Commodore Amiga A500 Technical Reference Manual page 238

List of B2000 Motherboard Jumpers
J101
This jumper determines the high-order address bit for Fat 3 2 1
3 2 1
Agnus. In its normal position shown the high-order bit is A23; in its
other position, this bit is A19. The current Fat Agnus chip requires
the A23 signal for proper management of the memory at $C00000.
Future Fat Agnus chips may map things differently.
This jumper is used to set the light-pen port number. In the normal
J200
position shown, the light pen input will be the FIRE input of
1
mouse/joystick port 1, as with the A500. With the jumper in the
2
other position, the light pen input will be the FIRE input of
3
mouse/joystick port 0, which is the scheme used on the A1000
machine.
J300
This jumper determines the time base used for the 50/ 60Hz CIA
1 2 3
timer chip. In the normal position, the 50/60HZ TICK clock, based
on AC 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 operate properly without one of these clocks.
This jumper is closed to add a second internal floppy
J301
drive, open to leave the second floppy out of the main unit
X X
box.
J500
This jumper is used to enable the 512K of RAM at
$C00000. It is normally closed; opening it will disable this
X-X
extra RAM.
235
Section 7.4