Intel microcode is microcode that runs inside x86 processors made by Intel. Since the P6 microarchitecture introduced in the mid-1990s, the microcode programs can be patched by the operating system or BIOS firmware to work around bugs found in the CPU after release.[1] Intel had originally designed microcode updates for processor debugging under its design for testing (DFT) initiative.[2]
Following the Pentium FDIV bug, the patchable microcode function took on a wider purpose to allow in-field updating without needing to do a product recall.[1]
In the P6 and later microarchitectures, x86 instructions are internally converted into simpler RISC-style micro-operations that are specific to a particular processor and stepping level.[1]
gwennap-20070915 was invoked but never defined (see the help page).Pentium Pro microprocessor ... Micropatching DFT feature. ... consists of two key elements: the microcode patch RAM and several pairs of Match and Destination registers. ... Microcode Instruction Pointer (UIP) matches the content of a Match register, the UIP will be reloaded with a new address from the Destination register. ... UIP for the reset subroutine can be set in the Match register ... thereby bypassing the reset subroutine altogether.