Download ppt - Overview of Assembly Language Chapter 9 S. Dandamudi

Overview of Assembly Language

Chapter 9

S. Dandamudi

2003

To be used with S. Dandamudi, “Fundamentals of Computer Organization and Design,” Springer, 2003.

S. Dandamudi Chapter 9: Page 2

Outline

• Assembly language statements

• Data allocation

• Where are the operands? Addressing modes

» Register

» Immediate

» Direct

» Indirect

• Data transfer instructions mov, xchg, and xlat PTR directive

• Overview of assembly language instructions Arithmetic Conditional Logical Shift Rotate

• Defining constants EQU and = directives

• Macros

• Illustrative examples

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Assembly Language Statements

• Three different classes Instructions

» Tell CPU what to do» Executable instructions with an op-code

Directives (or pseudo-ops)» Provide information to assembler on various aspects of the

assembly process» Non-executable

– Do not generate machine language instructions Macros

» A shorthand notation for a group of statements» A sophisticated text substitution mechanism with parameters

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Assembly Language Statements (cont’d)

• Assembly language statement format:

[label] mnemonic [operands] [;comment]

Typically one statement per line Fields in [ ] are optional label serves two distinct purposes:

» To label an instruction

– Can transfer program execution to the labeled instruction

» To label an identifier or constant

mnemonic identifies the operation (e.g., add, or) operands specify the data required by the operation

» Executable instructions can have zero to three operands

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Assembly Language Statements (cont’d)

comments» Begin with a semicolon (;) and extend to the end of the line

Examplesrepeat: inc result ; increment result

CR EQU 0DH ; carriage return character

• White space can be used to improve readabilityrepeat:

inc result

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Data Allocation

• Variable declaration in a high-level language such as C

char responseint valuefloat totaldouble average_value

specifies» Amount storage required (1 byte, 2 bytes, …)» Label to identify the storage allocated (response, value, …)» Interpretation of the bits stored (signed, floating point, …)

– Bit pattern 1000 1101 1011 1001 is interpreted as29,255 as a signed number 36,281 as an unsigned number

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Data Allocation (cont’d)

• In assembly language, we use the define directive Define directive can be used

» To reserve storage space

» To label the storage space

» To initialize

» But no interpretation is attached to the bits stored

– Interpretation is up to the program code

Define directive goes into the .DATA part of the assembly language program

• Define directive format

[var-name] D? init-value [,init-value],...

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• Five define directivesDB Define Byte ;allocates 1 byteDW Define Word ;allocates 2 bytesDD Define Doubleword ;allocates 4 bytesDQ Define Quadword ;allocates 8 bytesDT Define Ten bytes ;allocates 10 bytes

Examplessorted DB ’y’response DB ? ;no initializationvalue DW 25159float1 DD 1.234float2 DQ 123.456

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• Multiple definitions can be abbreviated

Example message DB ’B’ DB ’y’ DB ’e’ DB 0DH DB 0AH

can be written as

message DB ’B’,’y’,’e’,0DH,0AH

• More compactly asmessage DB ’Bye’,0DH,0AH

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• Multiple definitions can be cumbersome to initialize data structures such as arrays

ExampleTo declare and initialize an integer array of 8 elements marks DW 0,0,0,0,0,0,0,0

• What if we want to declare and initialize to zero an array of 200 elements? There is a better way of doing this than repeating zero

200 times in the above statement» Assembler provides a directive to do this (DUP directive)

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• Multiple initializations The DUP assembler directive allows multiple

initializations to the same value Previous marks array can be compactly declared as

marks DW 8 DUP (0)

Examplestable1 DW 10 DUP (?) ;10 words, uninitializedmessage DB 3 DUP (’Bye!’) ;12 bytes, initialized

; as Bye!Bye!Bye!Name1 DB 30 DUP (’?’) ;30 bytes, each

; initialized to ?

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• The DUP directive may also be nested

Examplestars DB 4 DUP(3 DUP (’*’),2 DUP (’?’),5 DUP (’!’))

Reserves 40-bytes space and initializes it as

***??!!!!!***??!!!!!***??!!!!!***??!!!!!

Examplematrix DW 10 DUP (5 DUP (0))

defines a 10X5 matrix and initializes its elements to 0

This declaration can also be done by

matrix DW 50 DUP (0)

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Symbol Table Assembler builds a symbol table so we can refer to the

allocated storage space by the associated label

Example.DATA name

offsetvalue DW 0 value 0

sum DD 0 sum 2

marks DW 10 DUP (?) marks 6

message DB ‘The grade is:’,0 message 26

char1 DB ? char1 40

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Correspondence to C Data Types

Directive C data type

DB char

DW int, unsigned

DD float, long

DQ double

DT internal intermediate

float value

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LABEL Directive LABEL directive provides another way to name a

memory location Format:

name LABEL type

type can beBYTE 1 byteWORD 2 bytesDWORD 4 bytesQWORD 8 bytesTWORD 10 bytes

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LABEL DirectiveExample

.DATAcount LABEL WORDLo-count DB 0Hi_count DB 0

.CODE...mov Lo_count,ALmov Hi_count,CL

count refers to the 16-bit value Lo_count refers to the low byte Hi_count refers to the high byte

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Where Are the Operands?

• Operands required by an operation can be specified in a variety of ways

• A few basic ways are: operand in a register

– register addressing mode operand in the instruction itself

– immediate addressing mode operand in memory

– variety of addressing modesdirect and indirect addressing modes

operand at an I/O port– discussed in Chapter 19

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Where Are the Operands? (cont’d)

Register addressing mode Operand is in an internal register

Examplesmov EAX,EBX ; 32-bit copy

mov BX,CX ; 16-bit copy

mov AL,CL ; 8-bit copy

The mov instruction

mov destination,source

copies data from source to destination

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Register addressing mode (cont’d) Most efficient way of specifying an operand

» No memory access is required

Instructions using this mode tend to be shorter» Fewer bits are needed to specify the register

• Compilers use this mode to optimize code total := 0

for (i = 1 to 400)

total = total + marks[i]

end for Mapping total and i to registers during the for loop optimizes

the code

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Immediate addressing mode Data is part of the instruction

» Ooperand is located in the code segment along with the instruction

» Efficient as no separate operand fetch is needed

» Typically used to specify a constant

Examplemov AL,75

This instruction uses register addressing mode for destination and immediate addressing mode for the source

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Direct addressing mode Data is in the data segment

» Need a logical address to access data

– Two components: segment:offset

» Various addressing modes to specify the offset component

– offset part is called effective address

The offset is specified directly as part of instruction We write assembly language programs using memory

labels (e.g., declared using DB, DW, LABEL,...)» Assembler computes the offset value for the label

– Uses symbol table to compute the offset of a label

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Direct addressing mode (cont’d)Examples

mov AL,response» Assembler replaces response by its effective address (i.e., its

offset value from the symbol table)

mov table1,56» table1 is declared as

table1 DW 20 DUP (0)

» Since the assembler replaces table1 by its effective address, this instruction refers to the first element of table1

– In C, it is equivalent totable1[0] = 56

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Direct addressing mode (cont’d)• Problem with direct addressing

Useful only to specify simple variables Causes serious problems in addressing data types such

as arrays» As an example, consider adding elements of an array

– Direct addressing does not facilitate using a loop structure to iterate through the array

– We have to write an instruction to add each element of the array

• Indirect addressing mode remedies this problem

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Indirect addressing mode• The offset is specified indirectly via a register

Sometimes called register indirect addressing mode For 16-bit addressing, the offset value can be in one of

the three registers: BX, SI, or DI For 32-bit addressing, all 32-bit registers can be used

Examplemov AX,[BX]

Square brackets [ ] are used to indicate that BX is holding an offset value

» BX contains a pointer to the operand, not the operand itself

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• Using indirect addressing mode, we can process arrays using loops

Example: Summing array elements Load the starting address (i.e., offset) of the array into

BX Loop for each element in the array

» Get the value using the offset in BX

– Use indirect addressing

» Add the value to the running total

» Update the offset in BX to point to the next element of the array

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Loading offset value into a register• Suppose we want to load BX with the offset value

of table1• We cannot write

mov BX,table1

• Two ways of loading offset value» Using OFFSET assembler directive

– Executed only at the assembly time» Using lea instruction

– This is a processor instruction– Executed at run time

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Loading offset value into a register (cont’d)• Using OFFSET assembler directive

The previous example can be written as

mov BX,OFFSET table1

• Using lea (load effective address) instruction The format of lea instruction is

lea register,source The previous example can be written as

lea BX,table1

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Loading offset value into a register (cont’d)Which one to use -- OFFSET or lea?

Use OFFSET if possible» OFFSET incurs only one-time overhead (at assembly time)» lea incurs run time overhead (every time you run the program)

May have to use lea in some instances» When the needed data is available at run time only

– An index passed as a parameter to a procedure» We can write

lea BX,table1[SI]to load BX with the address of an element of table1 whose index is in SI register

» We cannot use the OFFSET directive in this case

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Default Segments

• In register indirect addressing mode 16-bit addresses

» Effective addresses in BX, SI, or DI is taken as the offset into the data segment (relative to DS)

» For BP and SP registers, the offset is taken to refer to the stack segment (relative to SS)

32-bit addresses» Effective address in EAX, EBX, ECX, EDX, ESI, and EDI is

relative to DS

» Effective address in EBP and ESP is relative to SS

push and pop are always relative to SS

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Default Segments (cont’d)

• Default segment override Possible to override the defaults by using override

prefixes» CS, DS, SS, ES, FS, GS

Example 1» We can use

add AX,SS:[BX] to refer to a data item on the stack

Example 2» We can use

add AX,DS:[BP] to refer to a data item in the data segment

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Data Transfer Instructions

• We will look at three instructions mov (move)

» Actually copy xchg (exchange)

» Exchanges two operands xlat (translate)

» Translates byte values using a translation table

• Other data transfer instructions such asmovsx (move sign extended)movzx (move zero extended)

are discussed in Chapter 12

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Data Transfer Instructions (cont’d)

The mov instruction The format is

mov destination,source» Copies the value from source to destination»source is not altered as a result of copying

» Both operands should be of same size

»source and destination cannot both be in memory

– Most Pentium instructions do not allow both operands to be located in memory

– Pentium provides special instructions to facilitate memory-to-memory block copying of data

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The mov instruction Five types of operand combinations are allowed:

Instruction type Example

mov register,register mov DX,CX

mov register,immediate mov BL,100

mov register,memory mov BX,count

mov memory,register mov count,SI

mov memory,immediate mov count,23

The operand combinations are valid for all instructions that require two operands

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Ambiguous moves: PTR directive• For the following data definitions

.DATA

table1 DW 20 DUP (0)

status DB 7 DUP (1)

the last two mov instructions are ambiguous

mov BX,OFFSET table1

mov SI,OFFSET status

mov [BX],100

mov [SI],100 Not clear whether the assembler should use byte or word

equivalent of 100

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Ambiguous moves: PTR directive• The PTR assembler directive can be used to

clarify• The last two mov instructions can be written as

mov WORD PTR [BX],100mov BYTE PTR [SI],100

WORD and BYTE are called type specifiers

• We can also use the following type specifiers:DWORD for doubleword valuesQWORD for quadword valuesTWORD for ten byte values

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The xchg instruction• The syntax is

xchg operand1,operand2

Exchanges the values of operand1 and operand2

Examplesxchg EAX,EDXxchg response,CLxchg total,DX

• Without the xchg instruction, we need a temporary register to exchange values using only the mov instruction

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The xchg instruction• The xchg instruction is useful for conversion of

16-bit data between little endian and big endian forms Example:

mov AL,AHconverts the data in AX into the other endian form

• Pentium provides bswap instruction to do similar conversion on 32-bit data

bswap 32-bit register bswap works only on data located in a 32-bit register

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The xlat instruction• The xlat instruction translates bytes• The format is

xlatb

• To use xlat instruction» BX should be loaded with the starting address of the translation table

» AL must contain an index in to the table

– Index value starts at zero

» The instruction reads the byte at this index in the translation table and stores this value in AL

– The index value in AL is lost

» Translation table can have at most 256 entries (due to AL)

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The xlat instructionExample: Encrypting digits

Input digits: 0 1 2 3 4 5 6 7 8 9Encrypted digits: 4 6 9 5 0 3 1 8 7 2

.DATAxlat_table DB ’4695031872’...

.CODEmov BX,OFFSET xlat_tableGetCh ALsub AL,’0’ ; converts input character to indexxlatb ; AL = encrypted digit characterPutCh AL ...

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Pentium Assembly Instructions

• Pentium provides several types of instructions• Brief overview of some basic instructions:

Arithmetic instructions Jump instructions Loop instruction Logical instructions Shift instructions Rotate instructions

• These instructions allow you to write reasonable assembly language programs

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Arithmetic Instructions

INC and DEC instructions Format:

inc destination dec destination

Semantics:destination = destination +/-

1» destination can be 8-, 16-, or 32-bit operand, in memory

or registerNo immediate operand

• Examplesinc BX

dec value

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Arithmetic Instructions (cont’d)

Add instructions Format:

add destination,source

Semantics:destination = destination + source

• Examplesadd EBX,EAX

add value,35

inc EAX is better than add EAX,1– inc takes less space

– Both execute at about the same speed

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Add instructions Addition with carry Format:

adc destination,source

Semantics:destination = destination + source + CF

• Example: 64-bit additionadd EAX,ECX ; add lower 32 bits

adc EBX,EDX ; add upper 32 bits with carry

64-bit result in EBX:EAX

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Subtract instructions Format:

sub destination,source

Semantics:destination = destination - source

• Examplessub EBX,EAX

sub value,35

dec EAX is better than sub EAX,1– dec takes less space

– Both execute at about the same speed

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Subtract instructions Subtract with borrow Format:

sbb destination,source Semantics:

destination = destination - source - CF Like the adc, sbb is useful in dealing with more than

32-bit numbers• Negation

neg destination Semantics:

destination = 0 - destination

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CMP instruction Format:

cmp destination,source

Semantics:destination - source

destination and source are not altered Useful to test relationship (>, =) between two operands Used in conjunction with conditional jump instructions

for decision making purposes• Examples

cmp EBX,EAX cmp count,100

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Unconditional Jump

Format:jmp label

Semantics:» Execution is transferred to the instruction identified by label

• Target can be specified in one of two ways Directly

» In the instruction itself

Indirectly» Through a register or memory

» Discussed in Chapter 12

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Unconditional Jump (cont’d)

Example

• Two jump instructions Forward jump

jmp CX_init_done

Backward jumpjmp repeat1

• Programmer specifies target by a label

• Assembler computes the offset using the symbol table

. . .

mov CX,10

jmp CX_init_done

init_CX_20:

mov CX,20

CX_init_done:

mov AX,CX

repeat1:

dec CX

. . .

jmp repeat1

. . .

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Unconditional Jump (cont’d)

• Address specified in the jump instruction is not the absolute address Uses relative address

» Specifies relative byte displacement between the target instruction and the instruction following the jump instruction

» Displacement is w.r.t the instruction following jmp– Reason: IP points to this instruction after reading jump

Execution of jmp involves adding the displacement value to current IP

Displacement is a signed 16-bit number» Negative value for backward jumps

» Positive value for forward jumps

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Target Location

• Inter-segment jump Target is in another segment

CS = target-segment (2 bytes)IP = target-offset (2 bytes)

» Called far jumps (needs five bytes to encode jmp)

• Intra-segment jumps Target is in the same segment

IP = IP + relative-displacement (1 or 2 bytes) Uses 1-byte displacement if target is within 128 to +127

» Called short jumps (needs two bytes to encode jmp) If target is outside this range, uses 2-byte displacement

» Called near jumps (needs three bytes to encode jmp)

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Target Location (cont’d)

• In most cases, the assembler can figure out the type of jump For backward jumps, assembler can decide whether to

use the short jump form or not

• For forward jumps, it needs a hint from the programmer Use SHORT prefix to the target label If such a hint is not given

» Assembler reserves three bytes for jmp instruction» If short jump can be used, leaves one byte of nop (no

operation)– See the next example for details

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Example

. . . 8 0005 EB 0C jmp SHORT CX_init_done

0013 - 0007 = 0C

9 0007 B9 000A mov CX,10

10 000A EB 07 90 jmp CX_init_done

nop 0013 - 000D = 07

11 init_CX_20:

12 000D B9 0014 mov CX,20

13 0010 E9 00D0 jmp near_jump

00E3 - 0013 = D0

14 CX_init_done:

15 0013 8B C1 mov AX,CX

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Example (cont’d)

16 repeat1:

17 0015 49 dec CX

18 0016 EB FD jmp repeat1

0015 - 0018 = -3 = FDH

. . .

84 00DB EB 03 jmp SHORT short_jump

00E0 - 00DD = 3

85 00DD B9 FF00 mov CX, 0FF00H

86 short_jump:

87 00E0 BA 0020 mov DX, 20H

88 near_jump:

89 00E3 E9 FF27 jmp init_CX_20

000D - 00E6 = -217 = FF27H

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Conditional Jumps (cont’d)

Format:j<cond> lab

– Execution is transferred to the instruction identified by label only if <cond> is met

• Example: Testing for carriage returnread_char:

. . . cmp AL,0DH ; 0DH = ASCII carriage return je CR_received inc CL jmp read_char . . .CR_received:

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Some conditional jump instructions– Treats operands of the CMP instruction as signed numbers

je jump if equaljg jump if greaterjl jump if lessjge jump if greater or equaljle jump if less or equaljne jump if not equal

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Conditional jump instructions can also test values of the individual flags

jz jump if zero (i.e., if ZF = 1)jnz jump if not zero (i.e., if ZF = 0) jc jump if carry (i.e., if CF = 1)jnc jump if not carry (i.e., if CF = 0)

jz is synonymous for je jnz is synonymous for jne

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A Note on Conditional Jumps

target:

. . .

cmp AX,BX

je target

mov CX,10

. . .

traget is out of range for a short jump

• Use this code to get around

target:

. . .

cmp AX,BX

jne skip1

jmp target

skip1:

mov CX,10

. . .

• All conditional jumps are encoded using 2 bytes Treated as short jumps

• What if the target is outside this range?

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Loop Instructions

Unconditional loop instruction Format:

loop target

Semantics:» Decrements CX and jumps to target if CX 0

– CX should be loaded with a loop count value• Example: Executes loop body 50 times

mov CX,50

repeat:

<loop body>

loop repeat ...

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Loop Instructions (cont’d)

• The previous example is equivalent to mov CX,50

repeat:

<loop body>

dec CX

jnz repeat ...

Surprisingly, dec CX

jnz repeat

executes faster than loop repeat

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Loop Instructions (cont’d)

• Conditional loop instructions loope/loopz

» Loop while equal/zero

CX = CX – 1ff (CX = 0 and ZF = 1) jump to target

loopne/loopnz» Loop while not equal/not zero

CX = CX – 1ff (CX = 0 and ZF = 0) jump to target

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Logical Instructions

Format:and destination,sourceor destination,sourcexor destination,sourcenot destination

Semantics:» Performs the standard bitwise logical operations

– result goes to destination test is a non-destructive and instruction

test destination,source

Performs logical AND but the result is not stored in destination (like the CMP instruction)

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Logical Instructions (cont’d)

Example: . . .

and AL,01H ; test the least significant bit

jz bit_is_zero

<bit 1 code>

jmp skip1

bit_is_zero:

<bit 0 code>

skip1:

. . .

• test instruction is better in place of and

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Shift Instructions

• Two types of shifts» Logical» Arithmetic

Logical shift instructionsShift left

shl destination,count shl destination,CL

Shift rightshr destination,count shr destination,CL

Semantics:» Performs left/right shift of destination by the value in count or CL register

– CL register contents are not altered

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Shift Instructions (cont’d)

Logical shift Bit shifted out goes into the carry flag

» Zero bit is shifted in at the other end

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count is an immediate valueshl AX,5

Specification of count greater than 31 is not allowed» If a greater value is specified, only the least significant 5 bits

are used

CL version is useful if shift count is known at run time» Ex: when the shift count value is passed as a parameter in a

procedure call

» Only the CL register can be usedShift count value should be loaded into CL

mov CL,5

shl AX,CL

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Arithmetic shift Two versions as in logical shift

sal/sar destination,count

sal/sar destination,CL

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Double Shift Instructions

• Double shift instructions work on either 32- or 64-bit operands

• Format Takes three operands

shld dest,src,count ; left shift

shrd dest,src,count ; right shift dest can be in memory or register src must be a register count can be an immediate value or in CL as in other

shift instructions

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Double Shift Instructions (cont’d)

src is not modified by doubleshift instruction Only dest is modified Shifted out bit goes into the carry flag

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Rotate Instructions

Two types of ROTATE instructions Rotate without carry

» rol (ROtate Left)» ror (ROtate Right)

Rotate with carry» rcl (Rotate through Carry Left)» rcr (Rotate through Carry Right)

Format of ROTATE instructions is similar to the SHIFT instructions

» Supports two versions– Immediate count value– Count value in CL register

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Rotate Instructions (cont’d)

Bit shifted out goes into the carry flag as in SHIFT instructions

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Bit shifted out goes into the carry flag as in SHIFT instructions

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• Example: Shifting 64-bit numbers Multiplies a 64-bit value in EDX:EAX by 16

» Rotate versionmov CX,4

shift_left:shl EAX,1rcl EDX,1loop shift_left

» Doubleshift versionshld EDX,EAX,4shl EAX,4

• Division can be done in a similar a way

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Defining Constants

• Assembler provides two directives:» EQU directive

– No reassignment– String constants can be defined

» = directive– Can be reassigned– No string constants

• Defining constants has two advantages: Improves program readability Helps in software maintenance

» Multiple occurrences can be changed from a single place

• Convention» We use all upper-case letters for names of constants

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Defining Constants (cont’d)

The EQU directive• Syntax:

name EQU expression Assigns the result of expression to name The expression is evaluated at assembly time

Similar to #define in CExamples

NUM_OF_ROWS EQU 50NUM_OF_COLS EQU 10ARRAY_SIZE EQU NUM_OF_ROWS * NUM_OF_COLS

Can also be used to define string constantsJUMP EQU jmp

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Defining Constants (cont’d)

The = directive• Syntax:

name = expression Similar to EQU directive Two key differences:

» Redefinition is allowedcount = 0. . .count = 99

is valid» Cannot be used to define string constants or to redefine

keywords or instruction mnemonics

Example: JUMP = jmp is not allowed

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Macros

• Macros can be defined with MACRO and ENDM• Format

macro_name MACRO[parameter1, parameter2,...] macro body

ENDM

• A macro can be invoked usingmacro_name [argument1, argument2, …]

Example: Definition InvocationmultAX_by_16 MACRO ...

sal AX,4 mov AX,27

ENDM multAX_by_16

...

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Macros (cont’d)

• Macros can be defined with parameters» More flexible

» More useful

• Examplemult_by_16 MACRO operand

sal operand,4

ENDM To multiply a byte in DL register

mult_by_16 DL

To multiply a memory variable count

mult_by_16 count

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Macros (cont’d)

Example: To exchange two memory words Memory-to-memory transfer

Wmxchg MACRO operand1, operand2

xchg AX,operand1

xchg AX,operand2

xchg AX,operand1

ENDM

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Illustrative Examples

• Five examples in this chapter Conversion of ASCII to binary representation

(BINCHAR.ASM) Conversion of ASCII to hexadecimal by character

manipulation (HEX1CHAR.ASM) Conversion of ASCII to hexadecimal using the XLAT

instruction (HEX2CHAR.ASM) Conversion of lowercase letters to uppercase by

character manipulation (TOUPPER.ASM) Sum of individual digits of a number

(ADDIGITS.ASM)Last slide