Undocumented 8086 instructions, explained by the microcode

Where can one learn more about microcode, how it's implemented in silicon? As I understand it most machine code is actually "VM bytecode", and the "real" cpu is the microcode processor?

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.

> The undocumented C9 opcode is identical to the documented CB, far return instruction.

I remember this.

Once ... gosh, it's hard to believe how long ago that was now, but once I knew the entire Z80 opcode table off head. I could read the Z80 machine code and disassemble it. High school crowded that stuff out of my head, I went to a special math school, it was very very hard. Except... C9 was RET (and the Z80 is an extension of the Intel 8080). That has burned into me so deep I still remember, across more than 35 years. I will be 30 ;) years old in two weeks.

Having AdBlock extension, but like all my extensions also have "This can read and change site data > when you click the extension" while turn on to any domain that "misbehave" with sticky ads, videos or whatever annoying flashing things. Otherwise I'm just hurting content creators for no good reason. I would not enable an extension on any domain i input my email, password, or otherwise have sensitive private information.

I’m on iOS and the ad was unobtrusive. I’m very happy to support Ken. Unless the ads are more disruptive on other platforms I’d say leave ‘em in.