All You Need to Know About CISC

All You Need to Know About CISC

According to abbreviationfinder, CISC stands for Complex Instruction Set Computer. In Spanish (Complex Instruction Set Computer). In it the processor brings hundreds of registers and many steps and clock cycles are needed to perform a single operation.

All PC-compatible x86 CPUs are CISC processors, but newer Macs or some with complex engineering drawings probably have a RISC (Reduced Instruction Set Computer) CPU.


The practical difference between CISC and RISC is that CISCx86 processors run on DOS, Windows 3.1, and Windows 95 in native mode; that is, without software translation that will degrade performance.

But CISC and RISC also reflect two rival computing philosophies. RISC processing requires short software instructions of the same length, which are easy to process quickly and in tandem by a CPU.

CISC technology, one of the oldest and most common, is not as efficient as RISC, but it is the most widespread since it was used from the beginning by Intel, the largest processor manufacturer in the world, like AMD, its competition. There are millions of programs written for CISC that do not run on RISC; the difference between both architectures has been shortened a lot with the increasing speed reached by the CISC processors, however a RISC processor half the speed of a CISC will work almost the same as the latter and in many cases much more efficiently.

Nowadays, the programs are increasingly large and complex, they demand greater speed in the processing of information, which implies the search for faster and more efficient microprocessors.

CPUs combine elements of both and are not easy to pigeonhole. For example, the Pentium Pro translates the long CISC instructions of the x86 architecture into simple fixed-length micro-operations that run on a RISC-style kernel. The UltraSparc-II of Sun, accelerates MPEG decoding with special instructions for graphics; These instructions obtain results that in other processors would require 48 instructions.

Therefore, in the short term, RISC CPUs and RISC-CISC hybrid microprocessors will coexist in the market, but with increasingly diffuse differences between the two technologies. In fact, future processors will fight on four fronts:

  • Execute more instructions per cycle.
  • Execute the instructions in a different order from the original so that interdependencies between successive operations do not affect processor performance.
  • Rename the registries to alleviate the scarcity of them.
  • Help accelerate overall system performance in addition to CPU speed.


Microprogramming is an important and essential feature of almost all CISC architecture. Such as: Intel 8086, 8088, 80286, 80386, 80486, Motorola 68000, 68010, 620, 8030, 684.

Microprogramming means that each machine instruction is interpreted by a microprogram located in memory on the processor’s integrated circuit. In the sixties, micro-production, due to its characteristics, was the most appropriate technique for the memory technologies that existed at that time, and it also made it possible to develop processors with upward compatibility. Consequently, the processors were endowed with powerful instruction sets.

Composite instructions are internally decoded and executed with a series of microinstructions stored in an internal ROM. This requires several clock cycles (at least one per microinstruction). The fundamental goal of the CISC architecture is to complete a task in as few assembly lines as possible. This is achieved by building processor hardware that is capable of understanding and executing a series of operations.

For this particular task, a CISC processor would come prepared with a specific instruction called MULT. When this instruction is executed, it loads the two values ​​into separate registers, multiplies the operands in the unit of execution, and then stores the product in the appropriate register. Thus, the entire task of multiplying two numbers can be completed with one instruction.

MULT 2: 3, 5: 2

MULT is what is known as “complex instruction.


It runs directly into computer memory banks and does not require the programmer to explicitly call any loading or storing functions. It closely resembles a command in a high-level language. For example, if we let “a” represent the value of 2: 3 and “b” represent the value of 5: 2, then this command is identical to the declaration of C “a = a * B.”

One of the primary benefits of this system is that the compiler has to do very little work to translate a high-level language statement to the assembly. Because the length of the code is relatively short, very little RAM is required to store instructions. The emphasis is placed on building complex instructions directly on the hardware.

CISC does not represent a processor architecture proposal in the usual sense. CISC reflects the way in which they were developed and the improvements that had been made to processor architectures until about 1975. CISC, is the Computer with a Complex Instruction Set (Complex Instruction Set Computer), represents the name of the main current developed in computer architecture and, perhaps, we could understand that it is the name that was assigned to the tendency to which the movement RISC was opposed.


Comments are closed.