GNU-AS

as [-a[cdhlns][=file]] [-D] [–defsym sym=val] [-I dir] [-o objs] [-R] [-W] [–warn] [target-options]

[-mthumb] [-EB|-EL] [-mapcs-32|-mapcs-26|-mapcs-float|-mapcs-reentrant] [-mthumb-interwork] [-moabi] [-k]

-a[cdhlmns]
-ac	omit false conditionals	-ah	include high-level source
-ad	omit debugging directives	-al	include assembly
-an	omit forms processing	-am	include macro expansions
		-as	include symbols

--defsym sym=value	Define the symbol sym to be value before assembling the input file. value must be an integer constant
-I dir	Add directory dir to the search list for .include directives.
-o objfile	Name the object-file output from as objfile
--no-warn/-W	Suppress warning messages.
--fatal-warnings	Treat warnings as errors.
--warn	Don’t suppress warning messages or treat them as errors.
-mcpu=processor[+extension...]	specifies the target processor: arm7tdmi, arm8, strongarm, arm9, arm9tdmi, arm10t, xscale.
-march=architecture[+extension...]	specifies the target architecture: armv1, armv2, armv3, armv4, armv5, armv5te, armv5texp iwmmxt and xscale.
-mfpu=floating-point-format	specifies the floating point format: softfpa, fpe, fpa, fsoftvfp, softvfp+vfp, vfp, vfpxd, arm1020e.
-mapcs [26|32]	specifies that the output generated by the assembler should be marked as supporting the indicated version of the Arm Procedure Calling Standard.

internal preprocessor

removes extra whitespace: leaves one space or tab before the keywords on a line, and turns any other whitespace on the line into a single space. Whitespace is one or more blanks or tabs.

removes all comments, replaced with a single space, or an appropriate number of newlines. /* comment*/;  line comment character ,‘@’ on the ARM; ‘#’ on the i386

converts character constants into the appropriate numeric values.

symbol	one or more characters chosen from the set of all letters, digits and the three characters ‘_.$’. On most machines, you can also use $ in symbol names. ‘.’ refers to the current address. Expression ‘melvin: .long .’ defines melvin to contain its own address.
statement	ends at a newline (‘\n’) or line separator (‘;’). A statement begins with zero or more labels. If the symbol begins with a dot ‘.’ then the statement is an assembler directive. If a letter the statement is an assembly language instruction.
label	A label is a symbol immediately followed by a colon (:). It represents the current value of the active location counter.
constant	a number, written so that its value is known by inspection(Either ‘#’ or ‘$’ can be used to indicate immediate operands.). Like this: .byte 74, 0112, 092, 0x4A, 0X4a, ’J, ’\J           # All the same value. .ascii "Ring the bell\7"                        # A string constant. .octa 0x123456789abcdef0123456789ABCDEF0   # A bignum. .float 0f-314159265358979323846264338327\ 95028841971.693993751E-40                 # - pi, a flonum.

assembler directives
.abort	stops the assembly immediately
.err	it will print an error message and, it will not generate an object file.
.end	marks the end of the assembly file. It does not process anything in the file past the .end directive.
.print “string”	print string on the standard output during assembly.
.align abs-expr, abs-expr, abs-expr	Pad location counter to a particular storage boundary. The first expression is the alignment required. The second expression gives the fill value to be stored in the padding bytes. If it is omitted, the padding bytes are normally zero. The third expression , if it is present, it is the maximum number of bytes that should be skipped by this alignment directive.
.data subsection	Indicate the following statements onto the end of the data subsection subsection (absolute expression). If omitted, it defaults to zero.
.byte expressions	.byte expects zero or more expressions, separated by commas. Each expression is assembled into the next byte.
.ascii "string"	expects zero or more string separated by commas. It assembles each string (with no automatic trailing zero byte) into consecutive addresses.
.int expressions	Expect zero or more expressions, of any section, separated by commas.
.long expressions	the same as ‘.int’
.float flonums	expects zero or more flonums, separated by commas.
.double flonums	expects zero or more flonums, separated by commas.
.equ symbol, expr	sets the value of symbol to expression. It is synonymous with ‘.set’.
.global symbol, .globl symbol	makes the symbol visible to ld. If symbol defined in partial program, its value is made available to other partial programs that are linked with it.
.comm symbol , length	declares a common symbol named symbol. When linking, if ld does not see a definition for the symbol then it will allocate length bytes of uninitialized memory. If ld sees multiple common symbols with the same name, it will allocate space using the largest size.
.func name[,label]	.func emits debugging information to denote function name, and is ignored unless the file is assembled with debugging enabled. The function must be terminated with .endfunc.
.macro	.macro and .endm define macros that generate assembly output.
.include "file"	include supporting files at specified points in your source program.
.org new-lc , fill	Advance location counter of current section to new-lc. new-lc is either an absolute expr or an expr with same section as the current subsection.
.if absolute expr	marks the beginning of a section of code assembled if argument is nonzero. It ends with .endif
.ifeq absolute expr	Assembles the following section of code if the argument is zero.
.ifdef symbol	Assembles the following section of code if symbol has been defined.

ARM Machine Directives
name .req regName	This creates an alias for register name called name.( foo .req r0)
.code [16|32] .thumb .arm	This directive selects the instruction set being generated. The value 16 selects Thumb, with the value 32 selecting ARM.
nop	evaluate to a legal ARM instruction that does nothing(MOV r0, r0)
ldr <register> , = <expr>	If expr evaluates to a numeric constant then a MOV or MVN instruction will be used in place of the LDR instruction. If the constant can be generated by either of these instructions. Otherwise the constant will be placed into the nearest literal pool and a PC relative LDR instruction will be generated.
adr <register> <label>	Load the address of label into the indicated register. It will evaluate to a PC relative ADD or SUB instruction depending upon where the label is located.
adrl <register> <label>	Load the address of label into the indicated register. It will evaluate to one or two PC relative ADD or SUB instructions depending upon where the label is located. If a second instruction is not needed a NOP will be in its place, so that this instruction is always 8 bytes long.

section	A section is a range of addresses, with no gaps; all data “in” those addresses is treated the same for some particular purpose. The linker LD reads many object files (partial programs) and combines their contents to form a runnable program. When AS emits an object file, the partial program is assumed to start at address 0. LD assigns the final addresses for the partial program, so that different partial programs do not overlap. LD moves blocks of bytes of the program to their run-time addresses. These blocks slide to their run-time addresses as rigid units; their length does not change and neither does the order of bytes within them. Such a rigid unit is called a section. Assigning runtime addresses to sections is called relocation. It includes the task of adjusting mentions of object-file addresses so they refer to the proper run-time addresses. An object file written by AS has at least three sections, any of which may be empty. These are named text, data and bss sections. When it generates COFF or ELF output, AS can also generate whatever other named sections you specify using the ‘.section’ directive. Within the object file, the text section starts at address 0, the data section follows, and the bss section follows the data section. Any address whose section is unknown at assembly time is by definition rendered {undefined U}—where U is filled in later. The only way to generate an undefined address is to mention an undefined symbol.
relocation	• Where in the object file is the beginning of this reference to an address? • How long (in bytes) is this reference? • Which section does the address refer to? What is the numeric value of (address) − (start-address of section)? • Is the reference to an address “Program-Counter relative”? In fact, every address as ever uses is expressed as (section) + (offset into section) It use the notation {secname N} to mean “offset N into section secname.” Addresses in the absolute section remain unchanged. address {absolute 0} is “relocated” to run-time address 0 by ld.
bss section	Contains zeroed bytes when your program begins running. It is used to hold uninitialized variables or common storage.

Sub-Sections

Assembled bytes conventionally fall into two sections: text and data. You may have separate groups of data in named sections that you want to end up near to each other in the object file, even though they are not contiguous in the assembler source. AS allows you to use subsections for this purpose. Within each section, there can be numbered subsections with values from 0 to 8192. Objects assembled into the same subsection go into the object file together with other objects in the same subsection. For example, a compiler might want to store constants in the text section, but might not want to have them interspersed with the program being assembled. In this case, the compiler could issue a ‘.text 0’ before each section of code being output, and a ‘.text 1’ before each group of constants being output.

bss Section

The bss section is used for local common variable storage. You may allocate address space in the bss section, but you may not dictate data to load into it before your program executes. When your program starts running, all the contents of the bss section are zeroed bytes.