Commodore Amiga A500 Technical Reference Manual page 241

Section 7.4
List of B2000 Motherboard Jumpers
J101 Thisjumper determines the high-order address bit for Fat
3 2 1
Agnus. In its normal position shown the high-order bit is
A23: in its other position, this bit is A1 9. The current Fat
Agnus chip requires the A23 signal for proper manage-
ment of the memory at SC00000. Future Fat Agnus chips
may map things differently.
J200 This jumper is used to set the light-per, port number. In
the normal position shown, the light pen input will be the
FlRE input of mouse/joystick port
1,
as with the A500.
With the jumper in the other position, the light pen input
will be the FlRE input of mouse/joystick port 0, which is
the scheme used on the A1 000 machine.
J300 This jumper determines the time base used for the 501
1 2 3
60Hz CIA timer chip. In the normal position, the 5016OHz
W TICK clock, based on A C line frequency, is used as a time
base. In the alternate position, the vertical sync pulse
from the video section is used. The system will not oper-
ate properly without one of these clocks.
J301
This jumper is closed to add a second internal floppy
drive, open to leave the second floppy out of the main unit
X X box.
J500
This jumper is used to enable the 51 2K of RAM at
SC00000. It is normally closed; opening it will disable this
X-X extra RAM.