I don't know what do you exactly mean by XBYTE macro, but if one desires indexed access to absolutely positioned array, in SDCC the following:

__xdata __at 0 unsigned int a[0];

void main(void) {
  a[10] |= 20;
}
 

results in (some antique version of SDCC I happen to have on this computer, the current edition might work slightly differently as far as code generation goes)

  
   0000 90 00 14            134 	mov	dptr,#(_a + 0x0014)
   0003 E0                  135 	movx	a,@dptr
   0004 FA                  136 	mov	r2,a
   0005 A3                  137 	inc	dptr
   0006 E0                  138 	movx	a,@dptr
   0007 FB                  139 	mov	r3,a
   0008 43 02 14            140 	orl	ar2,#0x14
   000B 90 00 14            141 	mov	dptr,#(_a + 0x0014)
   000E EA                  142 	mov	a,r2
   000F F0                  143 	movx	@dptr,a
   0010 A3                  144 	inc	dptr
   0011 EB                  145 	mov	a,r3
   0012 F0                  146 	movx	@dptr,a
 

Is this what you meant by "XBYTE macro" (okay, in this particular case it would be XWORD perhaps)?

Jan