CPDR - Z80 instruction reference

cpdrCompare, decrement and repeat 
Repeats CPD, scanning down memory from HL for a byte equal to A, until a match is found or BC is zero.
2 bytes · 21/16 cycles·21 cycles for a pass that repeats, 16 for the last
SZHP/VNC ●●●●1–
● set from the result
1 always 1
– not changed
Forms  cpdr  
Compares the value of the memory location pointed to by HL with A. Then HL and BC are decremented. If BC is not zero and z is not set, this operation is repeated. p/v is reset if BC becomes zero and set otherwise, acting as an indicator that HL reached a memory location whose value equalled A before the counter went to zero. Interrupts can trigger while this instruction is processing.
         .org 0x8000
        ld a, 3
        ld hl, source + 3
        ld bc, 4
        cpdr
        halt
source: .byte 1, 2, 3, 4