Assembly Language for x86 Processors 7th Edition
Chapter 6: Conditional Processing
. Overview
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Boolean and Comparison Instructions
Conditional Jumps
Conditional Loop Instructions
Conditional Structures
Application: Finite-State Machines
Conditional Control Flow Directives
Boolean and Comparison Instructions
CPU Status Flags
AND Instruction
OR Instruction
XOR Instruction
NOT Instruction
Applications
TEST Instruction
CMP Instruction
Status Flags – Review
The Zero flag is set when the result of an operation equals zero.
The Carry flag is set when an instruction generates a result that is too large (or too small) for the destination operand.
The Sign flag is set if the destination operand is negative, and it is clear if the destination operand is positive.
The Overflow flag is set when an instruction generates an invalid signed result (bit 7 carry is XORed with bit 6 Carry).
The Parity flag is set when an instruction generates an even number of 1 bits in the low byte of the destination operand.
The Auxiliary Carry flag is set when an operation produces a carry out from bit 3 to bit 4
AND Instruction
Performs a Boolean AND operation between each pair of matching bits in two operands
AND destination, source
(same operand types as MOV)
111.unknown
OR Instruction
Performs a Boolean OR operation between each pair of matching bits in two operands
OR destination, source
112.unknown
XOR Instruction
Performs a Boolean exclusive-OR operation between each pair of matching bits in two operands
XOR destination, source
NOT Instruction
Performs a Boolean NOT operation on a single destination operand
NOT destination
Bit-Mapped Sets
Binary bits indicate set membership
Efficient use of storage
Also known as bit vectors
Bit-Mapped Set Operations
Set Complement
mov eax,SetX
Set Intersection
mov eax,setX
and eax,setY
mov eax,setX
or eax,setY
Applications (1 of 5)
mov al,’a’ ; AL = 01100001b
and al,11011111b ; AL = 01000001b
Task: Convert the character in AL to upper case.
Solution: Use the AND instruction to clear bit 5.
Task: Convert a binary decimal byte into its equivalent ASCII decimal digit. 6 = 54 IN ASCII
Solution: Use the OR instruction to set bits 4 and 5.
mov al,6 ; AL = 00000110b
or al,00110000b ; AL = 00110110b The ASCII digit ‘6’
Applications (3 of 5)
mov ax,40h ; BIOS segment
mov bx,17h ; keyboard flag byte
or BYTE PTR [bx],01000000b ; CapsLock on
Task: Turn on the keyboard CapsLock key
Solution: Use the OR instruction to set bit 6 in the keyboard flag byte at 0040:0017h in the BIOS data area.
This code only runs in Real-address mode, and it does not work under Windows NT, 2000, or XP.
Applications (4 of 5)
mov ax,wordVal
and ax,1 ; low bit set?
jz EvenValue ; jump if Zero flag set
Task: Jump to a label if an integer is even.
Solution: AND the lowest bit with a 1. If the result is Zero, the number was even.
JZ (jump if Zero) is covered in Section 6.3.
Applications (5 of 5)
jnz IsNotZero ; jump if not zero
Task: Jump to a label if the value in AL is not zero.
Solution: OR the byte with itself, then use the JNZ (jump if not zero) instruction.
TEST Instruction
Performs a nondestructive AND operation between each pair of matching bits in two operands
No operands are modified, but the Zero flag is affected.
Example: jump to a label if either bit 0 or bit 1 in AL is set.
test al,00000011b
jnz ValueFound
Example: jump to a label if neither bit 0 nor bit 1 in AL is set.
test al,00000011b
jz ValueNotFound
CMP Instruction (1 of 3)
Compares the destination operand to the source operand
Nondestructive subtraction of source from destination (destination operand is not changed)
Syntax: CMP destination, source
Example: destination == source
cmp al,5 ; Zero flag set
Example: destination < source
cmp al,5 ; Carry flag set
CMP Instruction (2 of 3)
Example: destination > source
cmp al,5 ; ZF = 0, CF = 0
(both the Zero and Carry flags are clear)
CMP Results ZF (ZR) CF (CY)
Destination < Source 0 1
Destination > Source 0 0
Destination = Source 1 0
CMP Instruction (3 of 3)
The comparisons shown here are performed with signed integers.
CMP Results SF (PL) OF (OV) Destination Source
Destination < Source
(SF !=OF) 0 1 -2 127
Destination > Source
(SF ==OF) 0 0 127 1
1 1 127 -1
Destination = Source ZF = 1
CMP Instruction (Examples)
Example: destination > source
cmp al,-5 ; Sign flag(0) == Overflow flag(0)
The comparisons shown here with signed integers.
mov al,-120
cmp al,10 ; Sign flag(0) != Overflow flag(1)
mov al, 120
cmp al, -10
mov al, 120
cmp al, 10
Example: destination < source
cmp al,5 ; Sign flag(1) != Overflow flag(0)
Set and Clear Flags
Set Zero flag
Clear Zero flag
Set Sign flag
Clear Sign flag
Set Carry flag
Clear Carry flag
Set Overflow flag
Clear Overflow flag
test al, 0
or al, 80h
and al, 7Fh
mov al, 7Fh
SF = 1 ZF = 0 AF = 1 PF = 0
Boolean Instructions in 64-Bit Mode
64-bit boolean instructions, for the most part, work the same as 32-bit instructions
If the source operand is a constant whose size is less than 32 bits and the destination is the lower part of a 64-bit register or memory operand, all bits in the destination operand are affected
When the source is a 32-bit constant or register, only the lower 32 bits of the destination operand are affected
Conditional Jumps
Jumps Based On . . .
Specific flags
Unsigned comparisons
Signed Comparisons
Applications
Encrypting a String
Bit Test (BT) Instruction
Jcond Instruction
A conditional jump instruction branches to a label when specific register or flag conditions are met
Specific jumps:
JB, JC - jump to a label if the Carry flag is set
JE, JZ - jump to a label if the Zero flag is set
JS - jump to a label if the Sign flag is set
JNE, JNZ - jump to a label if the Zero flag is clear
JECXZ - jump to a label if ECX = 0
Jcond Ranges
x86 processors:
32-bit offset permits jump anywhere in memory
Offset Encoding ASM Source
0040101A B0 80 mov al,80h L1: mov al, -128
0040101C 3C 0A cmp al,0Ah cmp al, 10
0040101E 74 FA je L1 (40101Ah) jz L1
00401020 8A D8 mov bl,al mov bl, al
FA: -6 00401020
+ FFFFFFFA
------------
Jumps Based on Specific Flags
Jumps Based on Equality
Jumps Based on Unsigned Comparisons
Jumps Based on Signed Comparisons
Applications (1 of 5)
cmp eax,ebx
ja Larger
Task: Jump to a label if unsigned EAX is greater than EBX
Solution: Use CMP, followed by JA
cmp eax,ebx
jg Greater
Task: Jump to a label if signed EAX is greater than EBX
Solution: Use CMP, followed by JG
Applications (2 of 5)
cmp eax,Val1
jbe L1 ; below or equal
Jump to label L1 if unsigned EAX is less than or equal to Val1
cmp eax,Val1
Jump to label L1 if signed EAX is less than or equal to Val1
Applications (3 of 5)
mov Large,bx
mov Large,ax
Compare unsigned AX to BX, and copy the larger of the two into a variable named Large
mov Small,ax
mov Small,bx
Compare signed AX to BX, and copy the smaller of the two into a variable named Small
Applications (4 of 5)
cmp WORD PTR [esi],0
Jump to label L1 if the memory word pointed to by ESI equals Zero
test DWORD PTR [edi],1
Jump to label L2 if the doubleword in memory pointed to by EDI is even
Applications (5 of 5)
and al,00001011b ; clear unwanted bits
cmp al,00001011b ; check remaining bits
je L1 ; all set? jump to L1
Task: Jump to label L1 if bits 0, 1, and 3 in AL are all set.
Solution: Clear all bits except bits 0, 1,and 3. Then compare the result with 00001011 binary.
test al,00001011b ; check bits
jnz L1 ; any set? jump to L1
Task: Jump to label L1 if any of bits 0, 1, or 3 in AL are set.
BT (Bit Test) Instruction, (Optional)
Copies bit n from an operand into the Carry flag
Syntax: BT bitBase, n
bitBase may be r/m16 or r/m32
n may be r16, r32, or imm8
Example: jump to label L1 if bit 9 is set in AX:
BTC, BTR, BTS: (Complement, Reset, Set)
bt AX,9 ; CF = bit 9
jc L1 ; jump if Carry
Semaphore WORD 10001000b
btc Semaphore, 6 ; CF=0, Semaphore WORD 11001000b
Conditional Loop Instructions
LOOPZ and LOOPE
LOOPNZ and LOOPNE
LOOPZ and LOOPE
LOOPE destination
LOOPZ destination
ECX ECX – 1
if ECX > 0 and ZF=1, jump to destination
In 32-bit mode, ECX is the loop counter register. In 16-bit real-address mode, CX is the counter, and in 64-bit mode, RCX is the counter.
LOOPNZ and LOOPNE
LOOPNZ (LOOPNE) is a conditional loop instruction
LOOPNZ destination
LOOPNE destination
ECX ECX – 1;
if ECX > 0 and ZF=0, jump to destination
LOOPNZ Example
array SWORD -3,-6,-1,-10,10,30,40,4
sentinel SWORD 0
mov esi,OFFSET array
mov ecx,LENGTHOF array
test WORD PTR [esi],8000h ; test sign bit
pushfd ; push flags on stack
add esi,TYPE array
Popfd ; pop flags from stack
loopnz next ; continue loop
jnz quit ; none found;
sub esi,TYPE array ; ESI points to value
The following code finds the first positive value in an array:
How to simplify?
If the loop fails to find a positive number, it stops when ECX equals zero.
Conditional Structures
Block-Structured IF Statements
Compound Expressions with AND
Compound Expressions with OR
WHILE Loops
Table-Driven Selection
Block-Structured IF Statements
Assembly language programmers can easily translate logical statements written in C++/Java into assembly language. For example:
mov eax,op1
cmp eax,op2
L1: mov X,2
if( op1 == op2 )
Compound Expression with AND (1 of 3)
When implementing the logical AND operator, consider that HLLs use short-circuit evaluation
In the following example, if the first expression is false, the second expression is skipped:
if (al > bl) AND (bl > cl)
Compound Expression with AND (2 of 3)
cmp al,bl ; first expression…
cmp bl,cl ; second expression…
L2: ; both are true
mov X,1 ; set X to 1
if (al > bl) AND (bl > cl)
This is one possible implementation . . .
Compound Expression with AND (3 of 3)
cmp al,bl ; first expression…
jbe next ; quit if false
cmp bl,cl ; second expression…
jbe next ; quit if false
mov X,1 ; both are true
if (al > bl) AND (bl > cl)
But the following implementation uses 29% less code by reversing the first relational operator. We allow the program to “fall through” to the second expression:
Some non-short-circuit evaluation (e.g., early BASIC)
Compound Expression with OR (1 of 2)
When implementing the logical OR operator, consider that HLLs use short-circuit evaluation
In the following example, if the first expression is true, the second expression is skipped:
if (al > bl) OR (bl > cl)
Compound Expression with OR (2 of 2)
cmp al,bl ; is AL > BL?
ja L1 ; yes
cmp bl,cl ; no: is BL > CL?
jbe next ; no: skip next statement
L1: mov X,1 ; set X to 1
We can use “fall-through” logic to keep the code as short as possible:
if (al > bl) OR (bl > cl)
WHILE Loops
while( eax < ebx)
eax = eax + 1;
A WHILE loop is really an IF statement followed by the body of the loop, followed by an unconditional jump to the top of the loop. Consider the following example:
top: cmp eax,ebx ; check loop condition
jae next ; false? exit loop
inc eax ; body of loop
jmp top ; repeat the loop
This is a possible implementation:
Table-Driven Selection (1 of 4)
Table-driven selection uses a table lookup to replace a multiway selection structure
Create a table containing lookup values and the offsets of labels or procedures
Use a loop to search the table
Suited to a large number of comparisons
Lookup Procedure
‘A’ Process_A
‘B’ Process_B
‘C’ Process_C
‘D’ Process_D
Table-Driven Selection (2 of 4)
CaseTable BYTE 'A' ; lookup value
DWORD Process_A ; address of procedure
EntrySize = ($ - CaseTable)
DWORD Process_B
DWORD Process_C
DWORD Process_D
NumberOfEntries = ($ - CaseTable) / EntrySize
Step 1: create a table containing lookup values and procedure offsets:
Table-Driven Selection (3 of 4)
Table of Procedure Offsets:
Table-Driven Selection (4 of 4)
mov ebx,OFFSET CaseTable ; point EBX to the table
mov ecx,NumberOfEntries ; loop counter
L1: cmp al,[ebx] ; match found?
jne L2 ; no: continue
call NEAR PTR [ebx + 1] ; yes: call the procedure
call WriteString ; display message
jmp L3 ; and exit the loop
L2: add ebx,EntrySize ; point to next entry
loop L1 ; repeat until ECX = 0
Step 2: Use a loop to search the table. When a match is found, call the procedure offset stored in the current table entry:
required for procedure pointers
Application: Finite-State Machines
A finite-state machine (FSM) is a graph structure that changes state based on some input. Also called a state-transition diagram.
We use a graph to represent an FSM, with squares or circles called nodes, and lines with arrows between the circles called edges.
Application: Finite-State Machines
A FSM is a specific instance of a more general structure called a directed graph.
Three basic states, represented by nodes:
Start state
Terminal state(s)
Nonterminal state(s)
Finite-State Machine
Accepts any sequence of symbols that puts it into an accepting (final) state
Can be used to recognize, or validate a sequence of characters that is governed by language rules (called a regular expression)
Advantages:
Provides visual tracking of program's flow of control
Easy to modify
Easily implemented in assembly language
Finite-State Machine Examples
FSM that recognizes strings beginning with 'x', followed by letters 'a'..'y', ending with 'z':
FSM that recognizes signed integers:
from state A to state B can only be accomplished if the letter x is read from the input stream.
from state B to itself is accomplished by the input of any letter of the alphabet except z.
A transition from state B to state C occurs only when the letter z is read from the input stream.
115.unknown
116.unknown
Implementing an FSM
call Getnext ; read next char into AL
cmp al,'+' ; leading + sign?
je StateB ; go to State B
cmp al,'-' ; leading - sign?
je StateB ; go to State B
call IsDigit ; ZF = 1 if AL = digit
jz StateC ; go to State C
call DisplayErrorMsg ; invalid input found
The following is code from State A in the Integer FSM:
View the Finite.asm source code.
Flowchart of State A
State A accepts a plus or minus sign, or a decimal digit.
122.unknown
Creating IF Statements
Runtime Expressions
Relational and Logical Operators
MASM-Generated Code
.REPEAT Directive
.WHILE Directive
Runtime Expressions
.IF eax > ebx
.IF, .ELSE, .ELSEIF, and .ENDIF can be used to evaluate runtime expressions and create block-structured IF statements.
MASM generates “hidden” code for you, consisting of code labels, CMP and conditional jump instructions.
.IF eax > ebx && eax > ecx
Relational and Logical Operators
Signed and Unsigned Comparisons
cmp eax,val1
jbe @C0001
mov result,1
val1 DWORD 5
result DWORD ?
.IF eax > val1
mov result,1
Generated code:
MASM automatically generates an unsigned jump (JBE) because val1 is unsigned.
Signed and Unsigned Comparisons
cmp eax,val1
jle @C0001
mov result,1
val1 SDWORD 5
result SDWORD ?
.IF eax > val1
mov result,1
Generated code:
MASM automatically generates a signed jump (JLE) because val1 is signed.
Signed and Unsigned Comparisons
cmp eax,ebx
jbe @C0001
mov result,1
result DWORD ?
.IF eax > ebx
mov result,1
Generated code:
MASM automatically generates an unsigned jump (JBE) when both operands are registers . . .
Signed and Unsigned Comparisons
cmp eax,ebx
jle @C0001
mov result,1
result SDWORD ?
.IF SDWORD PTR eax > ebx
mov result,1
Generated code:
. . . unless you prefix one of the register operands with the SDWORD PTR operator. Then a signed jump is generated.
.REPEAT Directive
; Display integers 1 – 10:
call WriteDec
.UNTIL eax == 10
Executes the loop body before testing the loop condition associated with the .UNTIL directive.
.WHILE Directive
; Display integers 1 – 10:
.WHILE eax < 10 call WriteDec Tests the loop condition before executing the loop body The .ENDW directive marks the end of the loop. MASM-Generated Code mov eax, 0 .while eax <10 00000026 B8 00000000 mov eax, 0 .while eax <10 0000002B EB 01 * jmp @C0001 0000002D 40 inc eax 0000002E 83 F8 0A * cmp eax, 00Ah 00000031 72 FA * jb @C0002 Bitwise instructions (AND, OR, XOR, NOT, TEST) manipulate individual bits in operands CMP – compares operands using implied subtraction sets condition flags Conditional Jumps & Loops equality: JE, JNE flag values: JC, JZ, JNC, JP, ... signed: JG, JL, JNG, ... unsigned: JA, JB, JNA, ... LOOPZ, LOOPNZ, LOOPE, LOOPNE Flowcharts – logic diagramming tool Finite-state machine – tracks state changes at runtime Test 2 in 11/03/2022 0 0 1 1 1 0 1 1 0 0 0 0 1 1 1 1 0 0 0 0 1 0 1 1 0 0 1 1 1 0 1 1 0 0 0 0 1 1 1 1 0 0 1 1 1 1 1 1 0 0 1 1 1 0 1 1 0 0 0 0 1 1 1 1 0 0 1 1 0 1 0 0 0 0 1 1 1 0 1 1 1 1 0 0 0 1 0 0 AL = '+' ? DisplayErrorMsg 程序代写 CS代考 加微信: powcoder QQ: 1823890830 Email: powcoder@163.com