A computer architecture book such as Hennessy and Patterson will describe microcode in detail. It's a bit confusing because microcode has changed meaning a bit

A computer architecture book such as Hennessy and Patterson will describe microcode in detail. It's a bit confusing because microcode has changed meaning a bit over time. "Classical" microcode, such as the 8086, replaces hard-wired control logic with micro-instructions. The processor steps through the appropriate micro-instructions, which are decoded to generate control signals. The Pentium Pro introduced a new model, where machine instructions are broken down into independent micro-ops, which are handed off to the core processor engine and processed independently, in parallel. At the end, the micro-ops are "retired" in a sequential order, so your program appears sequential. Most micro-ops are generated by decoders that convert a machine instruction into a small number of micro-ops. However, complicated machine instructions are converted into micro-ops by microcode. This is similar to classical microcode, except it's not executing micro-instructions but generating micro-ops that then get run by the underlying processor.