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                              Programmer's Manual

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        1.1.1  System requirements

        1.1.2  Executing compiler from command line

        1.1.3  Compiler messages

        1.1.4  Output formats

        1.2.1  Instruction syntax

        1.2.2  Data definitions

        1.2.3  Constants and labels

        1.2.4  Numerical expressions

        1.2.5  Jumps and calls

        1.2.6  Size settings

        2.1.1  Data movement instructions

        2.1.2  Type conversion instructions

        2.1.3  Binary arithmetic instructions

        2.1.4  Decimal arithmetic instructions

        2.1.5  Logical instructions

        2.1.6  Control transfer instructions

        2.1.7  I/O instructions

        2.1.8  Strings operations

        2.1.9  Flag control instructions

        2.1.10  Conditional operations

        2.1.11  Miscellaneous instructions

        2.1.12  System instructions

        2.1.13  FPU instructions

        2.1.14  MMX instructions

        2.1.15  SSE instructions

        2.1.16  SSE2 instructions

        2.1.17  SSE3 instructions

        2.1.18  AMD 3DNow! instructions

        2.1.19  The x86-64 long mode instructions

        2.2.1  Numerical constants

        2.2.2  Conditional assembly

        2.2.3  Repeating blocks of instructions

        2.2.4  Addressing spaces

        2.2.5  Other directives

        2.2.6  Multiple passes

        2.3.1  Including source files

        2.3.2  Symbolic constants

        2.3.3  Macroinstructions

        2.3.4  Structures

        2.3.5  Repeating macroinstructions

        2.3.6  Conditional preprocessing

        2.3.7  Order of processing

        2.4.1  MZ executable

        2.4.2  Portable Executable

        2.4.3  Common Object File Format

        2.4.4  Executable and Linkable Format

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using the flat assembler. If you are experienced assembly language programmer,

you should read at least this chapter before using this compiler.

processors, which does multiple passes to optimize the size of generated

machine code. It is self-compilable and versions for different operating

systems are provided. All the versions are designed to be used from the system

command line and they should not differ in behavior.

although they can produce programs for the x86 architecture 16-bit processors,

too. DOS version requires an OS compatible with MS DOS 2.0 and either true

real mode environment or DPMI. Windows version requires a Win32 console

compatible with 3.1 version.

parameters - first should be name of source file, second should be name of

destination file. If no second parameter is given, the name for output

file will be guessed automatically. After displaying short information about

the program name and version, compiler will read the data from source file and

compile it. When the compilation is successful, compiler will write the

generated code to the destination file and display the summary of compilation

process; otherwise it will display the information about error that occurred.

  The source file should be a text file, and can be created in any text

editor. Line breaks are accepted in both DOS and Unix standards, tabulators

are treated as spaces.

  In the command line you can also include "-m" option followed by a number,

which specifies how many kilobytes of memory flat assembler should maximally

use. In case of DOS version this options limits only the usage of extended

memory. The "-p" option followed by a number can be used to specify the limit

for number of passes the assembler performs. If code cannot be generated

within specified amount of passes, the assembly will be terminated with an

error message. The maximum value of this setting is 65536, while the default

limit, used when no such option is included in command line, is 100.

It is also possible to limit the number of passes the assembler

performs, with the "-p" option followed by a number specifying the maximum

number of passes.

  There are no command line options that would affect the output of compiler,

flat assembler requires only the source code to include the information it

really needs. For example, to specify output format you specify it by using

the "format" directive at the beginning of source.

the compilation summary. It includes the information of how many passes was

done, how much time it took, and how many bytes were written into the

destination file.

The following is an example of the compilation summary:

38 passes, 5.3 seconds, 77824 bytes.

error message. For example, when compiler can't find the input file, it will

display the following message:

error: source file not found.

that caused the error will be also displayed. Also placement of this line in

the source is given to help you finding this error, for example:

example.asm [3]:

        mob     ax,1

error: illegal instruction.

encountered an unrecognized instruction. When the line that caused error

contains a macroinstruction, also the line in macroinstruction definition

that generated the erroneous instruction is displayed:

example.asm [6]:

        stoschar 7

example.asm [3] stoschar [1]:

        mob     al,char

error: illegal instruction.

generated an unrecognized instruction with the first line of its definition.

assembler simply puts generated instruction codes into output, creating this

way flat binary file. By default it generates 16-bit code, but you can always

turn it into the 16-bit or 32-bit mode by using "use16" or "use32" directive.

Some of the output formats switch into 32-bit mode, when selected - more

information about formats which you can choose can be found in 2.4.

  All output code is always in the order in which it was entered into the

source file.

programmers that have been using some other assembly compilers before.

If you are beginner, you should look for the assembly programming tutorials.

  Flat assembler by default uses the Intel syntax for the assembly

instructions, although you can customize it using the preprocessor

capabilities (macroinstructions and symbolic constants). It also has its own

set of the directives - the instructions for compiler.

  All symbols defined inside the sources are case-sensitive.

instruction is expected to fill the one line of text. If a line contains

a semicolon, except for the semicolons inside the quoted strings, the rest of

this line is the comment and compiler ignores it. If a line ends with "\"

character (eventually the semicolon and comment may follow it), the next line

is attached at this point.

  Each line in source is the sequence of items, which may be one of the three

types. One type are the symbol characters, which are the special characters

that are individual items even when are not spaced from the other ones.

Any of the "+-*/=<>()[]{}:,|&~#`" is the symbol character. The sequence of

other characters, separated from other items with either blank spaces or

symbol characters, is a symbol. If the first character of symbol is either a

single or double quote, it integrates the any sequence of characters following

it, even the special ones, into a quoted string, which should end with the same

character, with which it began (the single or double quote) - however if there

are two such characters in a row (without any other character between them),

they are integrated into quoted string as just one of them and the quoted

string continues then. The symbols other than symbol characters and quoted

strings can be used as names, so are also called the name symbols.

  Every instruction consists of the mnemonic and the various number of

operands, separated with commas. The operand can be register, immediate value

or a data addressed in memory, it can also be preceded by size operator to

define or override its size (table 1.1). Names of available registers you can

find in table 1.2, their sizes cannot be overridden. Immediate value can be

specified by any numerical expression.

  When operand is a data in memory, the address of that data (also any

numerical expression, but it may contain registers) should be enclosed in

square brackets or preceded by "ptr" operator. For example instruction

"mov eax,3" will put the immediate value 3 into the EAX register, instruction

"mov eax,[7]" will put the 32-bit value from the address 7 into EAX and the

instruction "mov byte [7],3" will put the immediate value 3 into the byte at

address 7, it can also be written as "mov byte ptr 7,3". To specify which

segment register should be used for addressing, segment register name followed

by a colon should be put just before the address value (inside the square

brackets or after the "ptr" operator).

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  ³ Operator ³ Bits ³ Bytes ³

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  ³ byte     ³ 8    ³ 1     ³

  ³ word     ³ 16   ³ 2     ³

  ³ dword    ³ 32   ³ 4     ³

  ³ fword    ³ 48   ³ 6     ³

  ³ pword    ³ 48   ³ 6     ³

  ³ qword    ³ 64   ³ 8     ³

  ³ tbyte    ³ 80   ³ 10    ³

  ³ tword    ³ 80   ³ 10    ³

  ³ dqword   ³ 128  ³ 16    ³

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  ³ Type    ³ Bits ³                                                ³

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  ³         ³ 8    ³ al    cl    dl    bl    ah    ch    dh    bh   ³

  ³ General ³ 16   ³ ax    cx    dx    bx    sp    bp    si    di   ³

  ³         ³ 32   ³ eax   ecx   edx   ebx   esp   ebp   esi   edi  ³

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  ³ Segment ³ 16   ³ es    cs    ss    ds    fs    gs               ³

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  ³ Control ³ 32   ³ cr0         cr2   cr3   cr4                    ³

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  ³ Debug   ³ 32   ³ dr0   dr1   dr2   dr3               dr6   dr7  ³

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  ³ FPU     ³ 80   ³ st0   st1   st2   st3   st4   st5   st6   st7  ³

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  ³ MMX     ³ 64   ³ mm0   mm1   mm2   mm3   mm4   mm5   mm6   mm7  ³

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  ³ SSE     ³ 128  ³ xmm0  xmm1  xmm2  xmm3  xmm4  xmm5  xmm6  xmm7 ³

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table 1.3. The data definition directive should be followed by one or more of

numerical expressions, separated with commas. These expressions define the

values for data cells of size depending on which directive is used. For

example "db 1,2,3" will define the three bytes of values 1, 2 and 3

respectively.

  The "db" and "du" directives also accept the quoted string values of any

length, which will be converted into chain of bytes when "db" is used and into

chain of words with zeroed high byte when "du" is used. For example "db 'abc'"

will define the three bytes of values 61, 62 and 63.

  The "dp" directive and its synonym "df" accept the values consisting of two

numerical expressions separated with colon, the first value will become the

high word and the second value will become the low double word of the far

pointer value. Also "dd" accepts such pointers consisting of two word values

separated with colon, and "dt" accepts the word and quad word value separated

with colon, the quad word is stored first. The "dt" directive with single

expression as parameter accepts only floating point values and creates data in

FPU double extended precision format.

  Any of the above directive allows the usage of special "dup" operator to

make multiple copies of given values. The count of duplicates should precede

this operator and the value to duplicate should follow - it can even be the

chain of values separated with commas, but such set of values needs to be

enclosed with parenthesis, like "db 5 dup (1,2)", which defines five copies

of the given two byte sequence.

  The "file" is a special directive and its syntax is different. This

directive includes a chain of bytes from file and it should be followed by the

quoted file name, then optionally numerical expression specifying offset in

file preceded by the colon, and - also optionally - comma and numerical

expression specifying count of bytes to include (if no count is specified, all

data up to the end of file is included). For example "file 'data.bin'" will

include the whole file as binary data and "file 'data.bin':10h,4" will include

only four bytes starting at offset 10h.

  The data reservation directive should be followed by only one numerical

expression, and this value defines how many cells of the specified size should

be reserved. All data definition directives also accept the "?" value, which

means that this cell should not be initialized to any value and the effect is

the same as by using the data reservation directive. The uninitialized data

may not be included in the output file, so its values should be always

considered unknown.

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  ³ Size    ³ Define ³ Reserve ³

  ³ (bytes) ³ data   ³ data    ³

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  ³ 1       ³ db     ³ rb      ³

  ³         ³ file   ³         ³

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  ³ 2       ³ dw     ³ rw      ³

  ³         ³ du     ³         ³

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  ³ 4       ³ dd     ³ rd      ³

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  ³ 6       ³ dp     ³ rp      ³

  ³         ³ df     ³ rf      ³

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  ³ 8       ³ dq     ³ rq      ³

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  ³ 10      ³ dt     ³ rt      ³

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numbers. To define the constant or label you should use the specific

directives. Each label can be defined only once and it is accessible from the

any place of source (even before it was defined). Constant can be redefined

many times, but in this case it is accessible only after it was defined, and

is always equal to the value from last definition before the place where it's

used. When a constant is defined only once in source, it is - like the label -

accessible from anywhere.

  The definition of constant consists of name of the constant followed by the

"=" character and numerical expression, which after calculation will become

the value of constant. This value is always calculated at the time the

constant is defined. For example you can define "count" constant by using the

directive "count = 17", and then use it in the assembly instructions, like

"mov cx,count" - which will become "mov cx,17" during the compilation process.

  There are different ways to define labels. The simplest is to follow the

name of label by the colon, this directive can even be followed by the other

instruction in the same line. It defines the label whose value is equal to

offset of the point where it's defined. This method is usually used to label

the places in code. The other way is to follow the name of label (without a

colon) by some data directive. It defines the label with value equal to

offset of the beginning of defined data, and remembered as a label for data

with cell size as specified for that data directive in table 1.3.

  The label can be treated as constant of value equal to offset of labeled

code or data. For example when you define data using the labeled directive

"char db 224", to put the offset of this data into BX register you should use

"mov bx,char" instruction, and to put the value of byte addressed by "char"

label to DL register, you should use "mov dl,[char]" (or "mov dl,ptr char").

But when you try to assemble "mov ax,[char]", it will cause an error, because

fasm compares the sizes of operands, which should be equal. You can force

assembling that instruction by using size override: "mov ax,word [char]", but

remember that this instruction will read the two bytes beginning at "char"

address, while it was defined as a one byte.

  The last and the most flexible way to define labels is to use "label"

directive. This directive should be followed by the name of label, then

optionally size operator (it can be preceded by a colon) and then - also

optionally "at" operator and the numerical expression defining the address at

which this label should be defined. For example "label wchar word at char"

will define a new label for the 16-bit data at the address of "char". Now the

instruction "mov ax,[wchar]" will be after compilation the same as

"mov ax,word [char]". If no address is specified, "label" directive defines

the label at current offset. Thus "mov [wchar],57568" will copy two bytes

while "mov [char],224" will copy one byte to the same address.

  The label whose name begins with dot is treated as local label, and its name

is attached to the name of last global label (with name beginning with

anything but dot) to make the full name of this label. So you can use the

short name (beginning with dot) of this label anywhere before the next global

label is defined, and in the other places you have to use the full name. Label

beginning with two dots are the exception - they are like global, but they

don't become the new prefix for local labels.

  The "@@" name means anonymous label, you can have defined many of them in

the source. Symbol "@b" (or equivalent "@r") references the nearest preceding

anonymous label, symbol "@f" references the nearest following anonymous label.

These special symbol are case-insensitive.

constants or labels. But they can be more complex, by using the arithmetical

or logical operators for calculations at compile time. All these operators

with their priority values are listed in table 1.4.

The operations with higher priority value will be calculated first, you can

of course change this behavior by putting some parts of expression into

parenthesis. The "+", "-", "*" and "/" are standard arithmetical operations,

"mod" calculates the remainder from division. The "and", "or", "xor", "shl",

"shr" and "not" perform the same logical operations as assembly instructions

of those names. The "rva" performs the conversion of an address into the

relocatable offset and is specific to some of the output formats (see 2.4).

  The numbers in the expression are by default treated as a decimal, binary

numbers should have the "b" letter attached at the end, octal number should

end with "o" letter, hexadecimal numbers should begin with "0x" characters

(like in C language) or with the "$" character (like in Pascal language) or

they should end with "h" letter. Also quoted string, when encountered in

expression, will be converted into number - the first character will become

the least significant byte of number.

  The numerical expression used as an address value can also contain any of

general registers used for addressing, they can be added and multiplied by

appropriate values, as it is allowed for the x86 architecture instructions.

  There are also some special symbols that can be used inside the numerical

expression. First is "$", which is always equal to the value of current

offset, while "$$" is equal to base address of current addressing space. The

other one is "%", which is the number of current repeat in parts of code that

are repeated using some special directives (see 2.2). There's also "%t"

symbol, which is always equal to the current time stamp.

  Any numerical expression can also consist of single floating point value

(flat assembler does not allow any floating point operations at compilation

time) in the scientific notation, they can end with the "f" letter to be

recognized, otherwise they should contain at least one of the "." or "E"

characters. So "1.0", "1E0" and "1f" define the same floating point value,

while simple "1" defines an integer value.

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  ³ Priority ³ Operators    ³

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  ³ 0        ³ +  -         ³

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  ³ 1        ³ *  /         ³

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  ³ 2        ³ mod          ³

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  ³ 3        ³ and  or  xor ³

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  ³ 4        ³ shl  shr     ³

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  ³ 5        ³ not          ³

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  ³ 6        ³ rva          ³

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size operator, but also by one of the operators specifying type of the jump:

"short", "near" of "far". For example, when assembler is in 16-bit mode,

instruction "jmp dword [0]" will become the far jump and when assembler is

in 32-bit mode, it will become the near jump. To force this instruction to be

treated differently, use the "jmp near dword [0]" or "jmp far dword [0]" form.

  When operand of near jump is the immediate value, assembler will generate

the shortest variant of this jump instruction if possible (but won't create

32-bit instruction in 16-bit mode nor 16-bit instruction in 32-bit mode,

unless there is a size operator stating it). By specifying the jump type

you can force it to always generate long variant (for example "jmp near 0")

or to always generate short variant and terminate with an error when it's

impossible (for example "jmp short 0").

instruction is generated by using the short displacement if only address

value fits in the range. This can be overridden using the "word" or "dword"

operator before the address inside the square brackets (or after the "ptr"

operator), which forces the long displacement of appropriate size to be made.

In case when address is not relative to any registers, those operators allow

also to choose the appropriate mode of absolute addressing.

  Instructions "adc", "add", "and", "cmp", "or", "sbb", "sub" and "xor" with

first operand being 16-bit or 32-bit are by default generated in shortened

8-bit form when the second operand is immediate value fitting in the range

for signed 8-bit values. It also can be overridden by putting the "word" or

"dword" operator before the immediate value. The similar rules applies to the

"imul" instruction with the last operand being immediate value.

  Immediate value as an operand for "push" instruction without a size operator

is by default treated as a word value if assembler is in 16-bit mode and as a

double word value if assembler is in 32-bit mode, shorter 8-bit form of this

instruction is used if possible, "word" or "dword" size operator forces the

"push" instruction to be generated in longer form for specified size. "pushw"

and "pushd" mnemonics force assembler to generate 16-bit or 32-bit code

without forcing it to use the longer form of instruction.

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directives supported by flat assembler. Directives for defining labels were

already discussed in 1.2.3, all other directives will be described later in

this chapter.

purpose the assembly language instructions. If you need more technical

information, look for the Intel Architecture Software Developer's Manual.

  Assembly instructions consist of the mnemonic (instruction's name) and from

zero to three operands. If there are two or more operands, usually first is

the destination operand and second is the source operand. Each operand can be

register, memory or immediate value (see 1.2 for details about syntax of

operands). After the description of each instruction there are examples

of different combinations of operands, if the instruction has any.

  Some instructions act as prefixes and can be followed by other instruction

in the same line, and there can be more than one prefix in a line. Each name

of the segment register is also a mnemonic of instruction prefix, altough it

is recommended to use segment overrides inside the square brackets instead of

these prefixes.

destination operand. It can transfer data between general registers, from

the general register to memory, or from memory to general register, but it

cannot move from memory to memory. It can also transfer an immediate value to

general register or memory, segment register to general register or memory,

general register or memory to segment register, control or debug register to

general register and general register to control or debug register. The "mov"

can be assembled only if the size of source operand and size of destination

operand are the same. Below are the examples for each of the allowed

combinations:

    mov [char],al   ; general register to memory

    mov bl,[char]   ; memory to general register

    mov dl,32       ; immediate value to general register

    mov [char],32   ; immediate value to memory

    mov ax,ds       ; segment register to general register

    mov [bx],ds     ; segment register to memory

    mov ds,ax       ; general register to segment register

    mov ds,[bx]     ; memory to segment register

    mov eax,cr0     ; control register to general register

    mov cr3,ebx     ; general register to control register

two word operands or two double word operands. Order of operands is not

important. The operands may be two general registers, or general register

with memory. For example:

    xchg al,[char]  ; swap register with memory

the operand to the top of stack indicated by ESP. The operand can be memory,

general register, segment register or immediate value of word or double word

size. If operand is an immediate value and no size is specified, it is by

default treated as a word value if assembler is in 16-bit mode and as a double

word value if assembler is in 32-bit mode. "pushw" and "pushd" mnemonics are

variants of this instruction that store the values of word or double word size

respectively. If more operands follow in the same line (separated only with

spaces, not commas), compiler will assemble chain of the "push" instructions

with these operands. The examples are with single operands:

    push es         ; store segment register

    pushw [bx]      ; store memory

    push 1000h      ; store immediate value

This instruction has no operands. There are two version of this instruction,

one 16-bit and one 32-bit, assembler automatically generates the appropriate

version for current mode, but it can be overridden by using "pushaw" or

"pushad" mnemonic to always get the 16-bit or 32-bit version. The 16-bit

version of this instruction pushes general registers on the stack in the

following order: AX, CX, DX, BX, the initial value of SP before AX was pushed,

BP, SI and DI. The 32-bit version pushes equivalent 32-bit general registers

in the same order.

  "pop" transfers the word or double word at the current top of stack to the

destination operand, and then increments ESP to point to the new top of stack.

The operand can be memory, general register or segment register. "popw" and

"popd" mnemonics are variants of this instruction for restoring the values of

word or double word size respectively. If more operands separated with spaces

follow in the same line, compiler will assemble chain of the "pop"

instructions with these operands.

    pop ds          ; restore segment register

    popw [si]       ; restore memory

except for the saved value of SP (or ESP), which is ignored. This instruction

has no operands. To force assembling 16-bit or 32-bit version of this

instruction use "popaw" or "popad" mnemonic.

words, and double words into quad words. These conversions can be done using

the sign extension or zero extension. The sign extension fills the extra bits

of the larger item with the value of the sign bit of the smaller item, the

zero extension simply fills them with zeros.

  "cwd" and "cdq" double the size of value AX or EAX register respectively

and store the extra bits into the DX or EDX register. The conversion is done

using the sign extension. These instructions have no operands.

  "cbw" extends the sign of the byte in AL throughout AX, and "cwde" extends

the sign of the word in AX throughout EAX. These instructions also have no

operands.

  "movsx" converts a byte to word or double word and a word to double word

using the sign extension. "movzx" does the same, but it uses the zero

extension. The source operand can be general register or memory, while the

destination operand must be a general register. For example:

    movsx edx,dl        ; byte register to double word register

    movsx eax,ax        ; word register to double word register

    movsx ax,byte [bx]  ; byte memory to word register

    movsx edx,byte [bx] ; byte memory to double word register

    movsx eax,word [bx] ; word memory to double word register

destination operands and sets CF if overflow has occurred. The operands may

be bytes, words or double words. The destination operand can be general

register or memory, the source operand can be general register or immediate

value, it can also be memory if the destination operand is register.

    add ax,[si]     ; add memory to register

    add [di],al     ; add register to memory

    add al,48       ; add immediate value to register

    add [char],48   ; add immediate value to memory

operand with the result. Rules for the operands are the same as for the "add"

instruction. An "add" followed by multiple "adc" instructions can be used to

add numbers longer than 32 bits.

  "inc" adds one to the operand, it does not affect CF. The operand can be a

general register or memory, and the size of the operand can be byte, word or

double word.

    inc byte [bx]   ; increment memory by one

the destination operand with the result. If a borrow is required, the CF is

set. Rules for the operands are the same as for the "add" instruction.

  "sbb" subtracts the source operand from the destination operand, subtracts

one if CF is set, and stores the result to the destination operand. Rules for

the operands are the same as for the "add" instruction. A "sub" followed by

multiple "sbb" instructions may be used to subtract numbers longer than 32

bits.

  "dec" subtracts one from the operand, it does not affect CF. Rules for the

operand are the same as for the "inc" instruction.

  "cmp" subtracts the source operand from the destination operand. It updates

the flags as the "sub" instruction, but does not alter the source and

destination operands. Rules for the operands are the same as for the "sub"

instruction.

  "neg" subtracts a signed integer operand from zero. The effect of this

instructon is to reverse the sign of the operand from positive to negative or

from negative to positive. Rules for the operand are the same as for the "inc"

instruction.

  "xadd" exchanges the destination operand with the source operand, then loads

the sum of the two values into the destination operand. Rules for the operands

are the same as for the "add" instruction.

  All the above binary arithmetic instructions update SF, ZF, PF and OF flags.

SF is always set to the same value as the result's sign bit, ZF is set when

all the bits of result are zero, PF is set when low order eight bits of result

contain an even number of set bits, OF is set if result is too large for a

positive number or too small for a negative number (excluding sign bit) to fit

in destination operand.

  "mul" performs an unsigned multiplication of the operand and the

accumulator. If the operand is a byte, the processor multiplies it by the

contents of AL and returns the 16-bit result to AH and AL. If the operand is a

word, the processor multiplies it by the contents of AX and returns the 32-bit

result to DX and AX. If the operand is a double word, the processor multiplies

it by the contents of EAX and returns the 64-bit result in EDX and EAX. "mul"

sets CF and OF when the upper half of the result is nonzero, otherwise they

are cleared. Rules for the operand are the same as for the "inc" instruction.

  "imul" performs a signed multiplication operation. This instruction has

three variations. First has one operand and behaves in the same way as the

"mul" instruction. Second has two operands, in this case destination operand

is multiplied by the source operand and the result replaces the destination

operand. Destination operand must be a general register, it can be word or

double word, source operand can be general register, memory or immediate

value. Third form has three operands, the destination operand must be a

general register, word or double word in size, source operand can be general

register or memory, and third operand must be an immediate value. The source

operand is multiplied by the immediate value and the result is stored in the

destination register. All the three forms calculate the product to twice the

size of operands and set CF and OF when the upper half of the result is

nonzero, but second and third form truncate the product to the size of

operands. So second and third forms can be also used for unsigned operands

because, whether the operands are signed or unsigned, the lower half of the

product is the same. Below are the examples for all three forms:

    imul word [si]  ; accumulator by memory

    imul bx,cx      ; register by register

    imul bx,[si]    ; register by memory

    imul bx,10      ; register by immediate value

    imul ax,bx,10   ; register by immediate value to register

    imul ax,[si],10 ; memory by immediate value to register

The dividend (the accumulator) is twice the size of the divisor (the operand),

the quotient and remainder have the same size as the divisor. If divisor is

byte, the dividend is taken from AX register, the quotient is stored in AL and

the remainder is stored in AH. If divisor is word, the upper half of dividend

is taken from DX, the lower half of dividend is taken from AX, the quotient is

stored in AX and the remainder is stored in DX. If divisor is double word,

the upper half of dividend is taken from EDX, the lower half of dividend is

taken from EAX, the quotient is stored in EAX and the remainder is stored in

EDX. Rules for the operand are the same as for the "mul" instruction.

  "idiv" performs a signed division of the accumulator by the operand.

It uses the same registers as the "div" instruction, and the rules for

the operand are the same.

instructions (already described in the prior section) with the decimal

arithmetic instructions. The decimal arithmetic instructions are used to

adjust the results of a previous binary arithmetic operation to produce a

valid packed or unpacked decimal result, or to adjust the inputs to a

subsequent binary arithmetic operation so the operation will produce a valid

packed or unpacked decimal result.

  "daa" adjusts the result of adding two valid packed decimal operands in

AL. "daa" must always follow the addition of two pairs of packed decimal

numbers (one digit in each half-byte) to obtain a pair of valid packed

decimal digits as results. The carry flag is set if carry was needed.

This instruction has no operands.

  "das" adjusts the result of subtracting two valid packed decimal operands

in AL. "das" must always follow the subtraction of one pair of packed decimal

numbers (one digit in each half-byte) from another to obtain a pair of valid

packed decimal digits as results. The carry flag is set if a borrow was

needed. This instruction has no operands.

  "aaa" changes the contents of register AL to a valid unpacked decimal

number, and zeroes the top four bits. "aaa" must always follow the addition

of two unpacked decimal operands in AL. The carry flag is set and AH is

incremented if a carry is necessary. This instruction has no operands.

  "aas" changes the contents of register AL to a valid unpacked decimal

number, and zeroes the top four bits. "aas" must always follow the

subtraction of one unpacked decimal operand from another in AL. The carry flag

is set and AH decremented if a borrow is necessary. This instruction has no

operands.

  "aam" corrects the result of a multiplication of two valid unpacked decimal

numbers. "aam" must always follow the multiplication of two decimal numbers

to produce a valid decimal result. The high order digit is left in AH, the

low order digit in AL. The generalized version of this instruction allows

adjustment of the contents of the AX to create two unpacked digits of any

number base. The standard version of this instruction has no operands, the

generalized version has one operand - an immediate value specifying the

number base for the created digits.

  "aad" modifies the numerator in AH and AL to prepare for the division of two

valid unpacked decimal operands so that the quotient produced by the division

will be a valid unpacked decimal number. AH should contain the high order

digit and AL the low order digit. This instruction adjusts the value and

places the result in AL, while AH will contain zero. The generalized version

of this instruction allows adjustment of two unpacked digits of any number

base. Rules for the operand are the same as for the "aam" instruction.

complement of the operand. It has no effect on the flags. Rules for the

operand are the same as for the "inc" instruction.

  "and", "or" and "xor" instructions perform the standard

logical operations. They update the SF, ZF and PF flags. Rules for the

operands are the same as for the "add" instruction.

  "bt", "bts", "btr" and "btc" instructions operate on a single bit which can

be in memory or in a general register. The location of the bit is specified

as an offset from the low order end of the operand. The value of the offset

is the taken from the second operand, it either may be an immediate byte or

a general register. These instructions first assign the value of the selected

bit to CF. "bt" instruction does nothing more, "bts" sets the selected bit to

1, "btr" resets the selected bit to 0, "btc" changes the bit to its

complement. The first operand can be word or double word.

    bts word [bx],15 ; test and set bit in memory

    btr ax,cx        ; test and reset bit in register

    btc word [bx],cx ; test and complement bit in memory

and store the index of this bit into destination operand, which must be

general register. The bit string being scanned is specified by source operand,

it may be either general register or memory. The ZF flag is set if the entire

string is zero (no set bits are found); otherwise it is cleared. If no set bit

is found, the value of the destination register is undefined. "bsf" scans from

low order to high order (starting from bit index zero). "bsr" scans from high

order to low order (starting from bit index 15 of a word or index 31 of a

double word).

    bsr ax,[si]      ; scan memory reverse

in the second operand. The destination operand can be byte, word, or double

word general register or memory. The second operand can be an immediate value

or the CL register. The processor shifts zeros in from the right (low order)

side of the operand as bits exit from the left side. The last bit that exited

is stored in CF. "sal" is a synonym for "shl".

    shl byte [bx],1  ; shift memory left by one bit

    shl ax,cl        ; shift register left by count from cl

    shl word [bx],cl ; shift memory left by count from cl

specified in the second operand. Rules for operands are the same as for the

"shl" instruction. "shr" shifts zeros in from the left side of the operand as

bits exit from the right side. The last bit that exited is stored in CF.

"sar" preserves the sign of the operand by shifting in zeros on the left side

if the value is positive or by shifting in ones if the value is negative.

  "shld" shifts bits of the destination operand to the left by the number

of bits specified in third operand, while shifting high order bits from the

source operand into the destination operand on the right. The source operand

remains unmodified. The destination operand can be a word or double word

general register or memory, the source operand must be a general register,

third operand can be an immediate value or the CL register.

    shld [di],bx,1   ; shift memory left by one bit

    shld ax,bx,cl    ; shift register left by count from cl

    shld [di],bx,cl  ; shift memory left by count from cl

low order bits from the source operand into the destination operand on the

left. The source operand remains unmodified. Rules for operands are the same

as for the "shld" instruction.

  "rol" and "rcl" rotate the byte, word or double word destination operand

left by the number of bits specified in the second operand. For each rotation

specified, the high order bit that exits from the left of the operand returns

at the right to become the new low order bit. "rcl" additionally puts in CF

each high order bit that exits from the left side of the operand before it

returns to the operand as the low order bit on the next rotation cycle. Rules

for operands are the same as for the "shl" instruction.

  "ror" and "rcr" rotate the byte, word or double word destination operand

right by the number of bits specified in the second operand. For each rotation

specified, the low order bit that exits from the right of the operand returns

at the left to become the new high order bit. "rcr" additionally puts in CF

each low order bit that exits from the right side of the operand before it

returns to the operand as the high order bit on the next rotation cycle.

Rules for operands are the same as for the "shl" instruction.

  "test" performs the same action as the "and" instruction, but it does not

alter the destination operand, only updates flags. Rules for the operands are

the same as for the "and" instruction.

  "bswap" reverses the byte order of a 32-bit general register: bits 0 through

7 are swapped with bits 24 through 31, and bits 8 through 15 are swapped with

bits 16 through 23. This instruction is provided for converting little-endian

values to big-endian format and vice versa.

destination address can be specified directly within the instruction or

indirectly through a register or memory, the acceptable size of this address

depends on whether the jump is near or far (it can be specified by preceding

the operand with "near" or "far" operator) and whether the instruction is

16-bit or 32-bit. Operand for near jump should be "word" size for 16-bit

instruction or the "dword" size for 32-bit instruction. Operand for far jump

should be "dword" size for 16-bit instruction or "pword" size for 32-bit

instruction. A direct "jmp" instruction includes the destination address as

part of the instruction (and can be preceded by "short", "near" or "far"

operator), the operand specifying address should be the numerical expression

for near or short jump, or two numerical expressions separated with colon for

far jump, the first specifies selector of segment, the second is the offset

within segment. The "pword" operator can be used to force the 32-bit far call,

and "dword" to force the 16-bit far call. An indirect "jmp" instruction

obtains the destination address indirectly through a register or a pointer

variable, the operand should be general register or memory. See also 1.2.5 for

some more details.

    jmp 0FFFFh:0     ; direct far jump

    jmp ax           ; indirect near jump

    jmp pword [ebx]  ; indirect far jump

of the instruction following the "call" for later use by a "ret" (return)

instruction. Rules for the operands are the same as for the "jmp" instruction,

but the "call" has no short variant of direct instruction and thus it not

optimized.

  "ret", "retn" and "retf" instructions terminate the execution of a procedure

and transfers control back to the program that originally invoked the

procedure using the address that was stored on the stack by the "call"

instruction. "ret" is the equivalent for "retn", which returns from the

procedure that was executed using the near call, while "retf" returns from

the procedure that was executed using the far call. These instructions default

to the size of address appropriate for the current code setting, but the size

of address can be forced to 16-bit by using the "retw", "retnw" and "retfw"

mnemonics, and to 32-bit by using the "retd", "retnd" and "retfd" mnemonics.

All these instructions may optionally specify an immediate operand, by adding

this constant to the stack pointer, they effectively remove any arguments that

the calling program pushed on the stack before the execution of the "call"

instruction.

  "iret" returns control to an interrupted procedure. It differs from "ret" in

that it also pops the flags from the stack into the flags register. The flags

are stored on the stack by the interrupt mechanism. It defaults to the size of

return address appropriate for the current code setting, but it can be forced

to use 16-bit or 32-bit address by using the "iretw" or "iretd" mnemonic.

  The conditional transfer instructions are jumps that may or may not transfer

control, depending on the state of the CPU flags when the instruction

executes. The mnemonics for conditional jumps may be obtained by attaching

the condition mnemonic (see table 2.1) to the "j" mnemonic,

for example "jc" instruction will transfer the control when the CF flag is

set. The conditional jumps can be short or near, and direct only, and can be

optimized (see 1.2.5), the operand should be an immediate value specifying

target address.

  ÚÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿

  ³ Mnemonic ³ Condition tested      ³ Description            ³

  ÆÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍØÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍÍ͵

  ³ o        ³ OF = 1                ³ overflow               ³

  ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´

  ³ no       ³ OF = 0                ³ not overflow           ³

  ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´

  ³ c        ³                       ³ carry                  ³

  ³ b        ³ CF = 1                ³ below                  ³

  ³ nae      ³                       ³ not above nor equal    ³

  ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´

  ³ nc       ³                       ³ not carry              ³

  ³ ae       ³ CF = 0                ³ above or equal         ³

  ³ nb       ³                       ³ not below              ³

  ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´

  ³ e        ³ ZF = 1                ³ equal                  ³

  ³ z        ³                       ³ zero                   ³

  ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´

  ³ ne       ³ ZF = 0                ³ not equal              ³

  ³ nz       ³                       ³ not zero               ³

  ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´

  ³ be       ³ CF or ZF = 1          ³ below or equal         ³

  ³ na       ³                       ³ not above              ³

  ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´

  ³ a        ³ CF or ZF = 0          ³ above                  ³

  ³ nbe      ³                       ³ not below nor equal    ³

  ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´

  ³ s        ³ SF = 1                ³ sign                   ³

  ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´

  ³ ns       ³ SF = 0                ³ not sign               ³

  ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´

  ³ p        ³ PF = 1                ³ parity                 ³

  ³ pe       ³                       ³ parity even            ³

  ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´

  ³ np       ³ PF = 0                ³ not parity             ³

  ³ po       ³                       ³ parity odd             ³

  ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´

  ³ l        ³ SF xor OF = 1         ³ less                   ³

  ³ nge      ³                       ³ not greater nor equal  ³

  ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´

  ³ ge       ³ SF xor OF = 0         ³ greater or equal       ³

  ³ nl       ³                       ³ not less               ³

  ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´

  ³ le       ³ (SF xor OF) or ZF = 1 ³ less or equal          ³

  ³ ng       ³                       ³ not greater            ³

  ÃÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´

  ³ g        ³ (SF xor OF) or ZF = 0 ³ greater                ³

  ³ nle      ³                       ³ not less nor equal     ³

  ÀÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ

CX (or ECX) to specify the number of repetitions of a software loop. All

"loop" instructions automatically decrement CX (or ECX) and terminate the

loop (don't transfer the control) when CX (or ECX) is zero. It uses CX or ECX

whether the current code setting is 16-bit or 32-bit, but it can be forced to

us CX with the "loopw" mnemonic or to use ECX with the "loopd" mnemonic.

"loope" and "loopz" are the synonyms for the same instruction, which acts as

the standard "loop", but also terminates the loop when ZF flag is set.

"loopew" and "loopzw" mnemonics force them to use CX register while "looped"

and "loopzd" force them to use ECX register. "loopne" and "loopnz" are the

synonyms for the same instructions, which acts as the standard "loop", but

also terminate the loop when ZF flag is not set. "loopnew" and "loopnzw"

mnemonics force them to use CX register while "loopned" and "loopnzd" force

them to use ECX register. Every "loop" instruction needs an operand being an

immediate value specifying target address, it can be only short jump (in the

range of 128 bytes back and 127 bytes forward from the address of instruction

following the "loop" instruction).

  "jcxz" branches to the label specified in the instruction if it finds a

value of zero in CX, "jecxz" does the same, but checks the value of ECX

instead of CX. Rules for the operands are the same as for the "loop"

instruction.

  "int" activates the interrupt service routine that corresponds to the

number specified as an operand to the instruction, the number should be in

range from 0 to 255. The interrupt service routine terminates with an "iret"

instruction that returns control to the instruction that follows "int".

"int3" mnemonic codes the short (one byte) trap that invokes the interrupt 3.

"into" instruction invokes the interrupt 4 if the OF flag is set.

  "bound" verifies that the signed value contained in the specified register

lies within specified limits. An interrupt 5 occurs if the value contained in

the register is less than the lower bound or greater than the upper bound. It

needs two operands, the first operand specifies the register being tested,

the second operand should be memory address for the two signed limit values.

The operands can be "word" or "dword" in size.

    bound eax,[esi]  ; check double word for bounds

or EAX. I/O ports can be addressed either directly, with the immediate byte

value coded in instruction, or indirectly via the DX register. The destination

operand should be AL, AX, or EAX register. The source operand should be an

immediate value in range from 0 to 255, or DX register.

    in ax,dx         ; input word from port addressed by dx

or EAX. The program can specify the number of the port using the same methods

as the "in" instruction. The destination operand should be an immediate value

in range from 0 to 255, or DX register. The source operand should be AL, AX,

or EAX register.

    out dx,al        ; output byte to port addressed by dx

may be a byte, a word, or a double word. The string elements are addressed by

SI and DI (or ESI and EDI) registers. After every string operation SI and/or

DI (or ESI and/or EDI) are automatically updated to point to the next element

of the string. If DF (direction flag) is zero, the index registers are

incremented, if DF is one, they are decremented. The amount of the increment

or decrement is 1, 2, or 4 depending on the size of the string element. Every

string operation instruction has short forms which have no operands and use

SI and/or DI when the code type is 16-bit, and ESI and/or EDI when the code

type is 32-bit. SI and ESI by default address data in the segment selected

by DS, DI and EDI always address data in the segment selected by ES. Short

form is obtained by attaching to the mnemonic of string operation letter

specifying the size of string element, it should be "b" for byte element,

"w" for word element, and "d" for double word element. Full form of string

operation needs operands providing the size operator and the memory addresses,

which can be SI or ESI with any segment prefix, DI or EDI always with ES

segment prefix.

  "movs" transfers the string element pointed to by SI (or ESI) to the

location pointed to by DI (or EDI). Size of operands can be byte, word, or

double word. The destination operand should be memory addressed by DI or EDI,

the source operand should be memory addressed by SI or ESI with any segment

prefix.

    movs word [es:di],[ss:si]  ; transfer word

    movsd                      ; transfer double word

element and updates the flags AF, SF, PF, CF and OF, but it does not change

any of the compared elements. If the string elements are equal, ZF is set,

otherwise it is cleared. The first operand for this instruction should be the

source string element addressed by SI or ESI with any segment prefix, the

second operand should be the destination string element addressed by DI or

EDI.

    cmps word [ds:si],[es:di]  ; compare words

    cmps dword [fs:esi],[edi]  ; compare double words

(depending on the size of string element) and updates the flags AF, SF, ZF,

PF, CF and OF. If the values are equal, ZF is set, otherwise it is cleared.

The operand should be the destination string element addressed by DI or EDI.

    scasw                      ; scan word

    scas dword [es:edi]        ; scan double word

element. Rules for the operand are the same as for the "scas" instruction.

  "lods" places the source string element into AL, AX, or EAX. The operand

should be the source string element addressed by SI or ESI with any segment

prefix.

    lods word [cs:si]          ; load word

    lodsd                      ; load double word

by DX register to the destination string element. The destination operand

should be memory addressed by DI or EDI, the source operand should be the DX

register.

    ins word [es:di],dx        ; input word

    ins dword [edi],dx         ; input double word

DX register. The destination operand should be the DX register and the source

operand should be memory addressed by SI or ESI with any segment prefix.

    outsw                      ; output word

    outs dx,dword [gs:esi]     ; output double word

repeated string operation. When a string operation instruction has a repeat

prefix, the operation is executed repeatedly, each time using a different

element of the string. The repetition terminates when one of the conditions

specified by the prefix is satisfied. All three prefixes automatically

decrease CX or ECX register (depending whether string operation instruction

uses the 16-bit or 32-bit addressing) after each operation and repeat the

associated operation until CX or ECX is zero. "repe"/"repz" and

"repne"/"repnz" are used exclusively with the "scas" and "cmps" instructions

(described below). When these prefixes are used, repetition of the next

instruction depends on the zero flag (ZF) also, "repe" and "repz" terminate

the execution when the ZF is zero, "repne" and "repnz" terminate the execution

when the ZF is set.

    repe cmpsb       ; compare bytes until not equal

state of bits in the flag register. All instructions described in this

section have no operands.

  "stc" sets the CF (carry flag) to 1, "clc" zeroes the CF, "cmc" changes the

CF to its complement. "std" sets the DF (direction flag) to 1, "cld" zeroes