LDD - Z80 instruction reference

lddLoad and decrement 
Copies the byte at (HL) to (DE), then decrements HL, DE and BC, setting P/V while BC is not zero.
2 bytes · 16 cycles
SZHP/VNC ––0●0–
● set from the result
0 always 0
– not changed
Forms  ldd  
Transfers a byte of data from the memory location pointed to by HL to the memory location pointed to by DE. Then HL, DE, and BC are decremented. p/v is reset if BC becomes zero and set otherwise.
         .org 0x8000
        ld hl, source + 3
        ld de, 0x9000 + 3
        ld bc, 4
        ldd
        halt
source: .byte 1, 2, 3, 4