How does assembly version of putc work at boot time at low level?

Hello there,

I am looking at a file from an android kernel source: 
\arch\arm\boot\compressed\head.S

There I see a code which I suppose prints out a message to the screen, 
code snippet:

putc:
		mov	r2, r0
		mov	r0, #0
		loadsp	r3, r1
		b	2b

What I do not understand is that what sort of branch instruction "2b" 
suppose to mean? Is it may be a system call-like thing? If so, where 
could I obtain a full list of those available system calls? Thanks in 
advance