Understanding the Instruction Set

Overview

The SPU Mark II uses quite unique instruction set following a highly orthogonal programming style. In contrast to most cpus on the market, the SPU Mark II is a stack machine. This means that all operations move data to or from the stack. This does not mean that this cpu has no registers. There are auxiliary registers like a stack pointer SP or the instruction pointer IP.

Instructions

Each instruction is composed of a set of bit fields defining the behaviour:

The most relevant field is the command part. It defines what the CPU actually does with your data. There are 32 commands from which are some still reserved. They provide the ability to modify data (ALU), modify memory (load/store) or modify aux registers.

Each command has two input values: input 0 and input 1. These get fetched by the cpu and get passed two the command. It will then process both values and will yield an output. The cpu will then dispatch the output value as specified in the instruction.

An output can be discarded, pushed to the stack or used as a jump target. This means that actually all instructions can be jump instructions! Discarding an output might sound unnecessary, but it has its uses as well:

To remove a value from the stack, the command COPY is used and the output is just discarded. This means that we can fetch values from the stack and don't push new ones. We successfully implemented pop by assembling an instruction.

Each instruction has also a bit that, when set, will update the flags of the cpu according to the output of the command. There are two flags relevant for the normal program control flow: zero and negative. These flags will only be changed when an instruction has the modify flags bit set. Then, the zero flag will be set when output is zero and the negative flag will be set when the highest significant bit in output is set (and thus is negative in two's complement).

The flags can then be used to execute instructions conditionally. The execution field of an instruction defines for which combination of flags the instruction is actually executed. This is similar to other architectures like ARM or the Parallax Propeller that allow instructions to be executed conditionally instead of having a dedicated conditional jump, conditional move or similar. It is also different from instructions like the AVR skip which requires an additional fetch cycle.

Conditional execution checks for different combinations of flags that are semantically relevant. This yields 7 conditions: always, less-or-equal, greater-or-equal, less-than, greater-than, non-equal (or non-zero) and equal (or zero).

Now only two fields are unexplained: input 0 and input 1. Both fields have the same semantics and the same options: zero, immediate, peek and pop.

Zero is the simplest one and means that the input field gets set to 0. Immediate means that value for that field is encoded in the instruction itself and is not dependent on runtime properties. This is usually used for addresses, offsets, magic numbers or similar. Peek and pop both take the value of the stack top, and pop will remove the value from the stack whereas peek will keep the stack pointer itself untouched.

Registers

TODO: Explain what the four core registers do

Common Patterns

TODO: Explain how some coding patters are and how to write basic code for the processor.

; Check an an 8 bit value
sgxt [f:yes] [out:discard]