Christian,

Byte Sex refers to the order a multi-byte value is "stored" into memory. In regard to code store, the 8051 looks big-endian.

Now your example does not include a true multi-byte numberic value (although it is a multi-byte instruction) because it uses the ACALL format. An LCALL instruction uses a multi-byte numeric (the 16 bit address of the sub-routine).

If you code:

   LCALL  SUB@ABCD   ;A subroutine who's addrs = 0ABCDH

It assembles to (in Hex representation):

12ABCD

regards,
p