[wolfgke]

> it has a very simple 8-bit (risc) architecture

The 8080/Z80 variant that is used in the Game Boy is not a RISC architecture - it is a processor architecture from a time long before the term "RISC" was even used the first time.

While the terms RISC/CISC are defined somewhat vaguely, there are still some common properties of architectures that are considered RISC:

- Load-store architecture:

Counterexample: ADD A,(HL)

- Lots of general purpose registers:

The 8080 has rather few:

B,C,D,E,H,L,A as 8 bit registers and BC,DE,HL,SP as 16 bit registers

- Very orthogonal instruction set:

Counterexample: AND has always destination A

- Often pipelined architecture (though this is not very specific to RISC)

- Instructions typically aim to be run in one cycle (when pipelined)

Counterexample: Look at the instruction timing tables

- very uniform instruction format

Look at the instruction encoding

[userbinator]

- very uniform instruction format

Look at the instruction encoding

The instruction encoding is quite regular -- it follows the 2-3-3 pattern that came from the 8008 (if not earlier), and thus looks much better in octal than hex:

http://www.pastraiser.com/cpu/gameboy/gameboy_opcodes.html

http://www.z80.info/decoding.htm

As for everything else, I would agree with you --- it's not RISC.

[wolfgke]

> The instruction encoding is quite regular -- it follows the 2-3-3 pattern that came from the 8008 (if not earlier)

While it is true (and I am aware of it) that the Z80 instructions follow the 2-3-3 pattern, it is nevertheless not that regular. Best look at the "purest example" for this that is referred in the literature all the time and acted as a model for the criterion that in RISC the instruction format is very regular: the instruction format(s) used by MIPS. Here there only exist 3 different types of instruction (R-type, I-type and J-type) and all have a very regular pattern. One can find very suggestive pictures at

> http://homepage.divms.uiowa.edu/~ghosh/1-24-06.pdf

[0xcde4c3db]

> the 2-3-3 pattern that came from the 8008 (if not earlier)

If memory serves, that pattern has its roots in the PDP-11 instruction encoding.