作者：Jamey Pittman

永远的边缘

《吃豆人》是一款没有结局的游戏。Namco的开发者错误地认为提高游戏难度就足以阻止任何人无限期地玩游戏。当然，在《吃豆人》发行后的几年里，玩家已经发现21关卡之后的每一个关卡几乎都是相同的。对于那些通过了前面20个关卡的玩家来说，剩下的游戏将成为一种耐力测试，即检测你在失去关注力并犯错前能够赚的多少点数。甚至出现过上百万的高分，而大多数玩家也认为游戏将永远持续下去。最终，少数高技能玩家能够完成255个连续的游戏关卡（花费好几个小时获得300多万个点数），并发现等待他们的还有第256个关卡。这是没人知道的意外结果—-甚至连Namco的开发者也不清楚。（请点击此处阅读本文第一、二、、三、四章）

pac-man（from comcast）

第256个关卡呈现出了迷宫的左半部分，而右半部分是由随机颜色的字母，数字和符号所组合而成。我们可以注意到屏幕右下方的奖励计数器也不能正常工作。迷宫左半部分正常运行着，而右半部分却不是如此。尽管玩家和幽灵都能穿越屏幕的右半部分，但是最初的迷宫墙将不再可行。相反地，玩家必须在幽灵的追赶下将吃豆人引向一系列让人困惑的开放领域，隧道，单行交叉路口，单面墙以及所有看不见的领域。

为什么会出现这一分裂的关卡？罪魁祸首是沿着屏幕下方绘制奖励符号的程序。情况是：当玩家到达第256个关卡时，内部关卡计时器将提升到255（关卡计数器一开始是0），这时候绘制奖励符号的程序会被调用。程序加载现在的关卡计数器值（255）到CPU寄存器并在这一寄存器中增加1个数值。不幸的是，255是适应Z-80CPU寄存器单字节的最大数值，所以当数值提升时，溢出的内容将被舍弃，并在寄存器中留下0而不是我们所期望的256。这个0数值将让程序误以为这是早前的关卡，因为它低于7。程序将把这一误解的寄存器当成一个计数器去绘制奖励符号。在每次绘制循环最后，寄存器都会减少一个数值，然后检查是否等于0（游戏邦注：即表示程序停止绘制符号的标志）。因为寄存器已经拥有数值0，所以第一次递减将把数值带回255。这将减少寄存器的数值并不断绘制符号，直至寄存器再次回到0，让循环能够运行256次。这便意味着随着图像储存器位置的增加，奖励符号表范围外的内存位置将被分配到屏幕上。这种半分裂的关卡被玩家叫做“分区屏幕”；而开发者将其称为“乱码画面”。

尝试关卡

在屏幕的左半部分有114颗豆子，右半部分有9颗豆子，一个奖励钥匙，总共有6310个分数点。当所有的豆子都被清除时，什么都不会发生。游戏并未考虑到当吃豆人吃掉244颗豆子时一个关卡便算结束，所以除了让吃豆人挨饿便什么都不会发生。有趣的是，每当吃豆人丧失一个声明，屏幕右半部分的9颗豆子便会重置，并能再次被吃豆人吃进肚子，从而导致每个额外的生命能够获得额外的90个点数。在最佳场景中（即获得5个额外的生命），玩家最多能够获得6760个点数，但却只能吃掉168颗豆子—-不足以改变关卡，所以我们便会卡在这里。没有更多豆子能让玩家去吞食了。也没有最终的胜利等着吃豆人，只有半个充满幽灵的迷宫。在所有的这些内容之后游戏会有一个结局，但既不是愉快的结局也不是让人兴奋的结局。

pac-man（from comcast）

右半边屏幕中的9颗豆子中的4颗是看不见的，但在吃豆人吃到时会有声音响起。左边画面呈现了所有9颗豆子的位置。豆子1，5，6，9是看不见的，剩下的虽然都可见但是有些却是基于不同颜色。

任何人只要到达这一关起都会意识到：为了准确在屏幕右边标出内容，我们必须对幽灵做些事。在经过完善后，你们将发现，一旦幽灵太接近屏幕右边边缘，它便会“陷进去”。一旦幽灵受困，它便只能上下移动，而不能再左右移动了。通过引导幽灵靠近屏幕边缘，有技能的玩家最终能把幽灵逼入掘金，让自己专注于探索迷宫右半部分并努力收集豆子。

pac-man（from comcast）

困住幽灵存在许多方法。最简单的方法便是困住右边的3只重要的幽灵。黄线呈现的是吃豆人从关卡一开始到靠近右下端之间的路线。确切的指令是：一开始向右行走，直到到达一个蓝色大写之母“N”，然后向下行走，直到到达一个大写字母“F”，然后向右，直到到达一个黄色的“B”，然后再次向下。如果执行妥当，吃豆人将在最后一次转向时撞击到一堵看不到的墙。现在让我们等等。红色幽灵将最先受困。在几秒后粉色紧随其后。蓝色幽灵将继续自由移动，直至下一个散播模式出现。这时候，它将尝试着到达屏幕右边边缘附近的位置，与粉色幽灵和红色幽灵一起被困其中。现在，橙色幽灵是唯一能自由行走的（右下方）。Clyde并不是真正的威胁，然而，因为在吃豆人靠近时它会跑向角落，所以让吃豆人能够更轻松地清除所有豆子。要小心迷宫左下方的角落—-橙色幽灵还没有离开，随时有可能对你发动攻击。

不管你信不信

一些版本的《吃豆人》ROM带有一个“实验台”功能，即让橱柜的拥有者能够随时略过它而前往下一个关卡。到现在，唯一能够通过第256个关卡的已知方法是使用这些机器中的实验台。结果便是游戏会循环回第一个面板，但伴随着完整的分数并且幽灵仍然能够发挥作用，尽管这是在第21个关卡或更高。许多《吃豆人》ROM能够伴随着MAME模拟器进行使用，并且也带有这种实验台功能，让我们能够给更轻松地做到这些，而无需使用街机版本去创造这种分离屏幕。

几十年来，世界各地的《吃豆人》粉丝都听过有关一个能让玩家无需使用实验台便度过第256个关卡的“秘诀”。一些玩家夸口说自己已经掌握了这一秘诀，但却没人真正去证实它。在街机游戏社区中，这便成了一个大热门话题，而不相信有人能够做到这点的Billy Mitchell甚至宣称会给通过第256个关卡的首位玩家10万美元的奖励。但现在还没有人能够拿到这笔钱。

尽管有证据证明存在通过第256个关卡的方法，但在经典的游戏论坛上却仍存在各种谣言，也没人能够基于无可厚非的证据去支持自己的言论。我们很难想象为什么那些可以通过这个关卡的人却并未获得Mitchell的奖金。也许这只是人的天性，即宁愿相信可能性的存在—-就像相信圣诞老人或复活节兔子的存在那样。也许也有一些中年《吃豆人》忠实粉丝为了让自己好受点而抵制着一个30年的娱乐设备的秘密。这是一种很奇怪的现象。你必须学会进行判断。

（本文为游戏邦/gamerboom.com编译，拒绝任何不保留版权的转载，如需转载请联系：游戏邦）

CHAPTER 5:

Deciphering The Split Screen

By Jamey Pittman

On The Edge Of Forever

Pac-Man was always meant to be a game with no ending. The developers at Namco mistakenly assumed the game’s increasing difficulty was sufficient to prevent anyone from playing indefinitely. Of course, within a few years of Pac-Man’s release, players had discovered that every level beyond the 21st was identical. Patterns were quickly created to exploit this fact and, for any player able to get past the first 20 levels, the game now became a test of endurance to see how many points you could rack up before losing focus and making a mistake. High scores soared into the millions and most players agreed the game simply went on forever. Eventually, a few highly-skilled players were able to complete 255 consecutive levels of play (scoring over three million points and taking several hours to accomplish) and found a surprise waiting for them on level 256. It was a surprise no one knew about—not even the developers at Namco.

The 256th level displays the left half of the maze correctly, but the right half is a jumbled mess of randomly colored letters, numbers, and symbols. Notice the bonus counter in the lower-right of the screen is also malfunctioning. The left side of the maze plays normally, but the right side is a different story. Although both the player and the ghosts can navigate through the right half of the screen, the original maze walls no longer apply. Instead, Pac-Man must be guided through a confusing series of open areas, tunnels, one-way intersections, lone walls, and pass-throughs—all invisible to the player—while four ghosts are in hot pursuit.

Why does this broken level happen in the first place? The culprit is the routine responsible for drawing the bonus symbols along the bottom edge of the screen. Here’s what happens: when level 256 is reached, the internal level counter is incremented to 255 (the level counter starts at zero – not one) and the routine for drawing the bonus symbols is called. The routine loads the current level counter value (255) into a CPU register and increments that register by one. Unfortunately, 255 is the largest number that can fit in a single byte which is the size of the Z-80 CPU registers, so when the value is incremented the overflow is discarded leaving a zero in the register instead of the expected value of 256. This zero value leads the routine to believe this is an early level since its value is less than seven. The routine starts drawing bonus symbols using the confused register as a counter. At the end of every drawing loop, the register is decreased by one and then checked to see if it is zero (the signal for the routine to stop drawing symbols). Since the register already has a zero in it to start, the first decrement will roll the value back to 255. It will keep decrementing the register and drawing symbols until the register is reduced to zero again, causing the loop to run a total of 256 times. This means that memory locations outside the bounds of the bonus symbol table are drawn to the screen at increasing locations in video memory. This half-broken level was named the “split screen” by players; developers refer to it as a “kill screen”. Don Hodges’ website has an in-depth article analyzing this bug that includes source code and a proposed fix—click here to go there now.

Playing The Level

There are 114 dots on the left half of the screen, nine dots on the right, and one bonus key, totaling 6,310 points. When all of the dots have been cleared, nothing happens. The game does not consider a level to be completed until 244 dots have been eaten, so there is nothing left to do but sacrifice Pac-Man to a hungry ghost. Interestingly, every time a life is lost, the nine dots on the right half of the screen get reset and can be eaten again, resulting in an additional 90 points per extra man. In the best-case scenario (five extra men), 6,760 points is the maximum score possible, but only 168 dots can be harvested—not enough to change levels—so we are stuck. There are no more dots to gobble or energizers to eat. There is no final victory waiting for Pac-Man, only an empty half-maze full of ghosts. The game has an ending after all—just not a very happy or exciting one.

Four of the nine dots on the right half of the screen are invisible, but can be heard when eaten. The picture on the left shows all nine dot locations. Dots 1, 5, 6, and 9 are invisible; the rest can be seen but some are a different color than normal.

Anyone reaching this level quickly realized: to safely map out the right side of the screen something had to be done about the ghosts. After much tinkering, it was discovered that a ghost would get “trapped” on the right edge of the screen if he got too close to it. Once trapped, a ghost can only move up or down but never right or left again. By leading ghosts near the edge of the screen, a skilled player could eventually get the ghosts out of the way and concentrate on exploring the right half of the maze and collecting the dots.

There are many methods for trapping the ghosts. One of the easiest ways to trap the three important ghosts is shown in the picture to the right. The yellow line shows Pac-Man’s path from the start of the level to a spot near the bottom-right. The exact instructions are as follows: begin by going right until you reach a blue letter ‘N’, then go down. Keep going down until you reach a blue letter ‘F’, then go right. Keep going right until you reach a yellow ‘B’, then go down again. When executed properly, Pac-Man will hit an invisible wall almost immediately after the last turn is made. Now we wait. The red ghost will get stuck first. The pink ghost follows a few seconds later. The blue ghost will continue to move freely for several moments until the next scatter mode occurs. At that point, it will try to reach some location near the right edge of the screen and get stuck with the pink and red ghost instead. Now the orange ghost is the only one still on the loose (bottom-right). Clyde is no real threat, however, since he runs to his corner whenever Pac-Man gets close (see Chapter 4), making it relatively easy to clean up all the dots. Be sure to take care around the lower-left corner of the maze—the orange ghost will have nowhere left to run to and will be much more aggressive.

Click on the YouTube video below to watch this ghost-trapping method in action:

Believe It Or Not

Some versions of the Pac-Man ROMs have a “rack test” feature, allowing the cabinet owner to skip ahead to the next level of play whenever they want. To date, the only known way to legitimately get past level 256 is by using the rack test switch inside these machines. The result is that the game loops back around to the first board, but with the score intact and the ghosts still behaving as though it were level 21 or above. Many of the Pac-Man ROMs available for use with the MAME emulator also have this rack test feature, making it relatively easy for those without an arcade version handy to quickly get to the split-screen and beyond.

For decades, Pac-Man enthusiasts worldwide have heard the whispers about a “secret trick” allowing a player to get past level 256 and continue playing without using the aforementioned rack test. Several players have boasted having acquired this holy grail of Pac-Man knowledge over the years, but no one has been able to make good on their claims by actually proving it. This topic became so hotly debated in the upper echelons of the arcade gaming community that Billy Mitchell—who was convinced it was impossible—offered a $100,000 cash prize to the first player to prove they could legitimately get past level 256, leaving the challenge open for a full year. The prize money went unclaimed.

In spite of the evidence against there being a way to get past level 256, rumors still persist and can occasionally be found in classic gaming forums online, yet no one has been able to back up their words with indisputable proof. It’s hard to imagine why anyone who could legitimately get past the level did not collect Mr. Mitchell’s prize money to be sure. Still the occasional whispers can be heard. Perhaps it is simply natural for people to want to believe in the possibility as opposed to not—like Santa Claus or the Easter Bunny. Then again, maybe there is some middle-aged Pac-Man junkie out there who is withholding secrets to a 30 year-old amusement device for his or her own unfathomable reasons. Stranger things have happened. You be the judge.(source:comcast)