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What is RISC?

It's a term used for the processors that are made following certain principles, that were found around ten years ago. The best book about this was for a long time and probably still is: 'Computer architecture, a quantitive approach' by Hennessy and Peterson. One of them even came up with the term RISC.

Basically the philosophy is, that instructions are handled in parts:

• Fetch the instruction.
• Get the arguments
• Perform the action
• Write back the result

Once you do this, you can handle several instructions at the same time, each in one of the above stages. All steps in the process should be equally short and as short as possible, since the longest step will slow down the complete pipe-line.

This means, that the instructions should only do simple things. If you want to do something difficult, the compiler should generate several simple instructions. Instruction counts on real programs being executed on CISC* (complex instruction set computers) show, that around 90% of the instructions that are executed are simple anyway. (Compilers prefer them, because they are more basic.)

To keep the instructions simple, no fancy addressing modes are allowed. All instructions act on the internal register set. Except for some simple memory addressing instructions, that have the address to act on calculated in registers using the normal instructions.

Another viewpoint was that processors currently use a 32-bits databus anyway, so why not make every instruction 32-bits wide? Every instruction cycle fetches 32-bits anyway. (Also note that the fastest RISC processors use separate code and data buses.)
Using these 32-bits efficiently makes it logical, to use one byte to code the actual instruction and the other three bytes, to code the registers to act on. This makes using tryadic instructions logical: For example: 'add r0,r1,r2' which means: r0=r1+r2.

Another aspect is, that difficult instructions like call, push and return disappear, since they involve auto incrementing or decremting the stack pointer.
At a call, the old program counter is therefore put in another general register, so it can be saved on the stack with the normal process of saving the registers. (For a function that doesn't call other functions this save isn't even necessary.)
The registers aren't saved by pushing, but by decrementing the stack pointer a lot and saving the registers as a kind of local variables.
Because the program doesn't have to be pushed between the arguments and the local variables it is also possible to keep a certain number of arguments to the function in registers.

Because there is a byte to code a register, you could theoretically have 256 registers, of which AMD's 29000-series really has about 192, but generally RISC-processors have 32 registers, using only 5 bits to code them. The remaining bits are used for other useful things, like coding small immediate numbers in stead of a register.

A current criticisms about RISC is that the program code is to sparse, which spoils disk and memory space and code loading time (which isn't so terrible on normal computers) but when you're running from ROM in an embedded environment, it can be. It also makes caching much less efficient.
Hitachi has therefore made an embedded RISC processor, that uses 16 bit (dyadic) instructions and it claims, that it is currently the best sold RISC processor for embedded purposes.