...
    call ReturnFreeRAM
    ld a,l
    call DispHexA
    ld a,h
    call DispHexA
...

DispHexA:
   push ix
   push af
   push hl
   push bc
   push af
   rrca
   rrca
   rrca
   rrca
   call dispha
   pop af
   call dispha
   pop bc
   pop hl
    pop af
    pop ix
   ret
dispha:
   and 15
   cp 10
   jp nc,dhlet
   add a,48
   jp dispdh
dhlet:
   add a,55
dispdh:
   call PutChar
   ret