You can check this application note:
http://www.maxim-ic.com/appnotes....umber/3965

In general First you make a buffer of 8 bytes of BCD data
which is sent to ICM 7218A. In one port you can have 4lines for
BCD data and few lines such as _write , mode and so on.
You prepare the data to be sent in BCD format with binary to BCD converter and take a byte starting from digit0, format the
nibble and put it on the port. Then you give the write signal.
The ICM 7218A automatically gets prepared for next digit with
rising edge of write. Other program is reqd for initial setup
of ICM 7218A and giving mode instructions etc.
It is surprising that you are using ICM 7218A for LEDs for
only 8digits. I have just now made a display of 12digits
(3rows of 4digits) and that is entirely driven by 8051 and not using such costly IC as ICM 7218A.