For more information on the "simple instruction decoder" vs "complex instruction decoder", checkout https://www.hardwaresecrets.com/inside-pentium-m-architecture/4/ . In general, simple instruction decoder deals with frequently used, simple instructions that could be converted into one micro-op, while complex instruction decoder deals with complex ones that can be converted to up to 4 micro-ops.
For more information on the "simple instruction decoder" vs "complex instruction decoder", checkout https://www.hardwaresecrets.com/inside-pentium-m-architecture/4/ . In general, simple instruction decoder deals with frequently used, simple instructions that could be converted into one micro-op, while complex instruction decoder deals with complex ones that can be converted to up to 4 micro-ops.