XCHG RAX, RAX: 0x03, what does this code do and how does it work?

Question

"XCHG RAX, RAX" is a kind of riddle book that provides assembly code for you to reverse and undercover the meaning. Some of the examples calculate the Fibonacci sequence others bit-twiddle to toggle ASCII case. The snippet (riddle) on 0x03 is,

sub  rdx,rax
sbb  rcx,rcx
and  rcx,rdx
add  rax,rcx

How does this code work, and what does it do?

Answer 1

This code boils down to,

rax = min(rdx,rax)
rdx = sub(rdx,rax) ; store the difference in rdx

That is essentially,

• rdx - 0 (if rdx is the min)
• rdx - (rdx-rax) (if rax is the min)

The sbb and and here just move into rcx either

• 0
• rdx-rax

What determines what gets moved into rcx? That's determined by the result of the sbb. The sbb is doing reg - reg - CF. So you're either ANDing against all 1s or all 0s.

This is how I reasoned about it

# CF=0; rdx > rax
if ( rdx > rax ) {
  rdx -= rax
  rcx = 0     ; all bits off


              ; AND 0 (rcx) with anything (in rdx) is nop here.
              ; ADDing 0 (rcx) to rax is a nop
}



# CF=1; rax > rdx
else {
  rdx -= rax
  rcx = -1    ; all bits on


  rcx = rdx   ; code is rcx &= rdx
              ; remember -1 & x == x
  rax += rcx

}

Note regardless of the carry flag, this code will store the difference in rdx