zero. DS:SI specify the first memory location to read, and the low 4-bits

of the first operand specify the bit start position.  The bit field can
cross a byte boundary. After each complete data transfer, SI and the first
operand are automatically updated to point to the next bit field.

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

(Supplied by Anthony Naggs)