CPD - Z80 instruction reference

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