Before the RISC philosophy became prominent, many computer architects tried to bridge the so called semantic gap, i. e. to design instruction sets that directly supported high-level programming constructs such as procedure calls, loop control, and complex addressing modes, allowing data structure and array accesses to be combined into single instructions.

Due to inherently compact and semantically rich instructions, the average amount of work performed per machine code unit ( i. e. per byte or bit ) is higher for a CISC than a RISC processor, which may give it a significant advantage in a modern cache based implementation.

As CISC became a catch-all term meaning anything that's not a load-store ( RISC ) architecture, it's not the number of instructions, nor the complexity of the implementation or of the instructions themselves, that define CISC, but the fact that arithmetic instructions also perform memory accesses.

For instance, the PDP-8, having only 8 fixed-length instructions and no microcode at all, is a CISC because of how the instructions work, PowerPC, which has over 230 instructions ( more than some VAXes ) and complex internals like register renaming and a reorder buffer is a RISC, while Minimal CISC has 8 instructions, but is clearly a CISC because it combines memory access and computation in the same instructions.

Dhrystone tries to represent the result more meaningfully than MIPS ( million instructions per second ) because instruction count comparisons between different instruction sets ( e. g. RISC vs. CISC ) can confound simple comparisons.

For example, the same high-level task may require many more instructions on a RISC machine, but might execute faster than a single CISC instruction.

Each KSR1 processor was a custom 64-bit reduced instruction set computing ( RISC ) CPU clocked at 20 MHz and capable of peak output of 20 million instructions per second ( MIPS ) and 40 million floating-point operations per second ( MFLOPS ).

To save bits in the instruction word, RISC designs reduce the number of instructions to encode.

RISC design principles ensure that most instructions take the same number of cycles, helping avoid the need for most such continuation / restart logic.

Reduced instruction set computing, or RISC (), is a CPU design strategy based on the insight that simplified ( as opposed to complex ) instructions can provide higher performance if this simplicity enables much faster execution of each instruction.

Various suggestions have been made regarding a precise definition of RISC, but the general concept is that of a system that uses a small, highly-optimized set of instructions, rather than a more specialized set of instructions often found in other types of architectures.

In fact, over the years, RISC instruction sets have grown in size, and today many of them have a larger set of instructions than many CISC CPUs.

Some RISC processors such as the PowerPC have instruction sets as large as, say, the CISC IBM System / 370 ; and conversely, the DEC PDP-8 — clearly a CISC CPU because many of its instructions involve multiple memory accesses — has only 8 basic instructions, plus a few extended instructions.

In particular, RISC processors typically have separate instructions for I / O and data processing.

* Few data types in hardware, some CISCs have byte string instructions, or support complex numbers ; this is so far unlikely to be found on a RISC.

RISC designs are also more likely to feature a Harvard memory model, where the instruction stream and the data stream are conceptually separated ; this means that modifying the memory where code is held might not have any effect on the instructions executed by the processor ( because the CPU has a separate instruction and data cache ), at least until a special synchronization instruction is issued.

For any given level of general performance, a RISC chip will typically have far fewer transistors dedicated to the core logic which originally allowed designers to increase the size of the register set and increase internal parallelism.

Other features that are typically found in RISC architectures are:

In some architectures, notably most reduced instruction set computers ( RISC ), instructions are a fixed length, typically corresponding with that architecture's word size.

However, AMD64 still has fewer registers than many common RISC ISAs ( which typically have 32 – 64 registers ) or VLIW-like machines such as the IA-64 ( which has 128 registers ); note, however, that because of register renaming the number of physical registers is often much larger than the number of registers exposed by the instruction set.

This custom-made RISC processor was typically programmed to act like a graphics accelerator chip that would draw polygons to a frame buffer in the RAM that sat adjacent to it.

Historically, the effects of antisense RNA have often been confused with the effects of RNA interference ( RNAi ), a related process in which double-stranded RNA fragments called small interfering RNAs trigger catalytically mediated gene silencing, most typically by targeting the RNA-induced silencing complex ( RISC ) to bind to and degrade the mRNA.

Many GUI operations in RISC OS perform a related but slightly different function when right clicked.

The first highly ( or tightly ) pipelined x86 implementations, the 486 designs from Intel, AMD, Cyrix, and IBM, supported every instruction that their predecessors did, but achieved maximum efficiency only on a fairly simple x86 subset that was only a little more than a typical RISC instruction set ( i. e. without typical RISC load-store limitations ).

Later, it was noted that one of the most significant characteristics of RISC processors was that external memory was only accessible by a load or store instruction.

A reduced instruction set computer ( RISC ) simplifies the processor by only implementing instructions that are frequently used in programs ; unusual operations are implemented as subroutines, where the extra processor execution time is offset by their rare use.

For example, many implementations of the instruction pipeline only allow a single memory load or memory store per instruction, leading to a load-store architecture ( RISC ).

** RISC — arithmetic instructions use registers only, so explicit 2-operand load / store instructions are needed: load a, reg1 ; load b, reg2 ; add reg1 + reg2 -> reg3 ; store reg3, c ; unlike 2-operand or 1-operand, this leaves all three values a, b, and c in registers available for further reuse.

In keeping with the " load / store " model of the PowerPC's RISC design, the vector registers, like the scalar registers, can only be loaded from and stored to memory.

Unlike register-heavy load-store RISC CPUs, the transputer had only three data registers, which behaved as a stack.

For many RISC processors this is the only technique provided, but CISC architectures such as x86 support additional techniques.

RISC OS 3. 80 was limited, it only ran on ARM6 and ARM7 Risc PCs and not StrongARM based ones.

The first commercial RISC processors, the MIPS R2000 and R3000 and the earlier SPARC processors, did only trivial " not-taken " branch prediction.

The i960 RISC subset became popular for a time in the embedded processor market, but the high-end 960MC and the tagged-memory 960MX were marketed only for military applications.

Competition within and outside of Intel came not only from the i386 camp, but also from the i860 processor, yet another RISC processor design emerging within Intel at the time.

To avoid the performance issues that plagued the i432, the central i960 instruction-set architecture was a RISC design, only implemented in full in the i960MX, and the memory subsystem was made 33-bits wide — for a 32-bit word and a " tag " bit to indicate protected memory.

In the Berkeley RISC design, only eight registers out of a total of 64 are visible to the programs.

The processor instruction-set architecture was very efficient, predating RISC architectures with some similar features 20 years later, as there were only 68 instructions.

The register-reference instructions could execute multiple operations per instruction ( RISC processors generally execute only one operation per instruction )

Most RISC machines have only about five simple addressing modes, while CISC machines such as the DEC VAX supermini have over a dozen addressing modes, some of which are quite complicated.

The RISC idea was to include only those instructions that were really used, using those transistors to speed the system up instead.

The RISC designs, on the other hand, included only a single flavour of any particular instruction, the, for instance, would always use registers for all operands.