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Focus Energy glitch
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Miscellaneous glitches of Pokémon Red and Blue and Pokémon Yellow

Amazing Man | Billʼs Secret Garden | Cable Club escape glitch | Celadon looping map trick | Coastal Flooding | Cooltrainer move | Evolve without an Evolution Stone | Evolving Raichu | Expanded item pack | Focus Energy glitch | Get stuck in a wall | Glitch battle systems | Infinite Blaine Door | Introduction Nidorino glitch | Invisible PC | Invisible Tree | Item stack duplication glitch | Mute the music in the Pokémon League | Partial trapping move link battle glitch | Pokémon Tower Pokédex glitch | PP Reset Glitch | Recovery move glitch | See a Ghost without a Silph Scope | Silph Co. PC Glitch | Stand on a Tree | Statue behaviour glitch | Substitute glitch | Super effective move AI flaw | Super Glitch | Surfing glitch | Transform into a Ditto | Trick Zone | Walk around with only fainted Pokémon | Walking lag glitch | Walk on water through Surf | Walking Pikachu happiness glitch (Yellow only) | Wild appeared! | ZZAZZ Glitch

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The Focus Energy glitch is a glitch in Pokémon Red, Blue and Yellow.

Focus Energy is a move that, ever since Generation II, increased the user's critical hit rate.

In Generation I, due to a bug, Focus Energy will always effectively quarter the player's chances of getting a critical hit.

This glitch was fixed in Pokémon Stadium, where Focus Energy quadruples the critical hit rate instead.[citation needed]

Analysis

The relevant block of code for Focus Energy's effect is this, beginning at offset 3E055 in Pokémon Red and Blue (view 0F:6055 in a debugger). Thanks Wack0 for walking through this.

bit 2, a ; test for focus energy
jr nz, .focusEnergyUsed (6061)
sla b ; b=b*2
jr nc, .noFocusEnergyUsed ; if b was below 128, skip...
ld b, $ff ; a cap at 255/256
jr .noFocusEnergyUsed
.focusEnergyUsed
srl b ; whoops! b=b/2, and if you look, the jump to here is before the b=b*2, so b is effectively 1/4 of what it should be
.noFocusEnergyUsed ; no cap at 255/256 for focus energy. I wonder if, if done properly, this would have caused bugs related to integer wraparound..

... (check for high critical hit move here)

The relative jump to '.focusEnergyUsed' that happens when Focus Energy is used makes the game skip a 'sla b' (double b), and then a 'srl b' (half b) at '.focusEnergyUsed' halves the value in register 'b'.

If Focus Energy is not used, the value in 'b' is doubled and if it was used, the value in 'b' is halved. 2.0/0.5=4.0, meaning critical hits land four times less likely.