read this first
http://www.semiconductors.philips.com/pip/74HC1...HC138.html
(i m passing time doing YOUR job, since i have nothing todo ...)

i guess it should be something like

ABC is A10-A11-A12

line select (put one of) (beware of the inverter, you mmight have to invert some of your logic)
0001
0010
0100
1000
on the A0-7 port low nibble
make ABC = 010
make "wr" high
wich is a inverter/demultiplexer


have ABC = 000
have RD high
you will catch the P7-p14 first (AD0-7)
have rd low
have ABC = 001
have RD high
catch the P5-6 and P15-16 NIBBLE (on AD0-AD3)
have rd low

process that data (you ve been using 8052ASM for many years, already did the doc wont do the code) a few bitchecks should do the job

BTW : don't forget to activate the demultiplexer when you re reading or writing on the kb, putting line A15 high or lines A13 or A14 low (A13-A14 are inverting) or you may be deceived by the result ;-pp

Documentalized Regards,
Benjamin