GNU ARM Assembler pseudo-directive (assembler directives)

1. Introduction to the GNU ARM Assembly

Any assembly line is structured as follows: [<label>:] [<instruction or DIRECTIVE>} @ Comment
[< label;:] [< instruction;} @ Comment

In the GNU ARM assembly, any end of a colon is considered to be a label, not necessarily the beginning of a line. Here is a simple example of the assembler that defines an "add" function that returns the two parameters and:[CPP]View plain Copy. Section. Text, "X". Global add @ give the symbol add external linkage add:                                    Add R0, R0, r1 @ Add input arguments MOV pc, LR @ return from subroutine @ End of Program
2. GNU Arm Assembly pseudo-directive

GNU Arm Assembly pseudo-directives all begin with. Common Assembly pseudo-means are as follows:

Assembly Pseudo-Directives	Describe	Example
. ASCII "<string>"	Inserts a string into the destination file, but does not end with null	. ASCII "JNZ" @ Insert byte 0x4a 0x4E 0x5A
. Asciz "<string>"	Similar to. ASCII, but ends with null	. Asciz "JNZ" @ Insert byte 0x4a 0x4E 0x5A 0x00
. balign <power_of_2> {,<fill_value> {, <max_padding>}}	Make the following position <power_of_2> align	. Byte 0x55 @ Insert byte: 0x55 . balign 4,0 @ Insert 3 aligned bytes: 0x00 0x00 0x00 . Word 0xaa55ee11 @ Insert byte: 0x11 0xEE 0x55 0xAA (LSB order)
. Byte <byte1> {, <byte2>} ...	Insert byte to destination file	. Byte, ' A ' @ Inserts the bytes 0x40 0x41 . Byte 0x42 @ Inserts the byte 0x42 . Byte 0b1000011, 0104 @ Inserts the bytes 0x43 0x44
. Code <number_of_bits>	Set the number of instruction bits, 16-bit thumb or 32-bit arm	. code 16 @ Set the following assembly as thumb instructions
. else	Use with. if and. endif
. end	End of symbol assembly file, assembler no longer reads back content
. endif	If used in conjunction with the.
. endm	Flag macro definition end, used in conjunction with. macro
. Endr	End a loop, using the. rept and. IRP
. equ <symbol Name>, <value>	Set the value of a symbol	. Equ Adams, (5 * 8) + 2 @ set Adams to 42 . Set Adams, 0x2A @ set Adams to 42 Adams = 0b00101010 @ set Adams to 42
. err	Assembly when you encounter. Err, stop compiling
. exitm	Exit macro Definition Halfway
. Global <symbol>	Identity symbol is accessed by other modules of the program (assembly or C), Equivalent to Export_symbol in kernel	. Global _my_test_count @ It can be r/w by other modules
. Hword <short1> {, <short2>} ...	Insert 16-bit (half-character) value to the destination file	. Hword 0xaa55, 12345 @ Insert byte 0x55 0xAA 0x39 0x30 .2byte 0X55AA, 1 @ Insert byte 0xAA 0x55 0xFF 0xFF @ Least significant Byte (LSB) ordering assumed
. If <logical_expression>	Use with. endif
. ifdef <symbol>	Use with. endif
. ifndef <symbol>	Use with. endif
. include "<filename>"	Similar to C's # include
. IRP <param> {, <val_1>} {, <val_2>} ...	Repeats a block of code, once for each value In the value list. Mark the end of the block Using a. endr directive. In the repeated code block, Use \<param> to substitute the associated value in the Value list.
. macro <name> {<arg_1} {, <arg_2>} ... {, <arg_n>}	Define a compilation macro called Name, which has n parameters, and must end with. Endm.	. macro Shiftleft A, b . If \b < 0 MOV \a, \a, ASR #-\b . exitm . endif MOV \a, \a, LSL #\b . endm
. Section <section_name> {, "<flags>"}	To start a new section, the Section_name can be: . Text: Code snippet . Data: Initialized Segments . BSS: Uninitialized data segments The flags can be: A allowable section W writable section X executable section	. section. Text, "X"
. set <variable_name>, <variable_value>	Sets the value of the change, as with the. equ	. Set Adams, 0x2A @ set Adams to 42
. Space <number_of_bytes> {, <fill_byte>}	Reserve a given byte space and set to 0 or fill_byte	. Space 0x400,0
. Word <word1> {, <word2>} ...	Insert a 32-bit word list to the destination file	Head_ptr:. Word 0 @ Head pointer to within buffer (initially zero) Tail_ptr:. Word 0 @ tail pointer to within buffer (initially zero) . Word 0xDEADBEEF @inserts the bytes 0xEF 0xBE 0xAD 0xDE .4byte-42 @ Inserts the bytes 0xd6 0xFF 0xFF 0xFF @ Least significant Byte (LSB) ordering assumed
. rept <number_of_times>	Repeat code block number_of_times times, with C for Similar to the end of Endr
<register_name>. req <register_name>	Register Rename	Acc. req R0

3. Arm special characters and syntax

@	Comment symbols in line of code
#	Full line Comment symbol
;	Statement Detach symbol
#或 $	Prefix of direct operand
. arm	assembly using ARM directives
. Thumb	Assembly using Thumb Instructions
. code16	Assembly using Thumb Instructions
. code32	assembly using ARM directives
. force_thumb	Force use of thumb mode even if not supported
. thumb_func	Mark entry point as thumb coded (Force BX entry)
. ltorg	Start a new literal pool (text pool: an area embedded in code to hold constants)

0	8 binary
0x or 0X	16 binary
0b or 0 B	Binary
No leader character	10 binary

. Data	Identifies the subsequent statement into the destination file data segment
. text	Identifies the code snippet that puts the subsequent statement into the destination file
. extern symbol	Introduce symbols from other modules, like extern in C
. Skip Expression	The number of bytes specified by skip expression in the destination file Buffer:. Skip @Buffer of bytes, uninitialised