Epson 386SX - Manual (Page 162)

The illustration below shows the location of the SIMM card in
the computer.
The following illustration shows the location of jumper J1 on
the SlMM card. The jumper should be in position 1-2 if SIMMs
have been installed. If you are not using SIMMs, the jumper
should be set to position 2-3.
5-20 Installing Options