CISC is a type of computer architecture that can perform complex tasks using only one instruction. For example, one command can fetch data from memory, perform calculations, and store the results back. This happens all in one step. This way, the processor can have complex addressing modes (more flexible and capable of doing more with each command).
In summary, CISC architecture:
Focuses on complex, and multi-operation instructions
Reduces the number of instructions per program
Uses variable-length instructions with complex decoding
Enables direct operations on memory operands within a single instruction
This blog will go deeper into CISC architecture, from understanding the architecture, use cases, characteristics, how it works, to its pros and cons. So, without further ado, let’s begin.
What are CISC Processor?
CISC stands for Complex Instruction Set Computing. CISC processors were launched in 1970 and can perform complex tasks with relatively few instructions (without using multiple codes). The aim of this architecture is to create an instruction set that functions seamlessly with higher-level languages and data structures.
It can use instructions that are different lengths:
Register-based instructions can be as short as 2 bytes
Instructions with two memory addresses can be 5 bytes for the complete code
As instructions can be of different lengths, the execution time can also vary. Many will use more than one clock cycle.
Another unique feature is direct access to operands in memory. This reduces the difficulty of software (compilers) to convert high-level commands to machine-level instructions.
Imagine you have an instruction to add two numbers together. CISC processors can directly work with these numbers without requiring extra steps.
An instruction can add a number stored in memory and another in a register. And finally, save our result to some location in memory.
So, rather than working with small, fast storage areas called registers that are inside the processor, CISC processors work directly with computer memory to take in and send out information.
Architectural Overview: How CISC Processor Works
CISC processors perform their tasks through repeating cycles:
Program Counter (PC): It holds the 16-bit instruction address to be fetched.
Instruction Fetch: Then, the instruction is loaded from memory into the data bus and sent to the decoder.
Instruction Decoding: The decoder then figures out which operation to perform and the applicable operands.
Control Signals: The control unit sends the commands needed for fetching and executing further instructions.
Execution: Finally, the operation takes place. If required, it may fetch more data and store the result in memory.
PC Update: The Program Counter gets the next address, gets latched, and the cycle goes on.
How does this recurring cycle help? It ensures that most actions happen in memory – so only a few registers are required.
Unlike RISC, CISC can perform operations directly on data in memory without first loading it into registers. For example, the instructions MUL 2:1 and 3:5 multiply values and store the result directly in memory.
How it Works? Example of ADD and MUL Instructions
Let’s say you need to add two 8-bit numbers. In a CISC processor, you can get it done by using a single instruction only (instead of many).
For that, you can use commands like ADD and tell the processor where the numbers are stored in memory and where to put the results.
The processor will go to the memory, grab the two numbers, add them together, and store the result – within that one instruction.
Similarly, for multiplication, you can use this command: MUL 2:1, 3:5. How does it perform?
It multiplies the value at 2:1 by the value at 3:5, and stores the results WITHOUT any load/store commands. The entire operation is directly manipulated in memory.
Key Features of CISC Processors
CISC processors have several key features that set them apart. Let’s take a look at them and then understand with an example.
Instruction Set and Addressing
Large Instruction Set: Typically, 100 to 250 instructions are available.
Specialized Instructions: Some instructions are used rarely but support unique or advanced operations.
Several addressing modes: It generally ranges between 5 and 20 and can be used for flexible design.
Variable-Length Instructions: It can also vary depending on what kind of task you’re doing or how the data is being accessed.
Memory Operations: Many instructions can directly work with the data in memory. It means that you don’t always need to move the data to a faster register first. This saves time and simplifies the process.
Further Architectural Features
Complex Instruction Decoding: Due to variable-length and complex instructions, decoding logic is intricate.
General-Purpose Registers: CISC processors don’t need as many of these registers because they do most of their work directly with memory instead of using registers for temporary storage.
Special Purpose Registers: These are specific registers for special tasks. For example, some registers help manage things like the program’s stack (which keeps track of data), handle interruptions, or record important details like the result and status of operations (this is called the Condition Code Register).
Multiple Data Types: CISC processors handle varied data types with their extensive instruction sets.
Limited Register Use: Chip space is mostly dedicated to instruction decoding, execution, and microcode storage.
Multiple Clock Cycles: Some instructions require more than one clock cycle to complete.
Condition Code Register: It is used to store certain conditions, like whether the last result was negative, zero, or positive. It also flags errors if any.
Special Registers: They’re used for particular tasks, such as tracking the stack (where data is kept) or dealing with interruptions.
Why Use CISC Processors?
CISC design focuses on incorporating complexity directly into the CPU. By doing so, it takes stress off the software and other hardware. These processors are well-suited to tasks such as complex workloads generated by high-level programming languages.
Key points:
Makes compiler’s job easier: Each high-level language statement is directly into a single machine instruction.
Multithreading: CISC designs can benefit from running many tasks at the same time.
x86 Ecosystem: Started in the 80s by Intel, the x86 family is the biggest CISC architecture. Only Intel and AMD remain as leading manufacturers after decades of investment in PC hardware and software.
Market Status: ARM architecture has made progress in some areas, but x86 CISC chips still dominate IT infrastructure, especially in data centers.
Cooling Solutions: High power consumption and heat output are handled by innovations such as liquid and immersion cooling.
Advantages of CISC Processors
Smaller Code Size: The smaller the size of the code, lesser the amount of RAM an application with the same footprint will require.
Efficient Compilation: CISC can compile a programming language to machine code more easily and quickly.
Simpler Code: CISC processors can do some really complex things in a few instructions. This makes the code simpler.
Direct mapping: It means that complex commands in a program can be directly turned into instructions for the hardware to follow.
Disadvantages of CISC Processors
High hardware complexity can reduce computational speed and efficiency
Multiple instructions require multiple clock cycles, which can slow down operations
CISC architectures generally lack efficient pipelining support, limiting parallel execution
Solution for CISC-Based Legacy Architecture
Is your business dependent on CISC-based hardware that has reached its end-of-life status? The VAX and PDP are the most notable among them. Although they were instrumental in shaping the technology landscape a few decades ago, now, without support from vendors, they have become extremely risky and costly to maintain. In fact, the business continuity of the applications running on them is at stake. What if they fail tomorrow?
Stromasys Charon is the most cost-effective solution available. It emulates an aging hardware environment on modern systems, allowing related applications to run as if they are operating in their familiar setting. However, in reality, the physical hardware will be removed.
This way, businesses can eliminate the risks associated with VAX and PDP hardware while still running critical legacy applications.
No code changes and no year-long rewriting projects. Discover how we can achieve this in just a few days.
AMD’s mainstream CPUs are based on CISC x86 architecture.
RISC is faster than CISC because it uses fewer instructions. CISC, on the other hand, implies complex instructions.
Most laptops use CISC processors (those based on x86, like Intel and AMD). Some of the newer models have an RISC ARM chip, especially the ultra portables and some Macs.
Yes, it is the 8086, which is a CISC classic commencing the x86 legacy.
CISC remains popular because:
It permits a high level of code size density, thus minimizing memory and cache requirements.
It is a good fit for complexity and performance-oriented applications (such as desktops and servers).
It is easier to maintain backward compatibility with a large existing software ecosystem.
ARM is a RISC architecture that offers a more compact and efficient set of instructions compared to x86, delivering enhanced performance and better power efficiency, especially in battery-powered and portable devices.
About Author
Tuhin Das
Tuhin is a passionate writer with more than 7 years of experience in technical and marketing writing. With a unique ability to connect with his readers on a deeper level, he crafts content that not only captivates but also inspires action. Always on the cutting edge of industry trends, he excels at breaking down complex ideas into clear, engaging narratives that drive engagement and fuel business growth. Beyond his inherent inclination for writing, he is a sports enthusiast and a traveller, always seeking new experiences to enrich his perspective and creativity.
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