Chapter 4
Assembly Language for x86 Processors 6th Edition
Chapter 4: Data Transfers, Addressing, and Arithmetic
(c) Pearson Education, 2010. All rights reserved. You may modify and copy this slide show for your personal use, or for use in the classroom, as long as this copyright statement, the author’s name, and the title are not changed.
Kip Irvine
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Chapter Overview
Data Transfer Instructions
Addition and Subtraction
Data-Related Operators and Directives
Indirect Addressing
JMP and LOOP Instructions
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Data Transfer Instructions
Operand Types
Instruction Operand Notation
Direct Memory Operands
MOV Instruction
Zero & Sign Extension
XCHG Instruction
Direct-Offset Instructions
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Operand Types
Immediate – a constant integer (8, 16, or 32 bits)
value is encoded within the instruction
Register – the name of a register
register name is converted to a number and encoded within the instruction
Memory – reference to a location in memory
memory address is encoded within the instruction, or a register holds the address of a memory location
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Instruction Operand Notation
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Direct Memory Operands
A direct memory operand is a named reference to storage in memory
The named reference (label) is automatically dereferenced by the assembler
.data
var1 BYTE 10h
.code
mov al,var1 ; AL = 10h
mov al,[var1] ; AL = 10h
alternate format
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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MOV Instruction
.data
count BYTE 100
wVal WORD 2
.code
mov bl,count
mov ax,wVal
mov count,al
mov al,wVal ; error
mov ax,count ; error
mov eax,count ; error
Move from source to destination. Syntax:
MOV destination,source
No more than one memory operand permitted
CS, EIP, and IP cannot be the destination
No immediate to segment moves
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Your turn . . .
.data
bVal BYTE 100
bVal2 BYTE ?
wVal WORD 2
dVal DWORD 5
.code
mov ds,45
mov esi,wVal
mov eip,dVal
mov 25,bVal
mov bVal2,bVal
Explain why each of the following MOV statements are invalid:
immediate move to DS not permitted
size mismatch
EIP cannot be the destination
immediate value cannot be destination
memory-to-memory move not permitted
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Zero Extension
mov bl,10001111b
movzx ax,bl ; zero-extension
When you copy a smaller value into a larger destination, the MOVZX instruction fills (extends) the upper half of the destination with zeros.
The destination must be a register.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
79.unknown
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Sign Extension
mov bl,10001111b
movsx ax,bl ; sign extension
The MOVSX instruction fills the upper half of the destination with a copy of the source operand’s sign bit.
The destination must be a register.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
80.unknown
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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XCHG Instruction
.data
var1 WORD 1000h
var2 WORD 2000h
.code
xchg ax,bx ; exchange 16-bit regs
xchg ah,al ; exchange 8-bit regs
xchg var1,bx ; exchange mem, reg
xchg eax,ebx ; exchange 32-bit regs
xchg var1,var2 ; error: two memory operands
XCHG exchanges the values of two operands. At least one operand must be a register. No immediate operands are permitted.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Direct-Offset Operands
.data
arrayB BYTE 10h,20h,30h,40h
.code
mov al,arrayB+1 ; AL = 20h
mov al,[arrayB+1] ; alternative notation
A constant offset is added to a data label to produce an effective address (EA). The address is dereferenced to get the value inside its memory location.
Q: Why doesn’t arrayB+1 produce 11h?
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Direct-Offset Operands (cont)
.data
arrayW WORD 1234h,5678h,9ABCh
arrayD DWORD 1,2,3,4
.code
mov ax,[arrayW+2] ; AX = 5678h
mov ax,[arrayW+4] ; AX = 9ABCh
mov eax,[arrayD+4] ; EAX = 00000002h
A constant offset is added to a data label to produce an effective address (EA). The address is dereferenced to get the value inside its memory location.
; Will the following statements assemble?
mov ax,[arrayW-2] ; ??
mov eax,[arrayD+16] ; ??
What will happen when they run?
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Addition and Subtraction
INC and DEC Instructions
ADD and SUB Instructions
NEG Instruction
Implementing Arithmetic Expressions
Flags Affected by Arithmetic
Zero
Sign
Carry
Overflow
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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INC and DEC Instructions
Add 1, subtract 1 from destination operand
operand may be register or memory
INC destination
Logic: destination destination + 1
DEC destination
Logic: destination destination – 1
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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INC and DEC Examples
.data
myWord WORD 1000h
myDword DWORD 10000000h
.code
inc myWord ; 1001h
dec myWord ; 1000h
inc myDword ; 10000001h
mov ax,00FFh
inc ax ; AX = 0100h
mov ax,00FFh
inc al ; AX = 0000h
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Your turn…
Show the value of the destination operand after each of the following instructions executes:
.data
myByte BYTE 0FFh, 0
.code
mov al,myByte ; AL =
mov ah,[myByte+1] ; AH =
dec ah ; AH =
inc al ; AL =
dec ax ; AX =
FFh
00h
FFh
00h
FEFF
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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ADD and SUB Instructions
ADD destination, source
Logic: destination destination + source
SUB destination, source
Logic: destination destination – source
Same operand rules as for the MOV instruction
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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ADD and SUB Examples
.data
var1 DWORD 10000h
var2 DWORD 20000h
.code ; —EAX—
mov eax,var1 ; 00010000h
add eax,var2 ; 00030000h
add ax,0FFFFh ; 0003FFFFh
add eax,1 ; 00040000h
sub ax,1 ; 0004FFFFh
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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NEG (negate) Instruction
.data
valB BYTE -1
valW WORD +32767
.code
mov al,valB ; AL = -1
neg al ; AL = +1
neg valW ; valW = -32767
Reverses the sign of an operand. Operand can be a register or memory operand.
Suppose AX contains –32,768 and we apply NEG to it. Will the result be valid?
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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NEG Instruction and the Flags
.data
valB BYTE 1,0
valC SBYTE -128
.code
neg valB ; CF = 1, OF = 0
neg [valB + 1] ; CF = 0, OF = 0
neg valC ; CF = 1, OF = 1
The processor implements NEG using the following internal operation:
SUB 0,operand
Any nonzero operand causes the Carry flag to be set.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Implementing Arithmetic Expressions
Rval DWORD ?
Xval DWORD 26
Yval DWORD 30
Zval DWORD 40
.code
mov eax,Xval
neg eax ; EAX = -26
mov ebx,Yval
sub ebx,Zval ; EBX = -10
add eax,ebx
mov Rval,eax ; -36
HLL compilers translate mathematical expressions into assembly language. You can do it also. For example:
Rval = -Xval + (Yval – Zval)
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
Your turn…
mov ebx,Yval
neg ebx
add ebx,Zval
mov eax,Xval
sub eax,ebx
mov Rval,eax
Translate the following expression into assembly language.
Do not permit Xval, Yval, or Zval to be modified:
Rval = Xval – (-Yval + Zval)
Assume that all values are signed doublewords.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Flags Affected by Arithmetic
The ALU has a number of status flags that reflect the outcome of arithmetic (and bitwise) operations
based on the contents of the destination operand
Essential flags:
Zero flag – set when destination equals zero
Sign flag – set when destination is negative
Carry flag – set when unsigned value is out of range
Overflow flag – set when signed value is out of range
The MOV instruction never affects the flags.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Concept Map
status flags
ALU
conditional jumps
branching logic
arithmetic & bitwise operations
part of
used by
provide
attached to
affect
CPU
You can use diagrams such as these to express the relationships between assembly language concepts.
executes
executes
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Zero Flag (ZF)
mov cx,1
sub cx,1 ; CX = 0, ZF = 1
mov ax,0FFFFh
inc ax ; AX = 0, ZF = 1
inc ax ; AX = 1, ZF = 0
The Zero flag is set when the result of an operation produces zero in the destination operand.
Remember…
A flag is set when it equals 1.
A flag is clear when it equals 0.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Sign Flag (SF)
mov cx,0
sub cx,1 ; CX = -1, SF = 1
add cx,2 ; CX = 1, SF = 0
The Sign flag is set when the destination operand is negative. The flag is clear when the destination is positive.
The sign flag is a copy of the destination’s highest bit:
mov al,0
sub al,1 ; AL = 11111111b, SF = 1
add al,2 ; AL = 00000001b, SF = 0
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Signed and Unsigned Integers
A Hardware Viewpoint
All CPU instructions operate exactly the same on signed and unsigned integers
The CPU cannot distinguish between signed and unsigned integers
YOU, the programmer, are solely responsible for using the correct data type with each instruction
Added Slide. Gerald Cahill, Antelope Valley College
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Overflow and Carry Flags
A Hardware Viewpoint
How the ADD instruction affects OF and CF:
CF = (carry out of the MSb)
OF = CF XOR MSb
How the SUB instruction affects OF and CF:
CF = INVERT (carry out of the MSb)
negate the source and add it to the destination
OF = CF XOR MSb
MSb = Most Significant Bit (high-order bit)
XOR = eXclusive-OR operation
NEG = Negate (same as SUB 0,operand )
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Carry Flag (CF)
The Carry flag is set when the result of an operation generates an unsigned value that is out of range (too big or too small for the destination operand).
mov al,0FFh
add al,1 ; CF = 1, AL = 00
; Try to go below zero:
mov al,0
sub al,1 ; CF = 1, AL = FF
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
Your turn . . .
mov ax,00FFh
add ax,1 ; AX= SF= ZF= CF=
sub ax,1 ; AX= SF= ZF= CF=
add al,1 ; AL= SF= ZF= CF=
mov bh,6Ch
add bh,95h ; BH= SF= ZF= CF=
mov al,2
sub al,3 ; AL= SF= ZF= CF=
For each of the following marked entries, show the values of the destination operand and the Sign, Zero, and Carry flags:
0100h 0 0 0
00FFh 0 0 0
00h 0 1 1
01h 0 0 1
FFh 1 0 1
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Overflow Flag (OF)
The Overflow flag is set when the signed result of an operation is invalid or out of range.
; Example 1
mov al,+127
add al,1 ; OF = 1, AL = ??
; Example 2
mov al,7Fh ; OF = 1, AL = 80h
add al,1
The two examples are identical at the binary level because 7Fh equals +127. To determine the value of the destination operand, it is often easier to calculate in hexadecimal.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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A Rule of Thumb
When adding two integers, remember that the Overflow flag is only set when . . .
Two positive operands are added and their sum is negative
Two negative operands are added and their sum is positive
What will be the values of the Overflow flag?
mov al,80h
add al,92h ; OF =
mov al,-2
add al,+127 ; OF =
1
0
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
Your turn . . .
mov al,-128
neg al ; CF = OF =
mov ax,8000h
add ax,2 ; CF = OF =
mov ax,0
sub ax,2 ; CF = OF =
mov al,-5
sub al,+125 ; OF =
What will be the values of the given flags after each operation?
1 1
0 0
1 0
1
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
What’s Next
Data Transfer Instructions
Addition and Subtraction
Data-Related Operators and Directives
Indirect Addressing
JMP and LOOP Instructions
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
Data-Related Operators and Directives
OFFSET Operator
PTR Operator
TYPE Operator
LENGTHOF Operator
SIZEOF Operator
LABEL Directive
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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OFFSET Operator
OFFSET returns the distance in bytes, of a label from the beginning of its enclosing segment
Protected mode: 32 bits
Real mode: 16 bits
The Protected-mode programs we write use only a single segment (flat memory model).
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
81.unknown
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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OFFSET Examples
.data
bVal BYTE ?
wVal WORD ?
dVal DWORD ?
dVal2 DWORD ?
.code
mov esi,OFFSET bVal ; ESI = 00404000
mov esi,OFFSET wVal ; ESI = 00404001
mov esi,OFFSET dVal ; ESI = 00404003
mov esi,OFFSET dVal2 ; ESI = 00404007
Let’s assume that the data segment begins at 00404000h:
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Relating to C/C++
// C++ version:
char array[1000];
char * p = array;
The value returned by OFFSET is a pointer. Compare the following code written for both C++ and assembly language:
; Assembly language:
.data
array BYTE 1000 DUP(?)
.code
mov esi,OFFSET array
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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PTR Operator
.data
myDouble DWORD 12345678h
.code
mov ax,myDouble ; error – why?
mov ax,WORD PTR myDouble ; loads 5678h
mov WORD PTR myDouble,4321h ; saves 4321h
Overrides the default type of a label (variable). Provides the flexibility to access part of a variable.
Little endian order is used when storing data in memory (see Section 3.4.9).
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Little Endian Order
Little endian order refers to the way Intel stores integers in memory.
Multi-byte integers are stored in reverse order, with the least significant byte stored at the lowest address
For example, the doubleword 12345678h would be stored as:
When integers are loaded from memory into registers, the bytes are automatically re-reversed into their correct positions.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
82.unknown
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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PTR Operator Examples
.data
myDouble DWORD 12345678h
mov al,BYTE PTR myDouble ; AL = 78h
mov al,BYTE PTR [myDouble+1] ; AL = 56h
mov al,BYTE PTR [myDouble+2] ; AL = 34h
mov ax,WORD PTR myDouble ; AX = 5678h
mov ax,WORD PTR [myDouble+2] ; AX = 1234h
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
83.unknown
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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PTR Operator (cont)
.data
myBytes BYTE 12h,34h,56h,78h
.code
mov ax,WORD PTR [myBytes] ; AX = 3412h
mov ax,WORD PTR [myBytes+2] ; AX = 7856h
mov eax,DWORD PTR myBytes ; EAX = 78563412h
PTR can also be used to combine elements of a smaller data type and move them into a larger operand. The CPU will automatically reverse the bytes.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
Your turn . . .
.data
varB BYTE 65h,31h,02h,05h
varW WORD 6543h,1202h
varD DWORD 12345678h
.code
mov ax,WORD PTR [varB+2] ; a.
mov bl,BYTE PTR varD ; b.
mov bl,BYTE PTR [varW+2] ; c.
mov ax,WORD PTR [varD+2] ; d.
mov eax,DWORD PTR varW ; e.
Write down the value of each destination operand:
0502h
78h
02h
1234h
12026543h
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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TYPE Operator
The TYPE operator returns the size, in bytes, of a single element of a data declaration.
.data
var1 BYTE ?
var2 WORD ?
var3 DWORD ?
var4 QWORD ?
.code
mov eax,TYPE var1 ; 1
mov eax,TYPE var2 ; 2
mov eax,TYPE var3 ; 4
mov eax,TYPE var4 ; 8
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
LENGTHOF Operator
.data LENGTHOF
byte1 BYTE 10,20,30 ; 3
array1 WORD 30 DUP(?),0,0 ; 32
array2 WORD 5 DUP(3 DUP(?)) ; 15
array3 DWORD 1,2,3,4 ; 4
digitStr BYTE “12345678”,0 ; 9
.code
mov ecx,LENGTHOF array1 ; 32
The LENGTHOF operator counts the number of elements in a single data declaration.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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SIZEOF Operator
.data SIZEOF
byte1 BYTE 10,20,30 ; 3
array1 WORD 30 DUP(?),0,0 ; 64
array2 WORD 5 DUP(3 DUP(?)) ; 30
array3 DWORD 1,2,3,4 ; 16
digitStr BYTE “12345678”,0 ; 9
.code
mov ecx,SIZEOF array1 ; 64
The SIZEOF operator returns a value that is equivalent to multiplying LENGTHOF by TYPE.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Spanning Multiple Lines (1 of 2)
.data
array WORD 10,20,
30,40,
50,60
.code
mov eax,LENGTHOF array ; 6
mov ebx,SIZEOF array ; 12
A data declaration spans multiple lines if each line (except the last) ends with a comma. The LENGTHOF and SIZEOF operators include all lines belonging to the declaration:
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
Spanning Multiple Lines (2 of 2)
.data
array WORD 10,20
WORD 30,40
WORD 50,60
.code
mov eax,LENGTHOF array ; 2
mov ebx,SIZEOF array ; 4
In the following example, array identifies only the first WORD declaration. Compare the values returned by LENGTHOF and SIZEOF here to those in the previous slide:
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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LABEL Directive
Assigns an alternate label name and type to an existing storage location
LABEL does not allocate any storage of its own
Removes the need for the PTR operator
.data
dwList LABEL DWORD
wordList LABEL WORD
intList BYTE 00h,10h,00h,20h
.code
mov eax,dwList ; 20001000h
mov cx,wordList ; 1000h
mov dl,intList ; 00h
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
What’s Next
Data Transfer Instructions
Addition and Subtraction
Data-Related Operators and Directives
Indirect Addressing
JMP and LOOP Instructions
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
Indirect Addressing
Indirect Operands
Array Sum Example
Indexed Operands
Pointers
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Indirect Operands (1 of 2)
.data
val1 BYTE 10h,20h,30h
.code
mov esi,OFFSET val1
mov al,[esi] ; dereference ESI (AL = 10h)
inc esi
mov al,[esi] ; AL = 20h
inc esi
mov al,[esi] ; AL = 30h
An indirect operand holds the address of a variable, usually an array or string. It can be dereferenced (just like a pointer).
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
Indirect Operands (2 of 2)
.data
myCount WORD 0
.code
mov esi,OFFSET myCount
inc [esi] ; error: ambiguous
inc WORD PTR [esi] ; ok
Use PTR to clarify the size attribute of a memory operand.
Should PTR be used here?
add [esi],20
yes, because [esi] could point to a byte, word, or doubleword
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Array Sum Example
.data
arrayW WORD 1000h,2000h,3000h
.code
mov esi,OFFSET arrayW
mov ax,[esi]
add esi,2 ; or: add esi,TYPE arrayW
add ax,[esi]
add esi,2
add ax,[esi] ; AX = sum of the array
Indirect operands are ideal for traversing an array. Note that the register in brackets must be incremented by a value that matches the array type.
ToDo: Modify this example for an array of doublewords.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
Indexed Operands
.data
arrayW WORD 1000h,2000h,3000h
.code
mov esi,0
mov ax,[arrayW + esi] ; AX = 1000h
mov ax,arrayW[esi] ; alternate format
add esi,2
add ax,[arrayW + esi]
etc.
An indexed operand adds a constant to a register to generate an effective address. There are two notational forms:
[label + reg] label[reg]
ToDo: Modify this example for an array of doublewords.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
Index Scaling
.data
arrayB BYTE 0,1,2,3,4,5
arrayW WORD 0,1,2,3,4,5
arrayD DWORD 0,1,2,3,4,5
.code
mov esi,4
mov al,arrayB[esi*TYPE arrayB] ; 04
mov bx,arrayW[esi*TYPE arrayW] ; 0004
mov edx,arrayD[esi*TYPE arrayD] ; 00000004
You can scale an indirect or indexed operand to the offset of an array element. This is done by multiplying the index by the array’s TYPE:
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
Pointers
.data
arrayW WORD 1000h,2000h,3000h
ptrW DWORD arrayW
.code
mov esi,ptrW
mov ax,[esi] ; AX = 1000h
You can declare a pointer variable that contains the offset of another variable.
Alternate format:
ptrW DWORD OFFSET arrayW
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
What’s Next
Data Transfer Instructions
Addition and Subtraction
Data-Related Operators and Directives
Indirect Addressing
JMP and LOOP Instructions
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
JMP and LOOP Instructions
JMP Instruction
LOOP Instruction
LOOP Example
Summing an Integer Array
Copying a String
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
JMP Instruction
top:
.
.
jmp top
JMP is an unconditional jump to a label that is usually within the same procedure.
Syntax: JMP target
Logic: EIP target
Example:
A jump outside the current procedure must be to a special type of label called a global label (see Section 5.5.2.3 for details).
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
LOOP Instruction
The LOOP instruction creates a counting loop
Syntax: LOOP target
Logic:
ECX ECX – 1
if ECX != 0, jump to target
Implementation:
The assembler calculates the distance, in bytes, between the offset of the following instruction and the offset of the target label. It is called the relative offset.
The relative offset is added to EIP.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
LOOP Example
00000000 66 B8 0000 mov ax,0
00000004 B9 00000005 mov ecx,5
00000009 66 03 C1 L1: add ax,cx
0000000C E2 FB loop L1
0000000E
The following loop calculates the sum of the integers 5 + 4 + 3 +2 + 1:
When LOOP is assembled, the current location = 0000000E (offset of the next instruction). –5 (FBh) is added to the the current location, causing a jump to location 00000009:
00000009 0000000E + FB
offset machine code source code
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
Your turn . . .
If the relative offset is encoded in a single signed byte,
(a) what is the largest possible backward jump?
(b) what is the largest possible forward jump?
-128
+127
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
Your turn . . .
What will be the final value of AX?
mov ax,6
mov ecx,4
L1:
inc ax
loop L1
How many times will the loop execute?
mov ecx,0
X2:
inc ax
loop X2
10
4,294,967,296
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
Nested Loop
If you need to code a loop within a loop, you must save the outer loop counter’s ECX value. In the following example, the outer loop executes 100 times, and the inner loop 20 times.
.data
count DWORD ?
.code
mov ecx,100 ; set outer loop count
L1:
mov count,ecx ; save outer loop count
mov ecx,20 ; set inner loop count
L2: .
.
loop L2 ; repeat the inner loop
mov ecx,count ; restore outer loop count
loop L1 ; repeat the outer loop
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
Summing an Integer Array
.data
intarray WORD 100h,200h,300h,400h
.code
mov edi,OFFSET intarray ; address of intarray
mov ecx,LENGTHOF intarray ; loop counter
mov ax,0 ; zero the accumulator
L1:
add ax,[edi] ; add an integer
add edi,TYPE intarray ; point to next integer
loop L1 ; repeat until ECX = 0
The following code calculates the sum of an array of 16-bit integers.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
Copying a String
.data
source BYTE “This is the source string”,0
target BYTE SIZEOF source DUP(0)
.code
mov esi,0 ; index register
mov ecx,SIZEOF source ; loop counter
L1:
mov al,source[esi] ; get char from source
mov target[esi],al ; store it in the target
inc esi ; move to next character
loop L1 ; repeat for entire string
good use of SIZEOF
The following code copies a string from source to target:
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
Summary
Data Transfer
MOV – data transfer from source to destination
MOVSX, MOVZX, XCHG
Operand types
direct, direct-offset, indirect, indexed
Arithmetic
INC, DEC, ADD, SUB, NEG
Sign, Carry, Zero, Overflow flags
Operators
OFFSET, PTR, TYPE, LENGTHOF, SIZEOF, TYPEDEF
JMP and LOOP – branching instructions
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
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Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
*
46 69 6E 61 6C
Irvine, Kip R. Assembly Language for x86 Processors 6/e, 2010.
1 0 0 0 1 1 1 1
1 0 0 0 1 1 1 1
Source
Destination
0 0 0 0 0 0 0 0
0
1 0 0 0 1 1 1 1
1 0 0 0 1 1 1 1
Source
Destination
1 1 1 1 1 1 1 1
offset
myByte
data segment:
12345678
0000
5678
1234
78
56
34
12
0001
0002
0003
offset
doubleword
word
byte
myDouble
myDouble + 1
myDouble + 2
myDouble + 3