Hello again Jan
Thanks for your posts in this thread
I only send this message for clarifying

Then the procedure to set a command would be
- set HI/LO to 0
- write low byte of the command using MOVX to any address
- set HI/LO to 1
- write high byte of the command using MOVX to any address

your sentence is wrong!
You can't use movx,because movx send a data to an address
but in this structure you must send low byte at first,
high byte then and check status later
According to the figure 12-3 i write this code

write_byte:
clr write_pin
mov p0,a
setb write_pin
ret

write_command:
clr hilo
mov a,#high_byte
call write_Byte
setb hilo
mov a,#lowbyte
call write_byte
ret

Regards
Mehdi