SubSky is a simple, minimalist, and easy-to-use RISC ISA and computing environment. It strikes a reasonable balance with elegant simplicity and practical 32-bit range, intending to be the most amount of power you can get from the least amount of computer.
It is the project's ISA, used for the OS's userspace.
[README]

It has just 16 opcodes and 16 operands:

    Operations      Operands
  +0  +1  +2  +3   +0 +1 +2 +3
0 Ior Xor And Sft  GT BP IP FP
4 Add Sbf Mul Div  SP SE S1 S2
8 Rwr Rbr Lth Cpt  L1 L2 L3 L4
C Adf Adt Wwr Wbr  L5 L6 L7 L8
			

• Ior, Xor, And, Sft: bitwise operations.
• Add, Sbf, Mul, Div: math operations.
• Rwr, Rbr, Wwr, Wbr: memory operations.
• Lth: less-than (actually, DOZ (Difference Or Zero) -- lots of neat tricks with this!).
• Cpt, Adf, Adt: predicated instructions. Ad{t,f} are useful for conditional branching, like ADF IP <condition> &<destination>.
• ADT <Q> IP <B>: "silog2". Essentially Q = floor(log_2(B)).

The last row of operations (Adf through Wbr) can read from all three instruction operands.

• GT, General Temp: general-purpose register.
• BP, Begin Pointer: register holding the address of the currently-running program.
• IP, Instruction Pointer: register holding the address of the instruction to run next.
• FP, Frame Pointer: register holding the address of the "locals" L1 -- L8.
• SP, Stack Pointer: register holding the address of the stack top.
• SE, Stack Element: the stack. Setting SE pushes, and getting SE pops. A lot revolves around this operand.
• S1 -- S2: stack top and word under stack top. S1 is *(SP), and S2 is *(SP + 4).
• L1 -- L8: the "locals", used for most variables. L1 is *(FP), L2 is *(FP + 4), and so on.

BP through SP, when in the B field, specify to load an immediate to use as B; the B operand code itself indicates how many bytes long the immediate is (BP in the B field means load a 1-byte immediate, IP in the B field means load a 2-byte immediate, etc.).

See the spec for the actual definition (about 300 lines, excluding extraneous information).

• SubSky repository