There are two BCD formats. Packed (what you describe) and unpacked (where only a single digit is stored per byte). Unpacked seems to have the advantage that converting to ASCII is just a matter of setting two bits, and converting from ASCII is just a matter of masking those out.

I found this example very illuminating

    sub    AH,AH    ; clear AH
    mov    AL,'6'   ; AL := 36H
    add    AL,'7'   ; AL := 36H+37H = 6DH
    aaa             ; AX := 0103H
    or     AL,30H   ; AL := 33H

because aaa instruction clears the high nibble of AL, it works with either unpacked BCD or with ASCII (but always outputs BCD).

http://service.scs.carleton.ca/sivarama/asm_book_web/Instruc... (page 8)

But iiuc aaa didn't make it in the transation to 64bit processors.