What does `b .` mean in this ASSEMBLY code?

assembly arm gnu-assembler instruction-set
UndercoverCoder picture UndercoverCoder · Jan 3, 2018 · Viewed 11.8k times · Source

So I'm looking into the source code for Redox OS (An operating system made with Rust) just to see if I can learn something.

I'm reading the assembly file start.s in the bootloader folder. In the interrupt_vector_table label we have:

interrupt_vector_table:
    b . @ Reset
    b . 
    b . @ SWI instruction
    b . 
    b .
    b .
    b .
    b .

What exactly is b .?

I'm not a complete beginner in assembly, I just never came across this before.

Answer

Martin Rosenau picture Martin Rosenau · Jan 3, 2018

The b instruction for the ARM CPU is nearly the same as the jmp instruction for the x86 CPU: A jump instruction

Using the GNU tool chain . means: Address of the instruction itself.

So b . is equal to:

temporaryLabel:
    b temporaryLabel

or (for x86 CPUs):

temporaryLabel:
    jmp temporaryLabel

Related questions

internal relocation not fixed up

i recently started assembler programming for arm cores. My first little demos, only with the .text section, ran without any problems. As a logical extension i wanted to structure the assembler code into the usual sections: .text, .data, .bss . So …

Read More
How to call C functions from ARM Assembly?

I'm writing code targeting ARM Cortex-A on Android devices (using GNU assembler and compiler), and I'm trying to interface between Assembly and C. In particular, I'm interested in calling functions written in C from Assembly. I tried many things, including …

Read More
When are GAS ELF the directives .type, .thumb, .size and .section needed?

I'm working on an assembly program for an ARM Cortex-M3 based microcontroller (Thumb 2 instruction set), using GNU as. In some example code I find directives like .size, .section and .type which I understand are ELF directives. As an example: .section .…

Read More