If you work in the IT field and have anything to do with hardware or security, you need to have an awareness of CPU microcode.
What is CPU Microcode?
You can think of microcode as firmware for your CPU, and like other firmware it is updatable which means it is hackable. It is a layer of hardware-level instructions or firmware used to implement higher-level machine code instructions in a CPU. These micro-instructions are stored in special memory within the CPU and provide the control signals to execute machine code instructions.
CPU’s from before the Intel Pentium crisis of 1994 (see details at the end of this article), do not have microcode, but almost every CPU since then does.
Functionality of Microcode
Microcode acts as an intermediary between the hardware and the software, translating complex machine-level instructions into a sequence of simple operations that the hardware can execute. It essentially serves as a set of low-level instructions that define how the CPU should execute high-level machine code instructions. This allows for flexibility in the CPU’s design and enables updates and improvements to the processor’s functionality without requiring hardware changes.
How Microcode Works
When a machine code instruction is issued, the CPU’s instruction decoder translates it into a microcode sequence. This sequence consists of multiple micro-operations that control the various components of the CPU, such as the ALU (Arithmetic Logic Unit), registers, and memory. These micro-operations are executed in a specific order to perform the desired task.
Microcode is stored in a special read-only memory (ROM) or programmable read-only memory (PROM) within the CPU. When an instruction is executed, the corresponding microcode sequence is fetched from this memory and processed by the control unit of the CPU.
Benefits of Microcode
- Flexibility: Microcode allows for changes and updates to the CPU’s behavior without altering the hardware. This enables manufacturers to fix bugs, improve performance, and add new features through microcode updates.
- Simplified Design: By using microcode, the complexity of the CPU’s design is reduced. The control logic for executing instructions is implemented in software (microcode) rather than hardware, making it easier to design and maintain.
- Compatibility: Microcode helps maintain compatibility with legacy software by providing support for older instruction sets. This ensures that new CPU’s can still run older programs without modification.
Historical Context: The Intel Pentium FDIV Bug
In 1994, Intel faced a crisis with its Pentium processors known as the Pentium FDIV bug.
The Pentium FDIV bug is the most famous (or infamous) of the Intel microprocessor bugs. It was caused by an error in a lookup table that was a part of Intel’s SRT algorithm that was to be faster and more accurate.
With a goal to boost the execution of floating-point scalar code by 3 times and vector code by 5 times, compared to the 486DX chip, Intel decided to use the SRT algorithm that can generate two quotient bits per clock cycle, while the traditional 486 shift-and-subtract algorithm was generating only one quotient bit per cycle. This SRT algorithm uses a lookup table to calculate the intermediate quotients necessary for floating-point division. Intel’s lookup table consists of 1066 table entries, of which, due to a programming error, five were not downloaded into the programmable logic array (PLA). When any of these five cells is accessed by the floating point unit (FPU), it (the FPU) fetches zero instead of +2, which was supposed to be contained in the “missing” cells. This throws off the calculation and results in a less precise number than the correct answer
The discovery of this bug was a major event in the tech industry, and Intel initially downplayed the severity of the issue. However, due to public pressure and the potential impact on scientific and financial calculations, Intel eventually issued a recall and replaced the ALL of the affected processors at a cost of $475 million dollars. That is more than $1 billion 2025 dollars.
Relation to Microcode
The Pentium FDIV bug highlighted the importance of microcode in modern CPUs. If such a flaw were to be found in a microcoded component of the CPU, it could potentially be fixed through a microcode update, avoiding the need for a costly hardware recall. Microcode updates provide a mechanism to address issues and improve the CPU’s functionality without requiring physical changes to the hardware.
Modern Use of Microcode
Today, microcode updates are commonly used to address security vulnerabilities, optimize performance, and add new features to CPUs. These updates are typically distributed by CPU manufacturers (such as Intel and AMD) and can be applied through BIOS/UEFI updates or operating system patches.
In summary, microcode is a critical component of modern CPU design, providing flexibility, simplified design, and compatibility. It allows for updates and improvements to be made to the CPU’s functionality without requiring hardware changes, ensuring that processors remain reliable and secure.
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