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Game

Sonic the Hedgehog

Sega's 1991 Mega Drive/Genesis platformer that began the Sonic series.

draftAlso: Sonic 1, Sonic the Hedgehog (1991)Updated 2026-10-20
On this page
  1. Jumping
  2. Related
  3. References
  4. Sources

The atlas reads Sonic the Hedgehog through Sonic Retro’s s1disasm, a buildable community disassembly. The atlas checked every claim against the code, then against the Sonic Physics Guide, Sonic Retro’s write-up of the classic games’ physics. The guide agrees with the code on every point compared here, down to the motionless press frame1 and the 5 px2 shift when Sonic curls up.

#Jumping

Sonic jumps along the ground’s normal, so on flat ground all 6.5 px/frame ($680)3 goes upward4. He doesn’t move on the press frame1. One gravity of 0.21875 px/frame² ($38)5 applies, held or not. Variable jump height comes from the release, which clamps a fast rise to −4 px/frame (-$400)6.

Speeds are 8.8 and positions 16.16 fixed point, a 1/256 px subpixel resolution7. Nothing limits how fast Sonic falls8. Jumps start only from the ground modes, so Sonic has no coyote time9.

Uses

Linked from

Every claim about this game, by concept (12)
Coyote timeSonic gets no grace window after leaving a ledge: only the grounded modes, Sonic_MdNormal and Sonic_MdRoll, call Sonic_Jump.T3
GravityObjectFall's gravity equate adds $38 subpixels/frame to an airborne object's downward speed each frame, Sonic's included.T3
ObjectFall adds the pre-gravity speed, shifted left 8 bits, to the 32-bit position, so positions are 16.16 and speeds 8.8 fixed point.T3
Sonic 1 never caps falling speed in the air; with FixBugs = 0, Sonic_JumpHeight's only cap is upward (-$FC0) and skipped after jumps.T3
JumpSonic's jump speed, son_jumpspeed, is $680 subpixels per frame (6.5 px/frame) out of water.T3
Sonic_Jump adds the jump speed along the ground normal (CalcSine of angle − $40); on flat ground that is exactly -$680 to Y speed, nothing to X.T3
Sonic doesn't move on the press frame; Sonic_Jump drops its return address, skipping the rest of the grounded routine, SpeedToPos included.T3
A standing jump curls Sonic up, shrinking his height radius from 19 to 14 and adding 5 to Y, so his feet stay put.T3
Variable jump heightDuring a jump, with no jump button held, Sonic_JumpHeight clamps any rise faster than 4 px/frame to exactly -$400.T3
Gravity slows Sonic's 6.5 px/frame take-off below the 4 px/frame release cap within 12 airborne updates.T4
In the airborne-rolling mode (Sonic_MdJump2), the release check runs before ObjectFall moves Sonic and adds gravity.T3
Simulating the jump code (flat ground, out of water) gives apexes of 39 px tapped and 100 px held, airtimes 39 and 62 frames.T4

References

  1. ^ abSonic doesn't move on the press frame; Sonic_Jump drops its return address, skipping the rest of the grounded routine, SpeedToPos included. T3sonicretro/s1disasm · _incObj/01 Sonic.asm L1236, L317–323Sonic Retro, “Sonic Physics Guide: Jumping” · Bugs when Jumping
  2. ^A standing jump curls Sonic up, shrinking his height radius from 19 to 14 and adding 5 to Y, so his feet stay put. T3sonicretro/s1disasm · _incObj/01 Sonic.asm L1250–1254, _Constants.asm L312–314Sonic Retro, “Sonic Physics Guide: Characters” · Sonic
  3. ^Sonic's jump speed, son_jumpspeed, is $680 subpixels per frame (6.5 px/frame) out of water. T3sonicretro/s1disasm · _incObj/01 Sonic.asm L8, L1218Sonic Retro, “Sonic Physics Guide: Jumping” · Constants
  4. ^Sonic_Jump adds the jump speed along the ground normal (CalcSine of angle − $40); on flat ground that is exactly -$680 to Y speed, nothing to X. T3sonicretro/s1disasm · _incObj/01 Sonic.asm L1224–1233, _incObj/sub CalcSine.asm L53Sonic Retro, “Sonic Physics Guide: Jumping” · Jump Velocity
  5. ^ObjectFall's gravity equate adds $38 subpixels/frame to an airborne object's downward speed each frame, Sonic's included. T3sonicretro/s1disasm · _incObj/sub ObjectFall & SpeedToPos.asm L5, _incObj/01 Sonic.asm L363Sonic Retro, “Sonic Physics Guide: Air State” · Constants
  6. ^During a jump, with no jump button held, Sonic_JumpHeight clamps any rise faster than 4 px/frame to exactly -$400. T3sonicretro/s1disasm · _incObj/01 Sonic.asm L1273–1288Sonic Retro, “Sonic Physics Guide: Jumping” · Variable Jump Height
  7. ^ObjectFall adds the pre-gravity speed, shifted left 8 bits, to the 32-bit position, so positions are 16.16 and speeds 8.8 fixed point. T3sonicretro/s1disasm · _incObj/sub ObjectFall & SpeedToPos.asm L16–20Sonic Retro, “Sonic Physics Guide: Calculations (Pixel and Subpixel)” · Pixel and Subpixel
  8. ^Sonic 1 never caps falling speed in the air; with FixBugs = 0, Sonic_JumpHeight's only cap is upward (-$FC0) and skipped after jumps. T3sonicretro/s1disasm · _incObj/01 Sonic.asm L1291–1302Sonic Retro, “Sonic Physics Guide: Air State” · Top Y Speed
  9. ^Sonic gets no grace window after leaving a ledge: only the grounded modes, Sonic_MdNormal and Sonic_MdRoll, call Sonic_Jump. T3sonicretro/s1disasm · _incObj/01 Sonic.asm L113–116, L318, L348

Sources

Code

Secondary