The Sacred Armour of Antiriad: play

Loading the game's data…

Joystick: ← → ↑ ↓ move, Space fires. Fire at the title starts a game.
On foot: left or right walks, with fire jumps that way; up jumps; down crouches, and picks up what Tal crouches over; fire alone throws a stone.
In the armour: it floats any way the stick points, and once the Pulsar Beam is fitted fire shoots. Landed, left or right takes the armour off.
Q ends the game in play and brings back the title, as on a C64.

The game's own screen, drawn from the port's memory each frame as the video chip would draw it, at the C64's own pace. The game reads one joystick, in control port 2: the arrows and Space are that joystick here, and on a touch screen the pad is.

01 · Differences

What differs from a C64

The game plays as it does on a C64, at the C64's pace. These are the places where the page cannot be the machine, and what it does instead.

On a C64	On this page
The game loads from tape or disk, and its loader starts it at `$8009`.	There is no loading. The page starts at `$8009` with the memory as the loader leaves it, and the title screen and its tune come up at once.
Each instruction takes its cycles, and the video chip takes some of the processor's on the lines where it fetches characters and sprites.	The port executes no instructions. Each routine costs the cycles it costs the game, and one whose work varies costs it by the work: a room's character set and a tile's frame by the bytes they copy, the panel's colours by the entries looked at in their list, and each of the frame's tasks by the work it has that frame. The video chip takes its cycles as the beam passes those lines, so a room draws at the C64's pace.
An interrupt can fall between any two instructions. The handler lets the next one in early, so the raster's interrupts come in the middle of the frame's work and of the music, and a frame's work that runs past the start of the next frame costs a frame of play.	A handler runs whole when its interrupt comes, and then its time passes, in which the interrupts it let in are taken in turn; in the sound driver they come between its instructions, as on a C64. A handler reads the chips at the time it began, and the sound driver as far into its run as it has come. So the frames that run long run long here too, but a frame whose work ends within a few lines of the next one can lose its frame of play on one and not on the other.
The SID plays the music and the effects, from the driver the two timers of CIA 1 call.	The game's own sound driver runs on the site's 6502 in the port, as the How it works page's tune player runs it, called when the same timers run out, and the site's model of the SID, with the filter of the C64's original 6581, plays what it writes as each frame is shown.
The video chip draws the screen, and finds the sprites' collisions with each other and with the scenery as it draws them.	The site's model of the video chip draws each frame from the port's memory, with each change the port makes during the frame applied where the beam meets it, and the site's model of the collisions finds them as the clock passes each line.
The game reads a joystick in control port 2, and the keyboard only for `Q`.	The arrows and Space are the joystick in port 2, or the pad on a touch screen, and `Q` is the C64's Q. A tap shorter than a frame is held until the game has read it.