ES:DI. The length of the string, in BCD digits, is specified in CL. Unlike

Intel string operations CL, DI & SI are unchanged by the operation. The
Zero Flag (ZF) is set if the result is zero. The Carry Flag (CF) and
Overflow Flag (OF) appear to be set by the subtraction of the most
significant digits.

Note that 0F is treated as <POP CS> on the 88/86 and prefixes newer
instructions on 286+ CPUs.

(Supplied by Anthony Naggs)