Design goals

Simple to implement in other programming languages
Efficient to execute by emulator
Future extensibility (to support 128-bit data/address width or more)
Single instruction code for any data/address width
Register based load-store architecture rather than stack based machine or accumulator based machine

Other interesting instruction sets

CPU	Instruction count	Instruction size [bits]	Data size [bits]	Addressable memory [bits]	Registers
Tiny CPU Instruction Set	22	8-16	8	10	PC, SP, A, X, SR
A Quick Introduction to the ZipCPU Instruction Set	29	32	32	32	R0-R13, PC, CC
MINI-CPU Instruction Set Specification	12	4-12	8	8	PC, ACC, R
Simple Virtual Machine	18	8	32	32	PC, SP, FP
Simple virtual machine which interprets bytecode.	36	16-32	16-32	16-32	IP, FLAGS, R0-R9, SP
VAM Virtual Assembler Machine	58	16-32	16-32	16-32	R0-Rn (IP, SP)
Simple 8-bit Assembler Simulator	60	8	8	8	PC, SP, A, B, C, D
The RiSC-16 Instruction-Set Architecture	8	16	16	16	PC, R0-R7
TOY Machine	16	16	16	8 (256 words)	PC, R0-R15
MU0	8	16	16	12	PC, ACC
LC-3	15	16	16	16	PC, R0-R7
TOY/2 - a minimalist 16 bit CPU	15	16	16	16	PC, A, T
MARIE: An Introduction to a Simple Computer	9	16	16	12	PC, ACC
Building a 4-Bit Computer: The Instruction Set	16	4	4	12	R0-R5, S0, S1, PC, SP, IX, IV, TA
How to create your own virtual machine	35	8	8	16	A, B, X, Y, D
Little Man Computer	10	12	8	8	A
CRISC	32	struct	64	16-64
CHIP-8	35	16	8	12	V0-V15, I
Toy CPU	14	8	8	8	A
8bit virtual machine	28	8	8	8	PC, A, X

Types of architectures:

Direct - An address to memory is used as a parameter.
Accumulator - Accumulator based architecture which has special register called accumulator to hold value for operations. There can be more special registers in addition to accumulator.
Registers - Uses multiple general-purpose registers to hold value for operations.
Stack - Parameters and results for operations are kept on top of the stack.

Instruction	Meaning	Direct	Accumulator	Registers	Stack
SET	Set value n to address x	SET (x), n	LOAD A, n	LOAD Rx, n	PUSH n
COPY	Copy value from address y to address x	COPY (x), (y)	LOAD A, (y) STORE (x), A	LOAD Rx, (y) STORE (x), Rx	PUSH (y) POP (x)
COPY	Copy value from address loaded from address y to address x	COPY (x), ((y))	LOAD A, (y) LOAD A, (A) STORE (x), A	LOAD Rx, (y) LOAD Rx, (Rx) STORE (x), Rx	PUSH ((y)) POP (x)
INC	Increment value at address x	INC (x)	LOAD A, (x) INC A STORE (x), A	LOAD Rx, (x) INC Rx STORE (x), Rx	PUSH (x) INC POP (x)
ADD	Sum value at address x with value at address y and store to address x	ADD (x), (y)	LOAD A, (x) ADD A, (y) STORE (x), A	LOAD Rx, (x) LOAD Ry, (y) ADD Rx, Ry STORE (x), Rx	PUSH (x) PUSH (y) ADD POP (x)
ADD	Sum value at address y with value at address z and store to address x	ADD (x), (y), (z)	LOAD A, (y) ADD A, (z) STORE (x), A	LOAD Rx, (y) LOAD Ry, (z) ADD Rx, Ry STORE (x), Rx	PUSH (y) PUSH (z) ADD POP (x)