nasmdoc.txt

EDIT Assembler can for VC6 VC8 VC2005

       (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 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.

       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

   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: `+', `-', `~', `!' and `SEG'

       The highest-priority operators in NASM's expression grammar are
       those which only apply to one argument. `-' negates its operand, `+'
       does nothing (it's provided for symmetry with `-'), `~' computes the
       one's complement of its operand, `!' is the logical negation
       operator, and `SEG' provides the segment address of its operand
       (explained in more detail in section 3.6).

   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