Ctrl-Alt-Ins at boot up. 


Once you're in the CMOS editor, the machine should either recognize 
the memory (assuming you've got the right type of SIMMs, that is) or 
allow you to manually enter the correct value for your upper memory. 


A note on the types of SIMM the 6386/SX could use:  ISTR that 
the 6386/SX could ONLY use 256k X 9 and 1meg X 9 9-chip SIMMs, and 
that 1meg X 9 3-chip SIMMs and 4meg X 9 SIMMs were not compatible 
with it, meaning that the most memory you could install was 8meg.