NASM Manual

# The Netwide Assembler: NASM

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## Chapter 3: The NASM Language

### 3.1 Layout of a NASM Source Line

Like most assemblers, each NASM source line contains (unless it is a macro, a preprocessor directive or an assembler directive: see chapter 4 and chapter 6) some combination of the four fields

    label:    instruction operands        ; comment

As usual, most of these fields are optional; the presence or absence of any combination of a label, an instruction and a comment is allowed. Of course, the operand field is either required or forbidden by the presence and nature of the instruction field.

NASM uses backslash (\) as the line continuation character; if a line ends with backslash, the next line is considered to be a part of the backslash-ended line.

NASM places no restrictions on white space within a line: labels may have white space before them, or instructions may have no space before them, or anything. The colon after a label is also optional. (Note that this means that if you intend to code `lodsb` alone on a line, and type `lodab` by accident, then that's still a valid source line which does nothing but define a label. Running NASM with the command-line option `-w+orphan-labels` will cause it to warn you if you define a label alone on a line without a trailing colon.)

Valid characters in labels are letters, numbers, `_`, `$`, `#`, `@`, `~`, `.`, and `?`. The only characters which may be used as the first character of an identifier are letters, `.` (with special meaning: see section 3.9), `_` and `?`. An identifier may also be prefixed with a `$` to indicate that it is intended to be read as an identifier and not a reserved word; thus, if some other module you are linking with defines a symbol called `eax`, you can refer to `$eax` in NASM code to distinguish the symbol from the register. Maximum length of an identifier is 4095 characters.

The instruction field may contain any machine instruction: Pentium and P6 instructions, FPU instructions, MMX instructions and even undocumented instructions are all supported. The instruction may be prefixed by `LOCK`, `REP`, `REPE`/`REPZ` or `REPNE`/`REPNZ`, in the usual way. Explicit address-size and operand-size prefixes `A16`, `A32`, `A64`, `O16` and `O32`, `O64` are provided - one example of their use is given in chapter 10. You can also use the name of a segment register as an instruction prefix: coding `es mov [bx],ax` is equivalent to coding `mov [es:bx],ax`. We recommend the latter syntax, since it is consistent with other syntactic features of the language, but for instructions such as `LODSB`, which has no operands and yet can require a segment override, there is no clean syntactic way to proceed apart from `es lodsb`.

An instruction is not required to use a prefix: prefixes such as `CS`, `A32`, `LOCK` or `REPE` can appear on a line by themselves, and NASM will just generate the prefix bytes.

In addition to actual machine instructions, NASM also supports a number of pseudo-instructions, described in section 3.2.

Instruction operands may take a number of forms: they can be registers, described simply by the register name (e.g. `ax`, `bp`, `ebx`, `cr0`: NASM does not use the `gas`-style syntax in which register names must be prefixed by a `%` sign), or they can be effective addresses (see section 3.3), constants (section 3.4) or expressions (section 3.5).

For x87 floating-point instructions, NASM accepts a wide range of syntaxes: you can use two-operand forms like MASM supports, or you can use NASM's native single-operand forms in most cases. For example, you can code:

            fadd    st1             ; this sets st0 := st0 + st1
            fadd    st0,st1         ; so does this

            fadd    st1,st0         ; this sets st1 := st1 + st0
            fadd    to st1          ; so does this

Almost any x87 floating-point instruction that references memory must use one of the prefixes `DWORD`, `QWORD` or `TWORD` to indicate what size of memory operand it refers to.

### 3.2 Pseudo-Instructions

Pseudo-instructions are things which, though not real x86 machine instructions, are used in the instruction field anyway because that's the most convenient place to put them. The current pseudo-instructions are `DB`, `DW`, `DD`, `DQ`, `DT`, `DO` and `DY`; their uninitialized counterparts `RESB`, `RESW`, `RESD`, `RESQ`, `REST`, `RESO` and `RESY`; the `INCBIN` command, the `EQU` command, and the `TIMES` prefix.

#### 3.2.1 `DB` and Friends: Declaring Initialized Data

`DB`, `DW`, `DD`, `DQ`, `DT`, `DO` and `DY` are used, much as in MASM, to declare initialized data in the output file. They can be invoked in a wide range of ways:

          db    0x55                ; just the byte 0x55
          db    0x55,0x56,0x57      ; three bytes in succession
          db    'a',0x55            ; character constants are OK
          db    'hello',13,10,'$'   ; so are string constants
          dw    0x1234              ; 0x34 0x12
          dw    'a'                 ; 0x61 0x00 (it's just a number)
          dw    'ab'                ; 0x61 0x62 (character constant)
          dw    'abc'               ; 0x61 0x62 0x63 0x00 (string)
          dd    0x12345678          ; 0x78 0x56 0x34 0x12
          dd    1.234567e20         ; floating-point constant
          dq    0x123456789abcdef0  ; eight byte constant
          dq    1.234567e20         ; double-precision float
          dt    1.234567e20         ; extended-precision float

`DT`, `DO` and `DY` do not accept numeric constants as operands.

#### 3.2.2 `RESB` and Friends: Declaring Uninitialized Data

`RESB`, `RESW`, `RESD`, `RESQ`, `REST`, `RESO` and `RESY` are designed to be used in the BSS section of a module: they declare uninitialized storage space. Each takes a single operand, which is the number of bytes, words, doublewords or whatever to reserve. As stated in section 2.2.7, NASM does not support the MASM/TASM syntax of reserving uninitialized space by writing `DW ?` or similar things: this is what it does instead. The operand to a `RESB`-type pseudo-instruction is a critical expression: see section 3.8.

For example:

    buffer:         resb    64              ; reserve 64 bytes
    wordvar:        resw    1               ; reserve a word
    realarray       resq    10              ; array of ten reals
    ymmval:         resy    1               ; one YMM register

#### 3.2.3 `INCBIN`: Including External Binary Files

`INCBIN` is borrowed from the old Amiga assembler DevPac: it includes a binary file verbatim into the output file. This can be handy for (for example) including graphics and sound data directly into a game executable file. It can be called in one of these three ways:

        incbin  "file.dat"             ; include the whole file
        incbin  "file.dat",1024        ; skip the first 1024 bytes
        incbin  "file.dat",1024,512    ; skip the first 1024, and
                                       ; actually include at most 512

`INCBIN` is both a directive and a standard macro; the standard macro version searches for the file in the include file search path and adds the file to the dependency lists. This macro can be overridden if desired.

#### 3.2.4 `EQU`: Defining Constants

`EQU` defines a symbol to a given constant value: when `EQU` is used, the source line must contain a label. The action of `EQU` is to define the given label name to the value of its (only) operand. This definition is absolute, and cannot change later. So, for example,

    message         db      'hello, world'
    msglen          equ     $-message

defines `msglen` to be the constant 12. `msglen` may not then be redefined later. This is not a preprocessor definition either: the value of `msglen` is evaluated once, using the value of `$` (see section 3.5 for an explanation of `$`) at the point of definition, rather than being evaluated wherever it is referenced and using the value of `$` at the point of reference.

#### 3.2.5 `TIMES`: Repeating Instructions or Data

The `TIMES` prefix causes the instruction to be assembled multiple times. This is partly present as NASM's equivalent of the `DUP` syntax supported by MASM-compatible assemblers, in that you can code

    zerobuf:        times 64 db 0

or similar things; but `TIMES` is more versatile than that. The argument to `TIMES` is not just a numeric constant, but a numeric expression, so you can do things like

    buffer: db      'hello, world'
            times 64-$+buffer db ' '

which will store exactly enough spaces to make the total length of `buffer` up to 64. Finally, `TIMES` can be applied to ordinary instructions, so you can code trivial unrolled loops in it:

            times 100 movsb

Note that there is no effective difference between `times 100 resb 1` and `resb 100`, except that the latter will be assembled about 100 times faster due to the internal structure of the assembler.

The operand to `TIMES` is a critical expression (section 3.8).

Note also that `TIMES` can't be applied to macros: the reason for this is that `TIMES` is processed after the macro phase, which allows the argument to `TIMES` to contain expressions such as `64-$+buffer` as above. To repeat more than one line of code, or a complex macro, use the preprocessor `%rep` directive.

### 3.3 Effective Addresses

An effective address is any operand to an instruction which references memory. Effective addresses, in NASM, have a very simple syntax: they consist of an expression evaluating to the desired address, enclosed in square brackets. For example:

    wordvar dw      123
            mov     ax,[wordvar]
            mov     ax,[wordvar+1]
            mov     ax,[es:wordvar+bx]

Anything not conforming to this simple system is not a valid memory reference in NASM, for example `es:wordvar[bx]`.

More complicated effective addresses, such as those involving more than one register, work in exactly the same way:

            mov     eax,[ebx*2+ecx+offset]
            mov     ax,[bp+di+8]

NASM is capable of doing algebra on these effective addresses, so that things which don't necessarily look legal are perfectly all right:

        mov     eax,[ebx*5]             ; assembles as [ebx*4+ebx]
        mov     eax,[label1*2-label2]   ; ie [label1+(label1-label2)]

Some forms of effective address have more than one assembled form; in most such cases NASM will generate the smallest form it can. For example, there are distinct assembled forms for the 32-bit effective addresses `[eax*2+0]` and `[eax+eax]`, and NASM will generally generate the latter on the grounds that the former requires four bytes to store a zero offset.

NASM has a hinting mechanism which will cause `[eax+ebx]` and `[ebx+eax]` to generate different opcodes; this is occasionally useful because `[esi+ebp]` and `[ebp+esi]` have different default segment registers.

However, you can force NASM to generate an effective address in a particular form by the use of the keywords `BYTE`, `WORD`, `DWORD` and `NOSPLIT`. If you need `[eax+3]` to be assembled using a double-word offset field instead of the one byte NASM will normally generate, you can code `[dword eax+3]`. Similarly, you can force NASM to use a byte offset for a small value which it hasn't seen on the first pass (see section 3.8 for an example of such a code fragment) by using `[byte eax+offset]`. As special cases, `[byte eax]` will code `[eax+0]` with a byte offset of zero, and `[dword eax]` will code it with a double-word offset of zero. The normal form, `[eax]`, will be coded with no offset field.

The form described in the previous paragraph is also useful if you are trying to access data in a 32-bit segment from within 16 bit code. For more information on this see the section on mixed-size addressing (section 10.2). In particular, if you need to access data with a known offset that is larger than will fit in a 16-bit value, if you don't specify that it is a dword offset, nasm will cause the high word of the offset to be lost.

Similarly, NASM will split `[eax*2]` into `[eax+eax]` because that allows the offset field to be absent and space to be saved; in fact, it will also split `[eax*2+offset]` into `[eax+eax+offset]`. You can combat this behaviour by the use of the `NOSPLIT` keyword: `[nosplit eax*2]` will force `[eax*2+0]` to be generated literally.

In 64-bit mode, NASM will by default generate absolute addresses. The `REL` keyword makes it produce `RIP`-relative addresses. Since this is frequently the normally desired behaviour, see the `DEFAULT` directive (section 6.2). The keyword `ABS` overrides `REL`.

### 3.4 Constants

NASM understands four different types of constant: numeric, character, string and floating-point.

#### 3.4.1 Numeric Constants

A numeric constant is simply a number. NASM allows you to specify numbers in a variety of number bases, in a variety of ways: you can suffix `H` or `X`, `D` or `T`, `Q` or `O`, and `B` or `Y` for hexadecimal, decimal, octal and binary respectively, or you can prefix `0x`, for hexadecimal in the style of C, or you can prefix `$` for hexadecimal in the style of Borland Pascal or Motorola Assemblers. Note, though, that the `$` prefix does double duty as a prefix on identifiers (see section 3.1), so a hex number prefixed with a `$` sign must have a digit after the `$` rather than a letter. In addition, current versions of NASM accept the prefix `0h` for hexadecimal, `0d` or `0t` for decimal, `0o` or `0q` for octal, and `0b` or `0y` for binary. Please note that unlike C, a `0` prefix by itself does not imply an octal constant!

Numeric constants can have underscores (`_`) interspersed to break up long strings.

Some examples (all producing exactly the same code):

            mov     ax,200          ; decimal
            mov     ax,0200         ; still decimal
            mov     ax,0200d        ; explicitly decimal
            mov     ax,0d200        ; also decimal
            mov     ax,0c8h         ; hex
            mov     ax,$0c8         ; hex again: the 0 is required
            mov     ax,0xc8         ; hex yet again
            mov     ax,0hc8         ; still hex
            mov     ax,310q         ; octal
            mov     ax,310o         ; octal again
            mov     ax,0o310        ; octal yet again
            mov     ax,0q310        ; octal yet again
            mov     ax,11001000b    ; binary
            mov     ax,1100_1000b   ; same binary constant
            mov     ax,1100_1000y   ; same binary constant once more
            mov     ax,0b1100_1000  ; same binary constant yet again
            mov     ax,0y1100_1000  ; same binary constant yet again

#### 3.4.2 Character Strings

A character string consists of up to eight characters enclosed in either single quotes (`'...'`), double quotes (`"..."`) or backquotes (``...``). Single or double quotes are equivalent to NASM (except of course that surrounding the constant with single quotes allows double quotes to appear within it and vice versa); the contents of those are represented verbatim. Strings enclosed in backquotes support C-style `\`-escapes for special characters.

The following escape sequences are recognized by backquoted strings:

          \'          single quote (')
          \"          double quote (")
          \`          backquote (`)
          \\          backslash (\)
          \?          question mark (?)
          \a          BEL (ASCII 7)
          \b          BS  (ASCII 8)
          \t          TAB (ASCII 9)
          \n          LF  (ASCII 10)
          \v          VT  (ASCII 11)
          \f          FF  (ASCII 12)
          \r          CR  (ASCII 13)
          \e          ESC (ASCII 27)
          \377        Up to 3 octal digits - literal byte
          \xFF        Up to 2 hexadecimal digits - literal byte
          \u1234      4 hexadecimal digits - Unicode character
          \U12345678  8 hexadecimal digits - Unicode character

All other escape sequences are reserved. Note that `\0`, meaning a `NUL` character (ASCII 0), is a special case of the octal escape sequence.

Unicode characters specified with `\u` or `\U` are converted to UTF-8. For example, the following lines are all equivalent:

          db `\u263a`            ; UTF-8 smiley face
          db `\xe2\x98\xba`      ; UTF-8 smiley face
          db 0E2h, 098h, 0BAh    ; UTF-8 smiley face

#### 3.4.3 Character Constants

A character constant consists of a string up to eight bytes long, used in an expression context. It is treated as if it was an integer.

A character constant with more than one byte will be arranged with little-endian order in mind: if you code

              mov eax,'abcd'

then the constant generated is not `0x61626364`, but `0x64636261`, so that if you were then to store the value into memory, it would read `abcd` rather than `dcba`. This is also the sense of character constants understood by the Pentium's `CPUID` instruction.

#### 3.4.4 String Constants

String constants are character strings used in the context of some pseudo-instructions, namely the `DB` family and `INCBIN` (where it represents a filename.) They are also used in certain preprocessor directives.

A string constant looks like a character constant, only longer. It is treated as a concatenation of maximum-size character constants for the conditions. So the following are equivalent:

          db    'hello'               ; string constant
          db    'h','e','l','l','o'   ; equivalent character constants

And the following are also equivalent:

          dd    'ninechars'           ; doubleword string constant
          dd    'nine','char','s'     ; becomes three doublewords
          db    'ninechars',0,0,0     ; and really looks like this

Note that when used in a string-supporting context, quoted strings are treated as a string constants even if they are short enough to be a character constant, because otherwise `db 'ab'` would have the same effect as `db 'a'`, which would be silly. Similarly, three-character or four-character constants are treated as strings when they are operands to `DW`, and so forth.

#### 3.4.5 Unicode Strings

The special operators `__utf16__`, `__utf16le__`, `__utf16be__`, `__utf32__`, `__utf32le__` and `__utf32be__` allows definition of Unicode strings. They take a string in UTF-8 format and converts it to UTF-16 or UTF-32, respectively. Unless the `be` forms are specified, the output is littleendian.

For example:

    %define u(x) __utf16__(x)
    %define w(x) __utf32__(x)

          dw u('C:\WINDOWS'), 0       ; Pathname in UTF-16
          dd w(`A + B = \u206a`), 0   ; String in UTF-32

The UTF operators can be applied either to strings passed to the `DB` family instructions, or to character constants in an expression context.

#### 3.4.6 Floating-Point Constants

Floating-point constants are acceptable only as arguments to `DB`, `DW`, `DD`, `DQ`, `DT`, and `DO`, or as arguments to the special operators `__float8__`, `__float16__`, `__float32__`, `__float64__`, `__float80m__`, `__float80e__`, `__float128l__`, and `__float128h__`.

Floating-point constants are expressed in the traditional form: digits, then a period, then optionally more digits, then optionally an `E` followed by an exponent. The period is mandatory, so that NASM can distinguish between `dd 1`, which declares an integer constant, and `dd 1.0` which declares a floating-point constant.

NASM also support C99-style hexadecimal floating-point: `0x`, hexadecimal digits, period, optionally more hexadeximal digits, then optionally a `P` followed by a binary (not hexadecimal) exponent in decimal notation. As an extension, NASM additionally supports the `0h` and `$` prefixes for hexadecimal, as well binary and octal floating-point, using the `0b` or `0y` and `0o` or `0q` prefixes, respectively.

Underscores to break up groups of digits are permitted in floating-point constants as well.

Some examples:

          db    -0.2                    ; "Quarter precision"
          dw    -0.5                    ; IEEE 754r/SSE5 half precision
          dd    1.2                     ; an easy one
          dd    1.222_222_222           ; underscores are permitted
          dd    0x1p+2                  ; 1.0x2^2 = 4.0
          dq    0x1p+32                 ; 1.0x2^32 = 4 294 967 296.0
          dq    1.e10                   ; 10 000 000 000.0
          dq    1.e+10                  ; synonymous with 1.e10
          dq    1.e-10                  ; 0.000 000 000 1
          dt    3.141592653589793238462 ; pi
          do    1.e+4000                ; IEEE 754r quad precision

The 8-bit "quarter-precision" floating-point format is sign:exponent:mantissa = 1:4:3 with an exponent bias of 7. This appears to be the most frequently used 8-bit floating-point format, although it is not covered by any formal standard. This is sometimes called a "minifloat."

The special operators are used to produce floating-point numbers in other contexts. They produce the binary representation of a specific floating-point number as an integer, and can use anywhere integer constants are used in an expression. `__float80m__` and `__float80e__` produce the 64-bit mantissa and 16-bit exponent of an 80-bit floating-point number, and `__float128l__` and `__float128h__` produce the lower and upper 64-bit halves of a 128-bit floating-point number, respectively.

For example:

          mov    rax,__float64__(3.141592653589793238462)

... would assign the binary representation of pi as a 64-bit floating point number into `RAX`. This is exactly equivalent to:

          mov    rax,0x400921fb54442d18

NASM cannot do compile-time arithmetic on floating-point constants. This is because NASM is designed to be portable - although it always generates code to run on x86 processors, the assembler itself can run on any system with an ANSI C compiler. Therefore, the assembler cannot guarantee the presence of a floating-point unit capable of handling the Intel number formats, and so for NASM to be able to do floating arithmetic it would have to include its own complete set of floating-point routines, which would significantly increase the size of the assembler for very little benefit.

The special tokens `__Infinity__`, `__QNaN__` (or `__NaN__`) and `__SNaN__` can be used to generate infinities, quiet NaNs, and signalling NaNs, respectively. These are normally used as macros:

    %define Inf __Infinity__
    %define NaN __QNaN__

          dq    +1.5, -Inf, NaN         ; Double-precision constants

The `%use fp` standard macro package contains a set of convenience macros. See section 5.3.

#### 3.4.7 Packed BCD Constants

x87-style packed BCD constants can be used in the same contexts as 80-bit floating-point numbers. They are suffixed with `p` or prefixed with `0p`, and can include up to 18 decimal digits.

As with other numeric constants, underscores can be used to separate digits.

For example:

          dt 12_345_678_901_245_678p
          dt -12_345_678_901_245_678p
          dt +0p33
          dt 33p

### 3.5 Expressions

Expressions in NASM are similar in syntax to those in C. Expressions are evaluated as 64-bit integers which are then adjusted to the appropriate size.

NASM supports two special tokens in expressions, allowing calculations to involve the current assembly position: the `$` and `$$` tokens. `$` evaluates to the assembly position at the beginning of the line containing the expression; so you can code an infinite loop using `JMP $`. `$$` evaluates to the beginning of the current section; so you can tell how far into the section you are by using `($-$$)`.

The arithmetic operators provided by NASM are listed here, in increasing order of precedence.

#### 3.5.1 `|`: Bitwise OR Operator

The `|` operator gives a bitwise OR, exactly as performed by the `OR` machine instruction. Bitwise OR is the lowest-priority arithmetic operator supported by NASM.

#### 3.5.2 `^`: Bitwise XOR Operator

`^` provides the bitwise XOR operation.

#### 3.5.3 `&`: Bitwise AND Operator

`&` provides the bitwise AND operation.

#### 3.5.4 `<<` and `>>`: Bit Shift Operators

`<<` gives a bit-shift to the left, just as it does in C. So `5<<3` evaluates to 5 times 8, or 40. `>>` gives a bit-shift to the right; in NASM, such a shift is always unsigned, so that the bits shifted in from the left-hand end are filled with zero rather than a sign-extension of the previous highest bit.

#### 3.5.5 `+` and `-`: Addition and Subtraction Operators

The `+` and `-` operators do perfectly ordinary addition and subtraction.

#### 3.5.6 `*`, `/`, `//`, `%` and `%%`: Multiplication and Division

`*` is the multiplication operator. `/` and `//` are both division operators: `/` is unsigned division and `//` is signed division. Similarly, `%` and `%%` provide unsigned and signed modulo operators respectively.

NASM, like ANSI C, provides no guarantees about the sensible operation of the signed modulo operator.

Since the `%` character is used extensively by the macro preprocessor, you should ensure that both the signed and unsigned modulo operators are followed by white space wherever they appear.

#### 3.5.7 Unary Operators

The highest-priority operators in NASM's expression grammar are those which only apply to one argument. These are `+`, `-`, `~`, `!`, `SEG`, and the integer functions operators.

`-` negates its operand, `+` does nothing (it's provided for symmetry with `-`), `~` computes the one's complement of its operand, `!` is the logical negation operator.

`SEG` provides the segment address of its operand (explained in more detail in section 3.6).

A set of additional operators with leading and trailing double underscores are used to implement the integer functions of the `ifunc` macro package, see section 5.4.

### 3.6 `SEG` and `WRT`

When writing large 16-bit programs, which must be split into multiple segments, it is often necessary to be able to refer to the segment part of the address of a symbol. NASM supports the `SEG` operator to perform this function.

The `SEG` operator returns the preferred segment base of a symbol, defined as the segment base relative to which the offset of the symbol makes sense. So the code

            mov     ax,seg symbol
            mov     es,ax
            mov     bx,symbol

will load `ES:BX` with a valid pointer to the symbol `symbol`.

Things can be more complex than this: since 16-bit segments and groups may overlap, you might occasionally want to refer to some symbol using a different segment base from the preferred one. NASM lets you do this, by the use of the `WRT` (With Reference To) keyword. So you can do things like

            mov     ax,weird_seg        ; weird_seg is a segment base
            mov     es,ax
            mov     bx,symbol wrt weird_seg

to load `ES:BX` with a different, but functionally equivalent, pointer to the symbol `symbol`.

NASM supports far (inter-segment) calls and jumps by means of the syntax `call segment:offset`, where `segment` and `offset` both represent immediate values. So to call a far procedure, you could code either of

            call    (seg procedure):procedure
            call    weird_seg:(procedure wrt weird_seg)

(The parentheses are included for clarity, to show the intended parsing of the above instructions. They are not necessary in practice.)

NASM supports the syntax `call far procedure` as a synonym for the first of the above usages. `JMP` works identically to `CALL` in these examples.

To declare a far pointer to a data item in a data segment, you must code

            dw      symbol, seg symbol

NASM supports no convenient synonym for this, though you can always invent one using the macro processor.

### 3.7 `STRICT`: Inhibiting Optimization

When assembling with the optimizer set to level 2 or higher (see section 2.1.22), NASM will use size specifiers (`BYTE`, `WORD`, `DWORD`, `QWORD`, `TWORD`, `OWORD` or `YWORD`), but will give them the smallest possible size. The keyword `STRICT` can be used to inhibit optimization and force a particular operand to be emitted in the specified size. For example, with the optimizer on, and in `BITS 16` mode,

            push dword 33

is encoded in three bytes `66 6A 21`, whereas

            push strict dword 33

is encoded in six bytes, with a full dword immediate operand `66 68 21 00 00 00`.

With the optimizer off, the same code (six bytes) is generated whether the `STRICT` keyword was used or not.

### 3.8 Critical Expressions

Although NASM has an optional multi-pass optimizer, there are some expressions which must be resolvable on the first pass. These are called Critical Expressions.

The first pass is used to determine the size of all the assembled code and data, so that the second pass, when generating all the code, knows all the symbol addresses the code refers to. So one thing NASM can't handle is code whose size depends on the value of a symbol declared after the code in question. For example,

            times (label-$) db 0
    label:  db      'Where am I?'

The argument to `TIMES` in this case could equally legally evaluate to anything at all; NASM will reject this example because it cannot tell the size of the `TIMES` line when it first sees it. It will just as firmly reject the slightly paradoxical code

            times (label-$+1) db 0
    label:  db      'NOW where am I?'

in which any value for the `TIMES` argument is by definition wrong!

NASM rejects these examples by means of a concept called a critical expression, which is defined to be an expression whose value is required to be computable in the first pass, and which must therefore depend only on symbols defined before it. The argument to the `TIMES` prefix is a critical expression.

### 3.9 Local Labels

NASM gives special treatment to symbols beginning with a period. A label beginning with a single period is treated as a local label, which means that it is associated with the previous non-local label. So, for example:

    label1  ; some code

    .loop
            ; some more code

            jne     .loop
            ret

    label2  ; some code

    .loop
            ; some more code

            jne     .loop
            ret

In the above code fragment, each `JNE` instruction jumps to the line immediately before it, because the two definitions of `.loop` are kept separate by virtue of each being associated with the previous non-local label.

This form of local label handling is borrowed from the old Amiga assembler DevPac; however, NASM goes one step further, in allowing access to local labels from other parts of the code. This is achieved by means of defining a local label in terms of the previous non-local label: the first definition of `.loop` above is really defining a symbol called `label1.loop`, and the second defines a symbol called `label2.loop`. So, if you really needed to, you could write

    label3  ; some more code
            ; and some more

            jmp label1.loop

Sometimes it is useful - in a macro, for instance - to be able to define a label which can be referenced from anywhere but which doesn't interfere with the normal local-label mechanism. Such a label can't be non-local because it would interfere with subsequent definitions of, and references to, local labels; and it can't be local because the macro that defined it wouldn't know the label's full name. NASM therefore introduces a third type of label, which is probably only useful in macro definitions: if a label begins with the special prefix `..@`, then it does nothing to the local label mechanism. So you could code

    label1:                         ; a non-local label
    .local:                         ; this is really label1.local
    ..@foo:                         ; this is a special symbol
    label2:                         ; another non-local label
    .local:                         ; this is really label2.local

            jmp     ..@foo          ; this will jump three lines up

NASM has the capacity to define other special symbols beginning with a double period: for example, `..start` is used to specify the entry point in the `obj` output format (see section 7.4.6), `..imagebase` is used to find out the offset from a base address of the current image in the `win64` output format (see section 7.6.1). So just keep in mind that symbols beginning with a double period are special.

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