The Netwide Assembler: NASM

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Chapter 6: Assembler Directives

NASM, though it attempts to avoid the bureaucracy of assemblers like MASM and TASM, is nevertheless forced to support a few directives. These are described in this chapter.

NASM's directives come in two types: user-level directives and primitive directives. Typically, each directive has a user-level form and a primitive form. In almost all cases, we recommend that users use the user-level forms of the directives, which are implemented as macros which call the primitive forms.

Primitive directives are enclosed in square brackets; user-level directives are not.

In addition to the universal directives described in this chapter, each object file format can optionally supply extra directives in order to control particular features of that file format. These format-specific directives are documented along with the formats that implement them, in chapter 7.

6.1 BITS: Specifying Target Processor Mode

The BITS directive specifies whether NASM should generate code designed to run on a processor operating in 16-bit mode, 32-bit mode or 64-bit mode. The syntax is BITS XX, where XX is 16, 32 or 64.

In most cases, you should not need to use BITS explicitly. The aout, coff, elf, macho, win32 and win64 object formats, which are designed for use in 32-bit or 64-bit operating systems, all cause NASM to select 32-bit or 64-bit mode, respectively, by default. The obj object format allows you to specify each segment you define as either USE16 or USE32, and NASM will set its operating mode accordingly, so the use of the BITS directive is once again unnecessary.

The most likely reason for using the BITS directive is to write 32-bit or 64-bit code in a flat binary file; this is because the bin output format defaults to 16-bit mode in anticipation of it being used most frequently to write DOS .COM programs, DOS .SYS device drivers and boot loader software.

You do not need to specify BITS 32 merely in order to use 32-bit instructions in a 16-bit DOS program; if you do, the assembler will generate incorrect code because it will be writing code targeted at a 32-bit platform, to be run on a 16-bit one.

When NASM is in BITS 16 mode, instructions which use 32-bit data are prefixed with an 0x66 byte, and those referring to 32-bit addresses have an 0x67 prefix. In BITS 32 mode, the reverse is true: 32-bit instructions require no prefixes, whereas instructions using 16-bit data need an 0x66 and those working on 16-bit addresses need an 0x67.

When NASM is in BITS 64 mode, most instructions operate the same as they do for BITS 32 mode. However, there are 8 more general and SSE registers, and 16-bit addressing is no longer supported.

The default address size is 64 bits; 32-bit addressing can be selected with the 0x67 prefix. The default operand size is still 32 bits, however, and the 0x66 prefix selects 16-bit operand size. The REX prefix is used both to select 64-bit operand size, and to access the new registers. NASM automatically inserts REX prefixes when necessary.

When the REX prefix is used, the processor does not know how to address the AH, BH, CH or DH (high 8-bit legacy) registers. Instead, it is possible to access the the low 8-bits of the SP, BP SI and DI registers as SPL, BPL, SIL and DIL, respectively; but only when the REX prefix is used.

The BITS directive has an exactly equivalent primitive form, [BITS 16], [BITS 32] and [BITS 64]. The user-level form is a macro which has no function other than to call the primitive form.

Note that the space is neccessary, e.g. BITS32 will not work!

6.1.1 USE16 & USE32: Aliases for BITS

The `USE16' and `USE32' directives can be used in place of `BITS 16' and `BITS 32', for compatibility with other assemblers.

6.2 DEFAULT: Change the assembler defaults

The DEFAULT directive changes the assembler defaults. Normally, NASM defaults to a mode where the programmer is expected to explicitly specify most features directly. However, this is occationally obnoxious, as the explicit form is pretty much the only one one wishes to use.

Currently, DEFAULT can set REL & ABS and BND & NOBND.

6.2.1 REL & ABS: RIP-relative addressing

This sets whether registerless instructions in 64-bit mode are RIP-relative or not. By default, they are absolute unless overridden with the REL specifier (see section 3.3). However, if DEFAULT REL is specified, REL is default, unless overridden with the ABS specifier, except when used with an FS or GS segment override.

The special handling of FS and GS overrides are due to the fact that these registers are generally used as thread pointers or other special functions in 64-bit mode, and generating RIP-relative addresses would be extremely confusing.

DEFAULT REL is disabled with DEFAULT ABS.

6.2.2 BND & NOBND: BND prefix

If DEFAULT BND is set, all bnd-prefix available instructions following this directive are prefixed with bnd. To override it, NOBND prefix can be used.

     call foo            ; BND will be prefixed 
     nobnd call foo      ; BND will NOT be prefixed

DEFAULT NOBND can disable DEFAULT BND and then BND prefix will be added only when explicitly specified in code.

DEFAULT BND is expected to be the normal configuration for writing MPX-enabled code.

6.3 SECTION or SEGMENT: Changing and Defining Sections

The SECTION directive (SEGMENT is an exactly equivalent synonym) changes which section of the output file the code you write will be assembled into. In some object file formats, the number and names of sections are fixed; in others, the user may make up as many as they wish. Hence SECTION may sometimes give an error message, or may define a new section, if you try to switch to a section that does not (yet) exist.

The Unix object formats, and the bin object format (but see section 7.1.3), all support the standardized section names .text, .data and .bss for the code, data and uninitialized-data sections. The obj format, by contrast, does not recognize these section names as being special, and indeed will strip off the leading period of any section name that has one.

6.3.1 The __SECT__ Macro

The SECTION directive is unusual in that its user-level form functions differently from its primitive form. The primitive form, [SECTION xyz], simply switches the current target section to the one given. The user-level form, SECTION xyz, however, first defines the single-line macro __SECT__ to be the primitive [SECTION] directive which it is about to issue, and then issues it. So the user-level directive

        SECTION .text

expands to the two lines

%define __SECT__        [SECTION .text] 
        [SECTION .text]

Users may find it useful to make use of this in their own macros. For example, the writefile macro defined in section 4.3.3 can be usefully rewritten in the following more sophisticated form:

%macro  writefile 2+ 

        [section .data] 

  %%str:        db      %2 


        mov     dx,%%str 
        mov     cx,%%endstr-%%str 
        mov     bx,%1 
        mov     ah,0x40 
        int     0x21 


This form of the macro, once passed a string to output, first switches temporarily to the data section of the file, using the primitive form of the SECTION directive so as not to modify __SECT__. It then declares its string in the data section, and then invokes __SECT__ to switch back to whichever section the user was previously working in. It thus avoids the need, in the previous version of the macro, to include a JMP instruction to jump over the data, and also does not fail if, in a complicated OBJ format module, the user could potentially be assembling the code in any of several separate code sections.

6.4 ABSOLUTE: Defining Absolute Labels

The ABSOLUTE directive can be thought of as an alternative form of SECTION: it causes the subsequent code to be directed at no physical section, but at the hypothetical section starting at the given absolute address. The only instructions you can use in this mode are the RESB family.

ABSOLUTE is used as follows:

absolute 0x1A 

    kbuf_chr    resw    1 
    kbuf_free   resw    1 
    kbuf        resw    16

This example describes a section of the PC BIOS data area, at segment address 0x40: the above code defines kbuf_chr to be 0x1A, kbuf_free to be 0x1C, and kbuf to be 0x1E.

The user-level form of ABSOLUTE, like that of SECTION, redefines the __SECT__ macro when it is invoked.

STRUC and ENDSTRUC are defined as macros which use ABSOLUTE (and also __SECT__).

ABSOLUTE doesn't have to take an absolute constant as an argument: it can take an expression (actually, a critical expression: see section 3.8) and it can be a value in a segment. For example, a TSR can re-use its setup code as run-time BSS like this:

        org     100h               ; it's a .COM program 

        jmp     setup              ; setup code comes last 

        ; the resident part of the TSR goes here 
        ; now write the code that installs the TSR here 

absolute setup 

runtimevar1     resw    1 
runtimevar2     resd    20 


This defines some variables `on top of' the setup code, so that after the setup has finished running, the space it took up can be re-used as data storage for the running TSR. The symbol `tsr_end' can be used to calculate the total size of the part of the TSR that needs to be made resident.

6.5 EXTERN: Importing Symbols from Other Modules

EXTERN is similar to the MASM directive EXTRN and the C keyword extern: it is used to declare a symbol which is not defined anywhere in the module being assembled, but is assumed to be defined in some other module and needs to be referred to by this one. Not every object-file format can support external variables: the bin format cannot.

The EXTERN directive takes as many arguments as you like. Each argument is the name of a symbol:

extern  _printf 
extern  _sscanf,_fscanf

Some object-file formats provide extra features to the EXTERN directive. In all cases, the extra features are used by suffixing a colon to the symbol name followed by object-format specific text. For example, the obj format allows you to declare that the default segment base of an external should be the group dgroup by means of the directive

extern  _variable:wrt dgroup

The primitive form of EXTERN differs from the user-level form only in that it can take only one argument at a time: the support for multiple arguments is implemented at the preprocessor level.

You can declare the same variable as EXTERN more than once: NASM will quietly ignore the second and later redeclarations. You can't declare a variable as EXTERN as well as something else, though.

6.6 GLOBAL: Exporting Symbols to Other Modules

GLOBAL is the other end of EXTERN: if one module declares a symbol as EXTERN and refers to it, then in order to prevent linker errors, some other module must actually define the symbol and declare it as GLOBAL. Some assemblers use the name PUBLIC for this purpose.

The GLOBAL directive applying to a symbol must appear before the definition of the symbol.

GLOBAL uses the same syntax as EXTERN, except that it must refer to symbols which are defined in the same module as the GLOBAL directive. For example:

global _main 
        ; some code

GLOBAL, like EXTERN, allows object formats to define private extensions by means of a colon. The elf object format, for example, lets you specify whether global data items are functions or data:

global  hashlookup:function, hashtable:data

Like EXTERN, the primitive form of GLOBAL differs from the user-level form only in that it can take only one argument at a time.

6.7 COMMON: Defining Common Data Areas

The COMMON directive is used to declare common variables. A common variable is much like a global variable declared in the uninitialized data section, so that

common  intvar  4

is similar in function to

global  intvar 
section .bss 

intvar  resd    1

The difference is that if more than one module defines the same common variable, then at link time those variables will be merged, and references to intvar in all modules will point at the same piece of memory.

Like GLOBAL and EXTERN, COMMON supports object-format specific extensions. For example, the obj format allows common variables to be NEAR or FAR, and the elf format allows you to specify the alignment requirements of a common variable:

common  commvar  4:near  ; works in OBJ 
common  intarray 100:4   ; works in ELF: 4 byte aligned

Once again, like EXTERN and GLOBAL, the primitive form of COMMON differs from the user-level form only in that it can take only one argument at a time.

6.8 CPU: Defining CPU Dependencies

The CPU directive restricts assembly to those instructions which are available on the specified CPU.

Options are:

All options are case insensitive. All instructions will be selected only if they apply to the selected CPU or lower. By default, all instructions are available.

6.9 FLOAT: Handling of floating-point constants

By default, floating-point constants are rounded to nearest, and IEEE denormals are supported. The following options can be set to alter this behaviour:

The standard macros __FLOAT_DAZ__, __FLOAT_ROUND__, and __FLOAT__ contain the current state, as long as the programmer has avoided the use of the brackeded primitive form, ([FLOAT]).

__FLOAT__ contains the full set of floating-point settings; this value can be saved away and invoked later to restore the setting.

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