MIPS Breakdown

This article's for the beginner game hackers (or advanced game hackers who don't know anything about MIPS) who need help understanding the MIPS assembly language.

This section gets into some more complex stuff as you will learn about the MIPS assembly programming language (the language in which all PS2 games are written in), but is still meant for the beginners who want to understand the code they see in the ps2dis... lets get started.

when it comes to programming in 'assembly', there is NOT one type of assembly. ALL assembly languages are programming languages in which the source code deals directly with the processor chip. the PS2 runs off of a MIPS processor chip, and for this reason all PS2 games must be written in the MIPS assembly language. there are more than just MIPS assembly however... all of the types of processors have their own assembly language. MIPS assembly is the code you see when you open a dump file in the ps2dis. because assembly languages interact directly with the processor chip, they are EXTREMELY fast when it comes to program execution. in fact... when you read about a computer that has, lets say, a 2.4 GHz processor... this is telling you how fast the processor works. the 2.4 GHz is how many processes the processor chip makes per second... lets think about that. there's hertz, mega hertz, and giga hertz. about 1000 hertz in a mega hertz and about 1000 mega hertz in a giga hertz. that many processes per SECOND... that's REALLY fast. anyway, back to the part that matters.

there are some complex and key points to the MIPS assembly language which MUST be taken into account when reading MIPS assembly source code (or even more so... writing MIPS assembly source). ill start from the beginning.

each and every action done by the processor is done by a line of code called an 'instruction'. EVERY instruction in the MIPS assembly language is a 32-bit process. now, a single bit is a single binary digit that can be either '0' or '1' standing for 'false' and 'true'. there are 32 bits (or on/off digits) in every instruction. there are 8 bits in a single byte... and the 8 hex valued digits that make up an address are made up of 4 bytes. you can test this theory by multiplying 4 by 8. in other words you multiply the 8 bits that make up a byte by the number of bytes... the answer is 32, where you have 32 bits (hence the '32-bit' instructions).

MIPS assembly uses 'registers' to store data for operation in program execution. there are 31 general purpose registers, 30 double float registers, and 31 single float registers (if you don't know what i mean by 'float', read up on some c++... specifically the types of variables). the general purpose registers are broken down even more though... for instance, there are certain general purpose registers that should be used for certain things. (have you ever seen a register in the ps2dis that was identified with a 't'... i.e. t0, or t1??? these are 'temporary' registers and should ONLY be used within a function.) also, there are 2 (i believe) registers that are not meant to be used to store information... the zero register (known as $0 or zero) ALWAYS holds the value zero. so if you try to store data in it for an important comparison or for a branch... it'll compare the other register with the value zero instead of what you tried to store into register $0. and register 'ra' (i think) is the other register that is a special register. this register is used to hold the address for jumps and jump returns and things like that... not to hold values for comparison or anything else.

there are also little rules that one MUST abide by when using MIPS assembly. the first of the two major issues ill talk about is: the 'PC' (think of this as the 'program counter' which keeps track of which line the program is on) is incremented by 4 during the execution of each instruction (it increases by 4 because of the four bytes that make up an address). the PC in increased by four during the MIDDLE of the instruction... so when the program comes across a 'j' or jal' or any kind of jump, the PC is increased THEN it executes the line of code which contains the jump instruction. because of this, the program runs and executes the line of code after the line with the jump instruction. and, in the middle of that lines execution, the PC is finally set to the address in which the jump referred to. this is NOT a big deal at all... in fact, because of the one line delay, you can make good use of its time and put an important instruction after the jump. if you don't have an instruction after the jump... who knows what could happen (the program would crash most likely). this is why when you are viewing the code for the games, there is ALWAYS a line of code after the jump, even if its just a 'nop' (nop or no-op stands for 'no operation').

the second of the two key issues is the 'load/store delay time'. the MIPS assembly language (because it is 32-bit based) has addresses that range from 00000000 all the way to FFFFFFFF. BUT, the MIPS processor sections off certain ranges of addresses for certain usages... one of these usages is memory. MIPS has a section of addresses where you can store data and call upon it at a later time (if you are using the pcSPIM MIPS simulator, the 'memory' range starts at 10000000). there is, however, a delay time when it comes to loading or storing information in the memory. the delay time is only one instruction long... which is NOTHING considering how fast the programs execute. but, due to the delay time... you SHOULD NOT use the register for ANY reason after loading or storing information until at least one more instruction has already been executed. you will also see this in the ps2dis... there is always time between a load and store instruction, and an instruction that uses the registers that held (or hold) the data for loading or storing.

now ill go over a couple commands for the MIPS assembly language which should really help you when it comes to hacking psp games...

I take some, but not all credit for this TUT.