8 bits, 6 bytes

It was designed for a 8-bit computer which has 6 registers.

You can write 256 instructions at 1 time.

0~63: reg[0] = instruction
64-127 64+a: reg[3] = reg[2] [NAND,OR,AND,NOR,+,-][a] reg[1]
128~191 128+8a+b: reg[b] = reg[a]
192~255 192+a: if reg[3] [False,True,=,!=,<,>=,<=,>][a] 0 then goto reg[0]