Microcode is a layer of hardware-level instructions that implement higher-level machine code instructions or internal state machine sequencing in many digital processing elements. Microcode is used in general central processing units, in more specialized processors such as microcontrollers, digital signal processors, channel controllers, disk controllers, network interface controllers, network processors, graphics processing units, and in other hardware.
In Linux the CPU microcode can be stored in the initial ramdisk (initrd) e.g.
The initrd is a (compressed) cpio archive.
- Wikipedia: Microcode
- Early load microcode, by Fenghua Yu
- Security Analysis of x86 Processor microcode, by Daming D. Chen, Gail-Joon Ahn, December 11, 2014