GNU Arm Assembly pseudocommands (Directive er ctictives)

1. Introduction to GNU Arm Assembly

Any assembly line is structured as follows:

[<Label >:] [<instruction or Directive >}@ comment
[<Tag >:] [<command >}@ comment
 

In GNU arm assembly, any label ending with a colon is considered as a label, not necessarily at the beginning of a line. The following is a simple example. This assembler defines an "add" function, which returns the sum of two parameters:

.section .text, “x”.global add                      @ give the symbol add external linkageadd:         ADD r0, r0, r1          @ add input arguments         MOV pc, lr              @ return from subroutine                                 @ end of program

2. GNU Arm Assembly pseudocommands

The GNU Arm Assembly pseudocommands start. The following table lists the commonly used compilation artifacts:

Assembly of pseudo commands	Description	Example
. ASCII "<string>"	Inserts a string to the target file, but does not end with null.	. ASCII "jnz" @ insert byte 0x4a 0x4e 0x5a
. Asciz "<string>"	Similar to. ASCII, but ended with null	. Asciz "jnz" @ insert byte 0x4a 0x4e 0x5a 0x00
. Balign <power_of_2> {, <Fill_value> {, <Max_padding> }}	Align the following position <power_of_2>	. Byte 0x55 @ insert byte: 0x55 . Balign 4, 0 @ insert 3 alignment Bytes: 0x00 0x00 0x00 . Word 0xaa55ee11 @ insert byte: 0x11 0xEE 0x55 0xaa (LSB order)
. Byte <byte1 >{, <byte2> }...	Insert byte to target file	. Byte 64, 'A' @ inserts the bytes 0x40 0x41 . Byte 0x42 @ inserts the byte 0x42 . Byte 0b000011, 0104 @ inserts the bytes 0x43 0x44
. Code <number_of_bits>	Set the number of commands, 16-bit thumb or 32-bit arm	. Code 16 @ set the following assembly as the thumb command
. Else	Use with. If and. endif
. End	Indicates the end of the Assembly file. The assembler no longer reads the following content.
. Endif	Huai. If used in combination
. Endm	The macro definition is over and used with. Macro.
. Endr	End a loop and use it with. 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 = 0b001010 @ set Adams to 42
. Err	Stops compilation if. Err is encountered during assembly.
. Exitm	Exit macro definition halfway
. Global <symbol>	Identifies the symbol to be accessed by other modules (assembly or C) of the program, It is equivalent to export_symbol in the kernel.	. Global _ my_test_count @ It Can Be r/W by other modules
. Hword <short1 >{, <short2>} ...	Insert a 16-bit (half-word) value to the target file	. Hword 0xaa55, 12345 @ insert byte 0x55 0xaa 0x39 0x30 . 2 byte 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 # include of C
. 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>}	Defines an assembly macro named name, which has N parameters, It 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> "}	Start a new section. section_name can be: . Text: code segment . Data: initialized Data Segment . BSS: uninitialized data segment Flags can be: A allowable Section W writable Section X executable Section	. Section. Text, "X"
. Set <variable_name>, <Variable_value>	Set the changed value, which is the same as. equ.	. Set Adams, 0x2a @ set Adams to 42
. Space <number_of_bytes> {, <Fill_byte>}	Reserve the given byte space and set it to 0 or fill_byte.	. Space 0x400,0
. Word <word1 >{,< word2> }...	Insert a 32-bit word list to the target file	Head_ptr:. Word 0 @ head pointer to within buffer (initially zero) Tail_ptr:. Word 0 @ tail pointer to within buffer (initially zero) . Word 0 xdeadbeef @ inserts the bytes 0xef 0xbe 0xad 0xde . 4 byte-42 @ inserts the bytes 0xd6 0xff 0xff 0xff @ Least significant byte (LSB) Ordering assumed
. Rept <number_of_times>	The code block number_of_times is repeatedly executed. Similarly, end with endr
<Register_name>. req <Register_name>	Register renaming	ACC. req R0

3. Arm special characters and syntax

@	Annotator in the code line
#	Comments to the entire line
;	Statement Separator
# Or $	Direct operand prefix
. Arm	Assemble and use arm commands
. Thumb	Compile the thumb command
. Code16	Compile the thumb command
. Code32	Assemble and use arm commands
. Force_thumb	Use the thumb mode forcibly, even if not supported
. Thumb_func	Mark entry point as thumb coded (Force BX entry)
. Ltorg	Start a new literal pool (Text pool: The area embedded in the code to store constants)

0	Octal
0x or 0x	Hexadecimal
0b or 0b	Binary
No Leading Character	Decimal

. Data	Mark the subsequent statement into the data segment of the target file
. Text	Mark the code segment that puts subsequent statements into the target file
. Extern symbol	Introduce symbols from other modules, similar to extern in C
. Skip expression	Number of bytes specified by Skip expression in the target file Buffer:. Skip 512 @ buffer of 512 bytes, uninitialised