Prince of Persia (Apple II), read in Jordan Mechner's released 6502 source

Jordan Mechner's released Apple II source names its build in VERSION.S, "Prince of Persia 1.0 9/7/89", and its MASTER.S is headed as the 3.5-inch disk version; the game code the other claims here read is shared with the 5.25-inch build. pop/source-version

Value: 1.0

The 5.25-inch disk routines are in the repository's "02 POP Disk Routines" folder, which the atlas doesn't fetch; they load the same game code. The atlas hasn't built the source or compared it with a shipped disk.

Cited by: Prince of Persia

The kid's horizontal position, CharX, counts in units of 14 to a block: the left edges of the blocks (BlockEdge) step by 14, and the screen is 140 units wide, so it holds 10 blocks (computed). A floor is 63 px tall (BlockHeight). pop/block-grid

About: Prince of Persia Jump

Value: 14 units/block

The 140-unit screen is the Apple II's 280-pixel hi-res width at half resolution (CTRLSUBS.S calls it "140-res").

Cited by: Prince of Persia

Both of the kid's jumps are fixed sequences in SEQTABLE.S, with no speed or gravity: each cycle ANIMCHAR reads the sequence up to the next frame number, adding each chx byte to CharX in the direction he faces and each chy byte to his height. The frames of both jumps add no horizontal offset of their own (Fdx 0 in FRAMEDEF.S). pop/jump-sequence

About: Prince of Persia Jump

Other sequence bytes set the action (act), jump (goto), play footsteps (tap), and start a fall (setfall), which gives the speed that gravity works on (pop/freefall).

Cited by: Jump, Prince of Persia

The standing jump (standjump) runs frames 16 to 33. Its chx steps are 2, 2, 2, 2, 2, 7, 9, 5, 1, 4, −3, 5, and 1 units, 39 in all (computed); its only chy steps lift the kid 6 px and set him back down on the next frame. Frames 20 to 25 lack the floor-check bit, so onground doesn't test the floor under him there, and the steps between frame 19 and frame 26 move him 28 units, 2 blocks (computed). pop/standing-jump

About: Prince of Persia Jump

{
 "steps": [
  2,
  2,
  2,
  2,
  2,
  7,
  9,
  5,
  1,
  4,
  -3,
  5,
  1
 ],
 "total": 39,
 "unchecked": 28
}

A chx step sits after the frame number it follows, so it moves the kid on the next frame. The distances are CharX's: the floor test reads a foot point that each frame sets in the low bits of Fcheck (GETBASEX, CTRLSUBS.S), which these sums leave out. Frame 26 is where a jump over no floor turns into jumpfall (CTRL.S startfall). The comment ";chx 6?" is Mechner's.

Cited by: Prince of Persia

The running jump (runjump) runs frames 34 to 44. Its chx steps are 5, 6, 3, 5, 7, 12, 8, 8, 4, 4, and 5 units, 67 in all (computed), and its chy steps raise the kid 14 px over three frames and lower him 14 px over the next two. Frames 39 to 43 lack the floor-check bit, and the steps between frame 38 and frame 44 move him 43 units, 3 blocks and 1 unit (computed). pop/running-jump

About: Prince of Persia Jump

{
 "steps": [
  5,
  6,
  3,
  5,
  7,
  12,
  8,
  8,
  4,
  4,
  5
 ],
 "total": 67,
 "unchecked": 43
}

Screen Y grows downward, so the rise is the chy bytes −3, −9, and −2. The last step, 5, moves the kid onto the run cycle (runcyc1) after frame 44. As for the standing jump, the sums are CharX's, not the per-frame foot point the floor test reads.

Cited by: Prince of Persia

GENCTRL, which turns the player's input into moves, picks its branch by the frame the kid shows (standing, starting a run, running, starting a jump up, hanging, turning, or crouching). The frames of the standing and running jumps, 16 to 44, match none of them, and it returns without reading the joystick, so a jump can't be steered once it starts. pop/no-steering

About: Prince of Persia Air control

The direction comes from the facing: chx steps add in the direction the kid faces (pop/jump-sequence). A jump over no floor turns into a fall sequence, which reads only the button, for a ledge grab (pop/ledge-grab).

Cited by: Air control, Prince of Persia

In a run (frames 4 to 14), arunning reaches the running jump only while the joystick is held forward (centered stops the run, back turns it), and then only on a fresh press of up (clrU negative), with forward and up the names Mechner's comments give the joystick signs; in a full run (frame 7 or later) that calls DoRunjump, which Mechner's comment describes as "Calibrate jump so that foot will push off at edge". It looks for a gap or spikes in the next two blocks and measures from the kid's position 4 units on to that floor's end. When the end is 6 to 15 units off, it moves him so the jump starts with CharX 14 units (one block) short of the end (computed). Farther off, it returns without jumping (comment: "still too far away--wait till next frame"); nearer, it moves him 1 unit (computed) and jumps (comment: "He jumped too late; he'll miss edge"). With no gap in sight, he jumps where he is. pop/run-jump-takeoff

About: Prince of Persia Jump

Value: 14 units before the edge

The loop starts at the block after the kid's (projected) one and stops at the second floor block, so "the next two blocks" means the next block and the one after. The constants' comments call the units pixels; they are CharX units (pop/block-grid). Whether a press that waits is acted on in a later frame depends on whether anything sets clrU to 1 before then, which the atlas hasn't traced. That a negative JSTKY is up and a negative JSTKX forward (after facejstk turns a right-facing kid's input around) rests on Mechner's comments at CTRL.S L638-L643 and L616.

Cited by: Prince of Persia

A falling kid grabs a ledge while the button is down (either Apple II button, unless manual control splits them) and he is alive, his downward speed is under 32 px per cycle, and his feet are within 25 px above the floor plane he is falling toward. fallon then tests for a ledge 8 units behind him (grabreach), requiring clear space above it and a floor piece in front, and on a catch it aligns him to the block, stops his fall, and starts the hang. It runs on the first frames of a fall (102 to 105) and through the free fall. pop/ledge-grab

About: Prince of Persia Ledge grab

{
 "maxSpeed": 32,
 "lead": 25,
 "reach": 8
}

A fall's speed runs 15, 18, 21, 24, 27, 30, and then 33 (pop/freefall), so the speed test lets a grab through every speed below the cap. CHECKLEDGE also refuses a loose floor that is already falling and, facing left, a panel piece with floor. On a catch the kid is stunned for 12 cycles (stuntime), which the atlas hasn't traced further.

Cited by: Ledge grab, Prince of Persia

With the weightless flag clear, a fall (action 4) adds 3 px per cycle to the kid's downward speed, up to 33 px per cycle (AccelGravity, TermVelocity); with it set, gravity is 1 and the cap 4 (WtlessGravity, WtlessTermVel). The fall sequences that start it set the speed to 15 (setfall). In any other action, including the jumps' (action 1) and a fall's first frames (action 3), gravity does nothing. pop/freefall

About: Prince of Persia Gravity

{
 "accel": 3,
 "cap": 33,
 "start": 15
}

The jump falls, jumpfall and rjumpfall, have the same height steps and the same starting speed of 15.

Cited by: Gravity, Prince of Persia

A landing's cost goes by the kid's downward speed when he reaches a floor: under 22 (OofVelocity) he lands softly; from 22 up to 33 (DeathVelocity) he loses 1 hit point and dies if it was his last; at 33 he loses 100, and dies. A guard who lands at 22 or more dies, and the shadow lands softly at any speed under 33. pop/fall-damage

About: Prince of Persia Fall damage

{
 "hurt": 22,
 "kill": 33
}

Spikes come first: lethal spikes under the kid impale him whatever the speed. A kid already dead lands hard. The cap of 33 (pop/freefall) makes 33 the only killing speed.

Cited by: Fall damage, Prince of Persia

Computed from the fall sequence and gravity: a kid who steps off a ledge has fallen 30 px when the free fall starts at speed 15, and reaches the floor below at speed 21 after one floor (63 px), 30 after two, and 33 after three or more. One floor costs nothing, two cost a hit point, and three or more kill him. The count assumes weightless is clear and that a fall crossing to the screen below keeps its speed and its distance to the next floor, which the atlas hasn't read in the screen-change code. pop/fall-floors

About: Prince of Persia Fall damage

{
 "safe": 1,
 "hurt": 2,
 "kill": 3
}

The count starts from a kid standing on a floor (CharY at the floor's plane), the stepfall sequence; the jump falls, jumpfall and rjumpfall, step the same heights. A start up to 14 px lower gives the same three outcomes (computed): one floor then lands at 18, two at 27, and three still at 33.

Cited by: Fall damage, Prince of Persia