Open Game Source: Yaroid

by Dennis Payne

Name: Yaroid
Version: 0.2
Authors: Dennis Payne
License: GPL
Operating System: Net Yaroze

Important Note: The Net Yaroze license agreement restricts what information can be provided. Descriptions of the various library functions will have to be limited to a high-level discussion.


Yaroid is a simple asteroid-like game for the Net Yaroze. It has none of the "advanced" features of typical Asteroid clones. Some enhanced graphics are available but you can optionally run with images similar to those found on the Atari 2600. The gameplay hasn't been tweaked sufficiently to make the game challenging.

Net Yaroze

Although computer penetration has grown phenomenally with the advent of graphical user interfaces, they haven't matched the simplicity of a console game system. There are no questions about hardware compatibility or software versions. Just put in the CD or cartridge and play. Life for the game developer isn't as picturesque I'm afraid.

Development for a console machine poses some interesting challenges. The machines are typically much less powerful than a desktop computer. Memory is scarce. Cartridges have fast access speeds but limited space available while CDs reverse the restrictions. The development kits are expensive and restricted by the console manufacturers. As the hardware becomes more powerful, many of the restrictions aren't as big a concern as in the past. However, the cost and availability of development kits have remained problematic for hobbyist and learning game developers.

Sony developed the Net Yaroze specifically to address the aspiring developer. It plays all the regular Playstation games, plus it can be programmed from a personal computer. Yaroze means "let's do it together" or "let's work together" in Japanese. Sony wanted to develop a community spirit that supports each other similar to the Linux community. Unfortunately they are no longer accepting new members. Hopefully this article will provide some insight useful for any console project.

Remember Where You Parked

The Playstation has two megabytes of main memory. There is no hard drive on the Net Yaroze and reading recordable CDs is not possible. Some developers have created file servers over the serial link or other devices. In general they aren't heavily used because the servers are limited to a single operating system which reduces the number of players who can try the game. A large portion of active designers still feel that the full potential of the Net Yaroze has yet to be demonstrated.

Designing a game that fits on a floppy is the first step, but not the last. In the Makefile for Yaroid, the linker flags are defined as "-Xlinker -Ttext -Xlinker 80100000". The 80100000 is the hexadecimal memory address where the program is loaded. Additional hard coded memory addresses are found in images.c. The programmer needs to ensure that no object overlaps another. A similar problem occurs with video memory.

Some users have written little utilities to determine placement automatically but there are advantages to doing manual placement. For example, Yaroid has a couple of images: the ship, asteroids, explosions, and the title. In YInitialize, the program loads each image into video memory with LoadImage. Afterwards the images are never accessed again. Instead of wasting the memory, it can be used for variables that are only used after the images are loaded like GpuPacketArea. Another possibility is using it to store screenshots.

Grabbing Your Attention

Demonstrating Net Yaroze games is a difficult prospect since most people don't own the system. Screenshot are often the only visibility to the outside world. Developers with a tv tuner card for their computer can simply screen grab the game. Unfortunately those without such hardware are required to do a little more work.

One possibility is to simply use printf to output the image to the serial console. Although I haven't tried such a method, other users have commented on the unreliability of printf so I looked for another solution. The serial console program includes a command to download memory from the Net Yaroze. Since the Net Yaroze memory is not reset after execution of your game completes, a screenshot can be stored in memory for later recovery. The Yaroid patch is relatively simple. Unfortunately I discovered that siocons can only download memory at 9600 baud for some reason making the solution less than optimal.

The raw screen images still needs conversion to a usable form. Linux has many conversion utilities but none seem to handle 16-bit color raw images. Ideally a 16-bit color mode should be added to rawtoppm. To do a quick test, however, I modified the tim2tga program to process raw image files. Here are the results:

The third screenshot clearly shows an irregularity that deserves explanation. There are three asteroids near the bottom of the screen. If you look at the top of screen you'll notice that a portion of the two lowest asteroids is duplicated at the top. Television displays don't display screen borders well. The first fifteen pixels out from all edges may or may not be visible to the player. Sony even requires that commercial games not put any crucial details in those borders. For example on my television the score and lives appears at the top of screen. I purposely duplicated the asteroids before reaching the exact edge so that they would always be visible.

Drawing Fire

Yaroid has some other design oddities that likely go unnoticed by the player. Although the Playstation's central processor may be fast enough for a simple asteroids clone, it does not have the speed for the 3D graphics common in commercial games. Instead a separate graphics unit offloads the task from the main processor. Ideally a game should keep both processors running continuously. In Yaroid, the draw_screen function draws the main ship, shots, and asteroids. It loops through the various objects calling draw_apic or draw_epic. These two functions in turn use the Net Yaroze function GsSortFastSprite. After drawings all the game objects, draw_screen waits for the next synchronization period and switches to the new screen.

Between the drawing functions and synchronization, no collision detection or object movement is performed, so it appears that Yaroid fails to keep the two processors busy. A more persistent observer, however, would notice the GsDrawOt call at the bottom of draw_screen. The GsOT argument to the function can also be found as a parameter to all the GsSortFastSprite calls. The graphics processor doesn't go into action until the GsDrawOt procedure. The other functions simply bundle up the display instructions.

So what is the oddity I referred to earlier? Think about the drawing sequence. Let's say you just pressed thrusters to escape the path of an oncoming asteroid. While you're deciding what to do, the graphic unit is painting the results of your previous turn. So that means the image on screen does not reflect the current game state. If the player and asteroids moved every refresh the latency might be visible, but currently the player moves once every four display cycles making the discrepancy extremely difficult to spot.

Uncharted Territories

The Net Yaroze has such a vast array of capabilities that no single article could cover the system. Yaroid was never even designed to exploit its abilities. It does provide a good starting point with only limited use of the system library. There are still many improvements that could be made in Yaroid. Sound and music are obvious areas of improvement. If there are specific areas of interest let me know.

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Copyright (c) 2000 Dennis Payne / Identical Software