Jumper positions for normal A2000 operation
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A2000 rev 6, 1MB Agnus (8372A), 1MB on board (8 x 4bitx256 DRAM)
Jumper positions are shown in the default state where everything runs normally, no expansions, etc


J101 2-3  Agnus MSB address selection, set in conjuntion with J500

          With J101 in 1-2 and J500 fitted, then system will have 507904 bytes chip mem from $000000 to $07FFFF
          AND 524288 bytes 'other' (slow/ranger ) mem from $C00000 to $C7FFFF - the avail command calls this fast memory

          With J101 in 1-2 and J500 NOT fitted, then system will have 507904 (512k) bytes chip mem from $000000 to $07FFFF

          With J101 in 2-3 and J500 removed - system will have 1MB of chip memory [DEFAULT CONFIGURATION]
          With J101 in 2-3 and J500 fitted then system will not boot


J200 2-3  Light pen select, set to 1-2 for compatibility with A1000 light pen pinout


J300 1-2 = TICK from AC power supply,  2-3 = TICK generated onboard (use this if the PSU doesn't generate 50/60Hz ticks)


J301 Not fitted   (close J301 to enable internal DF1: floppy drive)


J500 Not fitted    Set in conjuntion with J101 [DEFAULT IS J500 NOT FITTED]
         With J101 in the default position of 1-2 this splits the first half of the memory (first 512k) as chip memory from $000000 to $07FFFF
         And with J500 fitted, the second half of the memory (second 512k) becomes 'other' (slow/ranger) memory from $C00000 to $C7FFFF

         i.e. with J101 = 1-2 and J500 fitted, the system will have 512k of chip memory AND 512k of other/slow/ranger memory

         Despite what online notes say, the real time clock appears to work normally regardless of the J500 position


J900 Fitted (linked)
         Enables Bus Grant Acknowledge to Gary input

         Notes from Dave Haynie:
         This, when on, routes BGACK* to the Gary chip.  When Gary sees BGACK* go low,
         it generates an extra wait state for DMA into Chip RAM.  This is needed
         because Gary can't guarantee proper 68000 timings going from the expansion
         bus, through Agnus and into Chip RAM.  It's possible that some cards won't
         care, so this minor slowdown can be eliminated by moving J900.  If you have
         Fast RAM, don't worry about it.