Extendable Instruction Set
In embedded applications, code density is a golden goal since higher code density means less memory traffic and die area for embedded memory can be reduced. To improve code density, compressed code architectures have appeared such as THUMB by ARM and MIPS16 by MIPS and so on. In short, compressed code architectures have shorter length instructions for controlling the data path. Many compressed code architectures suffer from insufficient immediate operand fields.
While achieving high code density and a low memory access rate, the EISC architecture adopts a novel and terse scheme to resolve the problem of insufficient immediate operand fields of the compressed code architectures. The EISC uses an efficient fixed length 16-bit instruction set for 32-bit data processing. To resolve the problem of insufficient immediate operand fields in a concise way, EISC uses an independent instruction called load extension register (LERI), which consists of a 2-bit opcode and a 14-bit immediate value.
The LERI instruction extends the immediate field by loading an immediate value to a special register called the extension register. By using LERI instructions, the EISC architecture can make the program code more compact than the competing architectures, since the frequency of LERI instructions is less than 20% in many programs. In addition, EISC does not require instructions for switching its processor mode between the compressed instruction mode and the normal instruction mode. (For competing architectures, extra mode-changing instructions are added to use specific instructions such as MAC instructions.)
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