D Source:

import core.stdc.stdio;

void main() {
  ulong stuff;
  asm {
    xor RDI, RDI;
    mov DIL, 1;
    mov stuff, RDI;
  }

  printf("%lld\n", stuff);
}

Output:  0

obj2asm disassembly of main():

		push	RBP
		mov	RBP,RSP
		sub	RSP,010h
		mov	qword ptr -8[RBP],0
		xor	RDI,RDI
		mov	BH,1
		mov	-8[RBP],RDI
		mov	RSI,-8[RBP]
		mov	RDI,offset FLAT:.rodata@64
		xor	EAX,EAX
		call	  printf@PC32
		xor	EAX,EAX
		mov	RSP,RBP
		pop	RBP
		ret

The opcode for BH is being generated when the assembly language code says DIL.

Similarly, DH is generated when SIL is in the source code.