol.

• Example:

COMMENT &This line is commentthis line is also comment

&

Program Template

TITEL Program Template ; Program Description: ; Author: ; Creation Date: ; Revisions: ; Date: Modified by:

INCLUDE Irvine32.inc .data

; (insert variables here) .code main PROC ; (insert executable instructions here)

exit main ENDP

; (insert additional procedure here) END main

Example: TITLE Hello Program ; Program Description: This program will display Hello World message on the ; screen ; Author: Sahar Mosleh ; Creation Date: August 30, 2005 INCLUDE Irvine32.inc .data

; (insert variables here) prompt1 byte “Hello World”,0 ; Store the prompt1 in

; memory .code main PROC ; (insert executable instructions here)

call clrscr ; clear the screenmov edx, offset prompt1 ; move the address of prompt to

edxcall writestring ; display the string that is stored at

;the address pointed by edx

call crlf ; move the courser to next lineexit

main ENDP; (insert additional procedure here)

END main

Example: TITLE Print Integer ; Program Description: This program will display integer number 216543 on the ; screen ; Author: Sahar Mosleh ; Creation Date: August 30, 2005 INCLUDE Irvine32.inc .data

; (insert variables here) .code main PROC ; (insert executable instructions here)

call clrscr ; clear the screenmov eax, 2156543 ; move the integer to eaxcall writeInt ; display the INT that is stored at

eax call crlf ; move the courser to next line

exit main ENDP

; (insert additional procedure here) END main

Example: TITLE Input Integer ; Program Description: This program will read integer from user and out put ;it on the screen ; Author: Sahar Mosleh ; Creation Date: August 30, 2005 INCLUDE Irvine32.inc .data

; (insert variables here) prompt1 byte “Please input an intege”,0 ; Store the prompt1 in

; memory .code main PROC mov edx, offset prompt1 ; move the address of prompt to edx

call writestring ; display the string that is stored at ;the address pointed by edx

call crlf ; move the courser to next linecall Readint ;read the int from user and put it in

eaxcall writeint ;display the content of eaxexit

main ENDP; (insert additional procedure here)

END main

LOOP Instruction

• The LOOP instruction provides a simple way to repeat a block of statements a specific number of times.

• ECX is automatically used as a counter and is decremented each time the loop repeats.

• The Loop instruction involves two steps:

• First, it subtracts 1 from ECX. • Next it compares ECX to zero. If ECX is not equal to zero, a

jump is taken to the label identified instruction following the loop.

• Example:

• In the following example, we add 1 to EAX each time the loop repeats. When the loop ends, EAX = 5 and ECX = 0

move eax,0move ecx5

L1:Inc eaxLoop L1

• A common programming error is to inadvertently initialize ecx to zero before beginning of the a loop.

• If this happens, the Loop instruction decrements ECX to FFFFFFFFh, and the loop repeats 4,294,967,296 times.

Nested loops• When You must create a loop inside another loop, the problem

arises of what to do with the counter in ECX. Saving the counter loop count in a variable is a good solution:

.datacount Dword ?

.codeMov ecx,100 ;set outer loop counter

L1:Mov count,ecx ;save outer loop counterMov ecx,20 ;set inner loop counter

L2:::Loop L2 ;repeat the inner loop count

Mov ecx, count ;restore outer loop countloopL1 ;repeat the outer loop

Direct memory operand

• Example:

.dataVar1 Dword 100h::.codeMov eax var1

Indirect operand• An indirect operand can be any 32-bit general purpose register

( EAX,EBX,ECX,EDX,ESI,EDI,EBP, and ESP) surrounded by brackets. The register is assumed to contain the offset of some data. For example ESI contains the offset of variable 1:

.data Val1 byte 10 h.codeMov esi, offset val1

• If a move instruction uses the indirect operand as the source, the pointer in ESI is dereferenced and a byte is moved to EAX

Mov EAX [esi] Eax = 10 h

• Or if the indirect operand is the destination operand, a new value is placed in memory at the location pointed to by the register:

Mov [esi] EBX

Array

• Indirect operands are practically useful when dealing with arrays because an indirect operand’s value can easily be modified.

• Similar to an array subscript, an indirect operand can point to different array elements.

• For example, ArrayB contain three bytes. We can increment ESI and make it to point each byte, in order:

.dataArrayB Byte 10h, 20h, 30h

.codeMov esi, offset ArrayBMov a1, [esi]

inc esiMov a1, [esi]Inc esiMov a1, [esi]

• MASM has a number of operators that are effective tools for describing and addressing variables:

• The Offset operator returns the distance of a variable from the beginning of it’s enclosing segment

• The DUP operator generates a repeated storage allocation.

• The TYPE operator returns the size ( in bytes ) of each element in an array

• LENGHTOF operator returns the number of elements in an array

• The SIZEOF operator returns the number of bytes used by an array initializer.

• These operators are only a small subset of the operators supported by MASM. You may want to view the complete list in Appendix D.

• Example:

ArrayB Byte 10h, 20h, 30h, 40h

ArrayDW Dword 1000,2000,3000,4000

• The pointers to the above arrays can be define as:

ptrB Dword ArrayB

PtrDW Dword ArrayDW

• Example:TITLE pointersInclude Irvine32.inc.data

arrayB Byte 10h,20h,30harrayD Dword 4,5,6

Ptr1 Dword arrayB ;pointer to arrayBPtr2 Dword arrayD ;pointer to arrayD

.codemain proc

mov esi, ptr1 ;move the address (pointer to arrayB) to esimov a1 [esi]mov esi, ptr2 ;move the address (pointer to arrayD) to esimov eax,[esi]

ExitMain ENDPEND main

.data

Buffer Byte 50 DUP(0) ;holds the characters

byteCount Dword ? ;holds counter for loop

.code

mov edx, offset buffer ;points to the buffer

mov ecx, (sizeof buffer)-1 ;specify max characters

call ReadString ;input the string

mov bytecount, eax

• The These color constants are defined in Irvine32.inc.

• The backgroung color must be multiplied by 16 before being added to the foreground color.

• The following for example, defines yellow characters on a blue background

Yellow + (blue * 16)

• Before calling SetTextColor, move the desired color to EAX

Mov eax, white + (blue*16)

Call setTextColor

Push operation• A 32-bit push operation decrements the stack pointer by 4 and

copies a value into the location in the stack pointed to by the stack pointer. In the following figure, we push 0000005A on the stack:

• Before the push, ESP=00001000h, and after the push, ESP=000000FFCh.

ESP

Pop Operation• A pop operation removes a value from the stack and places it in a

register or variable. After the value is popped from the stack, the stack pointer is incremented to point to the next highest location in the stack. The following diagram shows the stack before and after the value 00000002 is popped from the stack:

00001000

00000FFC

00000FF8

00000FF4

00000FF0

PUSH and PUP InstructionsPUSH Instruction:

• The PUSH instruction first decrements ESP and then copies either a 16 or 32 bit source operand into the stack. A 16 bit operation causes the ESP to be decremented by 2.

• A 32-bit operand causes ESP to be decremented by 4. There are Three instruction format

PUSH r/m16

Push r/m32

PUSH imm32• If your program calls procedures from the Irvine 32 library, you

should always push 32-bit values

Pop Instruction• The pop instruction first copies the contents of the stack element

pointed to by ESP into a 16- or 32-bit destination operand and then increments ESP.

• If the operand is 16 bits, ESP is incremented by 2.

• if the operand 32 bits, ESP is incremented by 4.

POP r/m16

POP r/m32

• When you create a procedures other than your program's main procedure, end it with a RET instruction. It forces the CPU to return to the location from the procedure was called:

SumOf PROCadd eax,ebxadd eax,ecx ; Return the Sum value

in EAXRet

SumOf ENDP

• The startup procedure ( main Procedure) is a special case because it ends with the exit statement. When you INCLUDE Irvine 32.inc statement, exit Is an alias for a call EXITProcess, a MS_Windows function call that terminates the program.

Call and ret instructions

• The CALL instruction call a procedures by directing the processor to begin execution at a new memory location.

• The call instruction pushes its return address on the stack and copies the called procedure’s address into the instruction pointer (EIP)

• When the procedures is ready it uses RET instruction to bring the Processor back to the point in the program where the procedure was called. By returning the address form stack to instruction pointer register EIP

• The CPU always executes the instruction in memory pointed to by EIP

• When the call instruction executes, The address following the call 00000025 is pushed on the stack and the address of MySub is located into as shown here:

Main proc00000020 Call MySub00000025 Mov ebx,eax

• All the instruction in MySub execute up to its RET instruction. When the RET instruction executes, the value in the stack pointed by ESP is popped into EIP.

00000040 MySub proc Mov eax, edx : RetMySub ENDP

.data

Array Dword 10000h, 20000h, 30000h, 40000h, 500000h

theSum dowrd ?

Main PROCmov esi, offset array ; ESI pointes to array

mov ecx, lenghtof Array ; ECX = array countcall ArraySum ; call calculate the summov theSum,eax ; returned sum in EAXcall writeint ; print the sumexit

main ENDP

;--------------------------------------------------------------------ArraySumPROC;;calculates and returns the array of 32-bit integers.; recieives: ESI = Array ; ECX= number of elements in the array; Returnes: EAX=Sum;-------------------------------------------------------------------

push esi ; Save ESI and ECXpush ecxmov eax,0 ; Set Sum to Zero

L1:add eax,[esi] ; add each integer to sumadd esi,4 ; point to next integerloop L1 ; repeat for array sizePop ecx : restore ecx and ESIPop esiret ; Return sum in EAX

ArraySum ENDPEnd mian

MLU Operations• The MUL instruction multiplies an 8-,16-,32-bits operand by

either AL,AX,or EAX. The instruction format is:

MUL r/m8MUL r/m16MUL r/m32

• The result is in EDX and EAX .

• EDX sets the flag register if it is not zero ( as the result of operation over flow situation is Eax carries the product of operation.

• Will we go through the detail of multiplication and division instruction later on this course.

• Example:TITLE Multiply (AddSub2.asm)

; This program Multiply two decimal numbers together. We need to put the first operand in eax, the ;second operand in another multi purpose register. The product goes to eax

; Last update: 2/1/02

INCLUDE Irvine32.inc

.dataval1 dword 10val2 dword 40

.codemain PROC

mov eax,val1 ; eax=10mov ebx,val2 ; ebx=40mul ebx ; eax=ebx*eaxcall writedec

exitmain ENDPEND main