Unsure how to incorporate edx

Question

I'm working on implementing the AddPacked procedure by modifying an example (AddPacked.asm is below) so that it adds two packed decimal integers of arbitrary size (both lengths must be the same). I have to use the following registers to pass information to the procedure:

• ESI: pointer to the first number
• EDI: pointer to the second number
• EDX: pointer to the sum
• ECX: number of bytes to add

I've referred to the example to write my code but I'm having trouble on utilizing edx, the pointer to the sum. The sum is stored in different variables based on the size so how do I move the value of eax outside of the AddPacked procedure? I've tried push/pop eax but neither work.

INCLUDE Irvine32.inc

.data
packed_1a   WORD    4536h
packed_1b   WORD    7297h
sum_1       DWORD   0

packed_2a   DWORD   67345620h
packed_2b   DWORD   54496342h
sum_2       DWORD   2 DUP (0)

packed_3a   QWORD   6734562000346521h
packed_3b   QWORD   5449634205738261h
sum_3       DWORD   3 DUP (0)

.code
main PROC
    mov esi, OFFSET packed_1a
    mov edi, OFFSET packed_1b
    mov edx, OFFSET sum_1
    mov ecx, 4
    call AddPacked
    mov  sum_1, eax
    call WriteHex
    call Crlf

    mov esi, OFFSET packed_2a
    mov edi, OFFSET packed_2b
    mov edx, OFFSET sum_2
    mov ecx, 8
    call AddPacked
    mov  sum_2, eax
    call WriteHex
    call Crlf
 
    mov esi, OFFSET packed_3a
    mov edi, OFFSET packed_3b
    mov edx, OFFSET sum_3
    mov ecx, 16
    call AddPacked
    mov  sum_3, eax
    call WriteHex
    call Crlf
    exit
main ENDP

;--------------------------------------------------------
;AddPacked PROC USES ecx esi edi,
;
; Adds two packed decimal integers of arbitrary size 
; (both lengths must be the same)
; Receives: 
;   esi: pointer to the first number
;   edi: pointer to the second number
;   edx: pointer to the sum
;   ecx: number of bytes to add
; Returns: nothing
;--------------------------------------------------------
AddPacked PROC USES esi, edi, edx, ecx
    xor  eax, eax       ; initialize sum to zero
    ;add  edi, ecx        last byte of sum
L1:
    mov  al, [esi]
    adc  al, [edi]      ; add corresponding byte of second number and carry
    daa
    mov  [edx], al      ; store result
    inc  esi
    inc  edi
    inc  edx
    dec  ecx
    jnz  L1             ; continue until byte count is zero
    

    ; Add final carry, if any
    inc esi
    ;mov    al, 0
    ;adc    al, 0

    ; add carry to most significant byte of sum
    mov [edx], al
    pop  eax
    ret
AddPacked ENDP

END main

----------------------------------------------------

; Packed Decimal Examples            (AddPacked.asm)

; This program adds two decimal integers.

INCLUDE Irvine32.inc

.data
packed_1 WORD 4536h
packed_2 WORD 7297h
sum DWORD ?

.code
main PROC
; Initialize sum and index.
    mov sum,0
    mov esi,0
    
; Add low bytes.
    mov al,BYTE PTR packed_1[esi]
    add al,BYTE PTR packed_2[esi]
    daa
    mov BYTE PTR sum[esi],al
    
; Add high bytes, include carry.
    inc esi
    mov al,BYTE PTR packed_1[esi]
    adc al,BYTE PTR packed_2[esi]
    daa
    mov BYTE PTR sum[esi],al

; Add final carry, if any.
    inc esi
    mov al,0
    adc al,0
    mov BYTE PTR sum[esi],al
    
; Display the sum in hexadecimal.   
    mov eax,sum
    call    WriteHex
    call    Crlf
    exit
main ENDP
END main