Where does one start to comprehend?
There is, contrary to common misconception, a vast difference between a n00b and a newb. n00bs are simply those dreamers with delusional aspirations, who are unwilling to put the effort into sticking at something until they 'get it'. So often you find them milling around on countless forums whining 'wot do I dooo? wea do I goooo??? HEEELLLLP', often followed up by someone giving them step by step instructions on how to use Google.
Newbs, on the other hand, are much smarter. They already know how to use Google, and best of all, they get started. They start actually doing things.
Starting is therefore the hardest step in almost any difficult task. But its also the most important step, because if you haven't started, its not likely that you're going to get much else done either, except the dreaming. But by starting, sooner or later you're going to amass enough knowledge of the fact that you're steering down the wrong path, and after a couple more false starts you should, with enough luck, converge on the right one.
So here is how I started.
Having a little knowledge of game programming as well as computers and their inner workings, I knew that first off I needed to know a little bit of how the PS1 worked. The storage media for the game data in this case is a cd-rom. I had little idea of how data on CDs were read or arranged. After obtaining a cue/bin image of the Clock Tower cd, I mounted it using Daemon Tools, and had a look at the contents.
|Figure 1. directory layout of Clock Tower cd.|
Boom. Right off the bat we have a little assortment of folders all with relatively descriptive names. That's nice, obviously not something you'd get if you were hacking a GBA rom, for example, which is just one big hunk of data referenced by offset (address number).
From just looking at the directory names, we can make a couple of assumptions about what sort of files or data they hold. For example, the BG# folders probably have some sort of background graphics in them, and probably not background music since there is already a folder called "SOUND".
|Figure 2. The Ritual Room|
|Figure 3. Director? Producer? ihavenoclue XP|
I also found out a couple of other interesting things, for example (hazarding a guess based on my rudimentary knowledge of Chinese characters) the credits appear to be images of text so translating them would simply require replacing the text in those images.
On a side note, in the folder called DA1, I found a suspicious music file called LIKE55.WAV and throughout the time I was listening to it I had plastered on my face an expression something like == so sincerely hoping its not the credits song or something like that.
Anyway, we now have a basic idea of how some of the in-game media is stored. How about the dialogue? Perhaps the first thing most people would do is start opening up files in a hex editor and start looking for text from the dialog. However I faced a bit of a dilemma, that is I didn't know how to read, and therefore write, Japanese in order to search for it. If a standard text encoding had been used, it either have to be some variant or subset of Unicode, which wouldn't be very likely since the encoding consisted of more than 100000 characters and I was quite sure it wasn't a widely used standard in Japan for encoding Japanese. The other option was Shift JIS which is a more commonly used encoding in Japan, if I have got my facts right.
Anyhow, there wasn't much point in trying to do any searches just yet. I needed to find out where they got their font from so that I could know how to change it to use English letters. There was little chance that the developers would have spent the time making an entire Japanese font for their game, what with all the Kanji they'd need, though there was a possibility they could have made a font for the subset of characters that they would use exclusively for the game. For the moment I was potentially ruling out that possibility because of the inconvenience that would have posed for the developers.
What I needed to do then was to somehow trace the code that was used to access the needed characters, and hopefully that would lead me to the place where it was stored. For this I would need some sort of debugger in order to log the machine instructions executed by the PSX (unofficial abbreviation for the PS1 console) at various times. Unsurprisingly I found a couple of debuggers built on top of the PCSX emulator. These would allow me to check for reads and writes to various places in memory. One of these, called Agemo, turned out to be an extremely nifty thing. Its main functionality includes the setting of breakpoints in the code when a certain area in memory (only 2MB of RAM!) is read or overwritten. For those that don't know, a breakpoint is something used when debugging a program, it pauses execution of the program when a specific place in the code is reached or a specific event happens. When the execution is paused, we can then dump the state of memory at that particular point in time, or we can 'step' through the following instructions of the process. The emulator is able to do this since it emulates a PSX processor, and can directly control its execution.
This was very cool and all, but it wasn't as I could just start dumping logs of assembly code and start rifling through them. Assembly code is a sort of extremely watered down version of your general programming language. Its probably the lowest level language you'd come across before you get into the realms of binary code, which the average human wouldn't sensibly try to comprehend. By low level, I mean that the assembly 'instructions' or operations almost directly reflect the actions taken by the processor itself. I don't quite mean on an electronic level, but more a conceptual level. There are instructions for adding, subtracting, loading and storing data, and different ways of manipulating bit-patterns, which are sequences of binary digits. There are also instructions for 'jumping' or 'branching' to different locations in the code, and since we don't truly have this high level concept of functions, we need to be able to jump to specific addresses in memory in order to repeat tasks or as the result of making decisions.
To give you a taster of the assembly language used by the processor in the PSX:
800158c4 : BEQ 00000082 (v0), 00000000 (r0), 80015a14
800158c8 : ADDU 00000154 (s3), 00000000 (r0), 00000000 (r0)
800158cc : ANDI 00000000 (s5), 00000001 (a3), ffff (65535)
800158d0 : ADDIU 00000000 (s7), 801ffe48 (sp), 0010 (16)
800158d4 : LUI 00000000 (s6), 8004 (32772)
Yes I was also utterly confuzzled when I first came across this. This is a sample of the logged output of the agemo debugger. It nicely provides the address of the instruction on the left, followed by the name of the instruction, and its parameters and their values.
The values are all in hex which is simply a base-16 counting system using the digits 0-9 and the letters A-F. To express a single hexadecimal digit would require 4 binary digits therefore its a very compact and nice way to express bit patterns and data values.
Reading these instruction sequentially from the top, we have:
BEQ v0 r0 80015a14
This is called a 'pseudo-instruction' which is basically a substitute for a commonly used sequence of basic instructions, in this case for carrying out the task of checking whether the value in the register v0 is equal to to that in r0, and jumping to the address 80015a14 in memory if it is. A register is simply a piece of memory in the processor that holds a bit pattern, and in the case of this processor, a 32-bit one. The jumping effect is actually delayed a single machine cycle (so even if the equality is true, the next instruction down is executed, and only then the jump made), but I won't go any more into that.
In the log sample above, the values of each of the registers at the time of the execution of each instruction are also logged, which makes it possible to trace where specific values came from.
The next instruction adds the value in register r0 to that in r0, i.e. it doubles the value of r0 and stores the result in s3. The next instruction performs a bitwise AND operation on the value in a3 with the value FFFF. A bitwise operation compares the same order bits of each value and places a 1 in the same position of the result if both bits are 1s. For example, the value in a3 is a 1, which in 32-bit binary is essentially 00000000000000000000000000000001, and FFFF is 1111111111111111. So the result will have a 1 in the low order bit, and all the other bits 0, just like the value in a3 at the moment. The result is stored in s5.
The next instruction adds the hex value 0x16 (I'll use the 0x prefix from now on to denote a hex value) to the value in the register sp. The result is stored in s7.
Finally the instruction LUI stores the 2-byte constant 0x8004 in the high order bytes of s6, and replaces the low order bytes with 0x0000. The resulting value passed to s6 is 0x80040000. For those who aren't familiar with it, a byte is an 8-bit binary sequence, which can be expressed using 2 digits in hex, e.g. the largest value able to be expressed by a single byte is the value 255 in the decimal system, which is 0xFF in hex and 11111111 in binary.
Anyway, I won't go too indepth about this because its a topic complex enough to write a textbook about.
But suffice to say that I found out somewhere that the processor in the PSX was extremely similar to a MIPS processor, so that meant that if I learned how MIPS assembly language worked, I would be able to understand the output of the debugger. Consequently I spent about a week reading through half of the MIPS assembly language tutorial on this site:
This tutorial is superb, and was actually all I needed in order to be able to read through the logged output of the debugger actually knowing what each line meant.
However, one does not simply 'read' through a log of assembly instructions from cover to cover. There are a lot of lines of code there, and as I found out, between the mere drawing of two frames, over 500000 instructions were executed.
I had to be a little more clever.
And that's it for Part 2. In the next part I'll apologize profusely for this cliffhanger ending, and I'll also go over how I ended up finding where the font was stored, and how that affected my sanity.
Stay tyoond ;)