extended asm - what does this option do?

[dw <limegreensocks at yahoo dot com>]

I was reading a bit of inline assembler the other day, and I saw 
something in it that I didn't understand.  Here's the code:

#define __cpuid(level, a, b, c, d)            \
  __asm__ __volatile__ ("xchg{l}\t{%%}ebx, %k1\n\t"            \
       "cpuid\n\t"                    \
       "xchg{l}\t{%%}ebx, %k1\n\t"            \
       : "=a" (a), "=&r" (b), "=c" (c), "=d" (d)    \
       : "0" (level))

At first blush, looks pretty basic.  Input values here, output values 
there, constraints blah blah blah.

But what caught my eye was this bit in the actual assembler: %k1

Now, I know about letters that can be used in input/output constraints.  
And I know about how %digits get replaced in the asm string.  But I 
wasn't aware of any qualifiers(?) that could be embedded into the asm 
statement's arguments.  I'm not seeing anything in the docs about it 
either.

Q1: So what's the difference between %k1 and just %1?

FWIW, here's the asm that gets output from -S:

    xchgl    %ebx, %r8d
    cpuid
    xchgl    %ebx, %r8d

Removing the 'k' seems to produce the same code, but the fact that 
neither the compiler nor the assembler objects to having the k there 
suggests that it means something to somebody.

Q2: And while I'm asking, what's with the {braces} in the asm?

I'm assuming those are an att thing (I'm an intel asm guy myself). 
Because again, they are apparently being consumed by the compiler (-S 
doesn't show them).  I thought that "[the compiler] does not parse the 
assembler instruction template and does not know what it means or even 
whether it is valid assembler input."  Isn't -S just the output that is 
being sent to the assembler?  Because it seems more is happening here 
than simply writing the unparsed string to the asm file.

Q3: So are the docs wrong, and the c++ compiler really is parsing the 
asm (beyond just for the purpose of replacing the arguments)?

Using rubenvb-4.8.0 on x64.

Thanks,
dw