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 5) 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.

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, O16 and O32 are provided - one example of their use is given in chapter 9. 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 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. Details of all forms of each supported instruction are given in appendix B. 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 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 and DT, their uninitialised counterparts RESB, RESW, RESD, RESQ and REST, the INCBIN command, the EQU command, and the TIMES prefix.

3.2.1 DB and friends: Declaring Initialised Data

DB, DW, DD, DQ and DT are used, much as in MASM, to declare initialised 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'                 ; 0x41 0x00 (it's just a number) 
      dw    'ab'                ; 0x41 0x42 (character constant) 
      dw    'abc'               ; 0x41 0x42 0x43 0x00 (string) 
      dd    0x12345678          ; 0x78 0x56 0x34 0x12 
      dd    1.234567e20         ; floating-point constant 
      dq    1.234567e20         ; double-precision float 
      dt    1.234567e20         ; extended-precision float

DQ and DT do not accept numeric constants or string constants as operands.

3.2.2 RESB and friends: Declaring Uninitialised Data

RESB, RESW, RESD, RESQ and REST are designed to be used in the BSS section of a module: they declare uninitialised 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 uninitialised 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

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

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. Note that the operand to an EQU is also a critical expression (section 3.8).

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, like that of EQU and those of RESB and friends, 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 9.2). 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.

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, Q and B for hex, octal and binary, or you can prefix 0x for hex in the style of C, or you can prefix $ for hex in the style of Borland Pascal. 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.

Some examples:

        mov     ax,100          ; decimal 
        mov     ax,0a2h         ; hex 
        mov     ax,$0a2         ; hex again: the 0 is required 
        mov     ax,0xa2         ; hex yet again 
        mov     ax,777q         ; octal 
        mov     ax,10010011b    ; binary

3.4.2 Character Constants

A character constant consists of up to four characters enclosed in either single or double quotes. The type of quote makes no difference to NASM, except of course that surrounding the constant with single quotes allows double quotes to appear within it and vice versa.

A character constant with more than one character 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 (see section B.4.34).

3.4.3 String Constants

String constants are only acceptable to some pseudo-instructions, namely the DB family and INCBIN.

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 as an operand to db, a constant like 'ab' is treated as a string constant despite being 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.

3.4.4 Floating-Point Constants

Floating-point constants are acceptable only as arguments to DD, DQ and DT. They 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.

Some examples:

      dd    1.2                     ; an easy one 
      dq    1.e10                   ; 10,000,000,000 
      dq    1.e+10                  ; synonymous with 1.e10 
      dq    1.e-10                  ; 0.000 000 000 1 
      dt    3.141592653589793238462 ; pi

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.

3.5 Expressions

Expressions in NASM are similar in syntax to those in C.

NASM does not guarantee the size of the integers used to evaluate expressions at compile time: since NASM can compile and run on 64-bit systems quite happily, don't assume that expressions are evaluated in 32-bit registers and so try to make deliberate use of integer overflow. It might not always work. The only thing NASM will guarantee is what's guaranteed by ANSI C: you always have at least 32 bits to work in.

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, 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

        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 2.1.16), NASM will usee size specifiers (BYTE, WORD, DWORD, QWORD, or TWORD), 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

A limitation of NASM is that it is a two-pass assembler; unlike TASM and others, it will always do exactly two assembly passes. Therefore it is unable to cope with source files that are complex enough to require three or more passes.

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; for the same reason, the arguments to the RESB family of pseudo-instructions are also critical expressions.

Critical expressions can crop up in other contexts as well: consider the following code.

                mov     ax,symbol1 
symbol1         equ     symbol2 
symbol2:

On the first pass, NASM cannot determine the value of symbol1, because symbol1 is defined to be equal to symbol2 which NASM hasn't seen yet. On the second pass, therefore, when it encounters the line mov ax,symbol1, it is unable to generate the code for it because it still doesn't know the value of symbol1. On the next line, it would see the EQU again and be able to determine the value of symbol1, but by then it would be too late.

NASM avoids this problem by defining the right-hand side of an EQU statement to be a critical expression, so the definition of symbol1 would be rejected in the first pass.

There is a related issue involving forward references: consider this code fragment.

        mov     eax,[ebx+offset] 
offset  equ     10

NASM, on pass one, must calculate the size of the instruction mov eax,[ebx+offset] without knowing the value of offset. It has no way of knowing that offset is small enough to fit into a one-byte offset field and that it could therefore get away with generating a shorter form of the effective-address encoding; for all it knows, in pass one, offset could be a symbol in the code segment, and it might need the full four-byte form. So it is forced to compute the size of the instruction to accommodate a four-byte address part. In pass two, having made this decision, it is now forced to honour it and keep the instruction large, so the code generated in this case is not as small as it could have been. This problem can be solved by defining offset before using it, or by forcing byte size in the effective address by coding [byte ebx+offset].

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 6.2.6).

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