What is .arm in assembly code ? -

the following code taken https://github.com/xilinx/linux-xlnx/blob/master/arch/arm/kernel/head.s

i have never done arm assembly programming can me understand going on in these lines? .ar.? etc:

.arm                __head entry(stext)   thumb( adr     r9, bsym(1f)    )   @ kernel entered in arm.  thumb( bx  r9              )   @ if thumb-2 kernel,  thumb( .thumb                  )   @ switch thumb now.  thumb(1:                       ) 

also kindly point me tutorials getting starting with.

plenty of arm microcontrollers have 2 different instruction sets:

1. the default 32 bit arm instruction set
2. the lightweight 16 bit thumb instruction set

during program execution, arm chip can switch between 2 modes in order run instructions of these sets.

the purpose of these lines seems selection of right mode (i.e., .arm or .thumb) in order execute subsequent code.

edit: sorry, made mistake. real purpose specify set of instruction used in generated code. example, if write add r0, r1, #3, binary instruction produced belong either arm or thumb instruction set, depending on directive chose between .arm , .thumb.

from https://sourceware.org/binutils/docs/as/arm-directives.html :

.arm

this performs same action .code 32.

-

.thumb

this performs same action .code 16.

-

.code [16|32]

this directive selects instruction set being generated. value 16 selects thumb, value 32 selecting arm.