CISC (Complex Instruction Set Computer)

What is CISC?
CISC, which stands for Complex Instruction Set Computer, is a philosophy for designing chips that are easy to program and which make efficient use of memory. Each instruction in a CISC instruction set might perform a series of operations inside the processor. This reduces the number of instructions required to implement a given program, and allows the programmer to learn a small but flexible set of instructions.

Since the earliest machines were programmed in assembly language and memory was slow and expensive, the CISC philosophy made sense, and was commonly implemented in such large computers as the PDP-11 and the DECsystem 10 and 20 machines.

Most common microprocessor designs --- including the Intel(R) 80x86 and Motorola 68K series --- also follow the CISC philosophy.

As we shall see, recent changes in software and hardware technology have forced a re-examination of CISC. But first, let's take a closer look at the decisions which led to CISC.

CISC philosophy 1: Use Microcode
The earliest processor designs used dedicated (hardwire) logic to decode and execute each instruction in the processor's instruction set. This worked well for simple designs with few registers, but made more complex architectures hard to build, as control path logic can be hard to implement. So, designers switched tactics --- they built some simple logic to control the data paths between the various elements of the processor, and used a simplified microcode instruction set to control the data path logic. This type of implementation is known as a microprogrammed implementation.

In a microprogrammed system, the main processor has some built-in memory (typically ROM) which contains groups of microcode instructions which correspond with each machine-language instruction. When a machine language instruction arrives at the central processor, the processor executes the corresponding series of microcode instructions.

Because instructions could be retrieved up to 10 times faster from a local ROM than from main memory, designers began to put as many instructions as possible into microcode. In fact, some processors could be ordered with custom microcode which would replace frequently used but slow routines in certain application. 

There are some real advantages to a microcoded implementation:

• since the microcode memory can be much faster than main memory, an instruction set can be implemented in microcode without losing much speed over a purely hard-wired implementation.
• new chips are easier to implement and require fewer transistors than implementing the same instruction set with dedicated logic, and...
• a microprogrammed design can be modified to handle entirely new instruction sets quickly.

Using microcoded instruction sets, the IBM 360 series was able to offer the same programming model across a range of different hardware configurations.

Some machines were optimized for scientific computing, while others were optimized for business computing. However, since they all shared the same instruction set, programs could be moved from machine to machine without re-compilation (but with a possible increase or decrease in performance depending on the underlying hardware.)

This kind of flexibility and power made microcoding the preferred way to build new computers for quite some time.

CISC philosophy 2: Build "rich" instruction sets
One of the consequences of using a microprogrammed design is that designers could build more functionality into each instruction. This not only cut down on the total number of instructions required to implement a program, and therefore made more efficient use of a slow main memory, but it also made the assembly-language programmer's life simpler.
Soon, designers were enhancing their instruction sets with instructions aimed specifically at the assembly language programmer. Such enhancements included string manipulation operations, special looping constructs, and special addressing modes for indexing through tables in memory.

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For example:
ABCD Add Decimal with Extend
ADDA Add Address
ADDX Add with Extend
ASL Arithmentic Shift Left
CAS Compare and Swap Operands
NBCD Negate Decimal with Extend
EORI Logical Exclusive OR Immediate
TAS Test Operand and Set

CISC philosophy 3: Build high-level instruction sets
Once designers started building programmer-friendly instruction sets, the logical next step was to build instruction sets which map directly from high-level languages. Not only does this simplify the compiler writer's task, but it also allows compilers to emit fewer instructions per line of source code.
Modern CISC microprocessors, such as the 68000, implement several such instructions, including routines for creating and removing stack frames with a single call.

For example:
DBcc Test Condition, Decrement and Branch
ROXL Rotate with Extend Left
RTR Return and Restore Codes
SBCD Subtract Decimal with Extend
SWAP Swap register Words
CMP2 Compare Register against Upper and Lower Bounds

The rise of CISC
CISC Design Decisions:

• use microcode
• build rich instruction sets
• build high-level instruction sets

Taken together, these three decisions led to the CISC philosophy which drove all computer designs until the late 1980s, and is still in major use today. (Note that "CISC" didn't enter the computer designer's vocabulary until the advent of RISC --- it was simply the way that everybody designed computers.)
The next lesson discusses the common characteristics that all CISC designs share, and how those characteristics affect the operation of a CISC machine.

Characteristics of a CISC design

Introduction
While the chips that emerged from the 1970s and 1980s followed their own unique design paths, most were bound by what we are calling the "CISC Design Decisions". These chips all have similar instruction sets, and similar hardware architectures.
In general terms, the instruction sets are designed for the convenience of the assembly-language programmer and the hardware designs are fairly complex.

Instruction sets
The design constraints that led to the development of CISC (small amounts of slow memory, and the fact that most early machines were programmed in assembly language) give CISC instruction sets some common characteristics:

• A 2-operand format, where instructions have a source and a destination. For example, the add instruction "add #5, D0" would add the number 5 to the contents of register D0 and place the result in register D0.
• Register to register, register to memory, and memory to register commands.
• Multiple addressing modes for memory, including specialized modes for indexing through arrays
• Variable length instructions where the length often varies according to the addressing mode

Instructions which require multiple clock cycles to execute. If an instruction requires additional information before it can run (for example, if the processor needs to read in two memory locations before operating on them), collecting the extra information will require extra clock cycles. As a result, some CISC instructions will take longer than others to execute. 

Hardware architectures
Most CISC hardware architectures have several characteristics in common:

• Complex instruction-decoding logic, driven by the need for a single instruction to support multiple addressing modes.
• A small number of general purpose registers. This is the direct result of having instructions which can operate directly on memory and the limited amount of chip space not dedicated to instruction decoding, execution, and microcode storage.
• Several special purpose registers. Many CISC designs set aside special registers for the stack pointer, interrupt handling, and so on. This can simplify the hardware design somewhat, at the expense of making the instruction set more complex.
• A "Condition code" register which is set as a side-effect of most instructions. This register reflects whether the result of the last operation is less than, equal to, or greater than zero, and records if certain error conditions occur.

The ideal CISC machine
CISC processors were designed to execute each instruction completely before beginning the next instruction. Even so, most processors break the execution of an instruction into several definite stages; as soon as one stage is finished, the processor passes the result to the next stage:

• An instruction is fetched from main memory.
• The instruction is decoded: the controlling code from the microprogram identifies the type of operation to be performed, where to find the data on which to perform the operation, and where to put the result. If necessary, the processor reads in additional information from memory.
• The instruction is executed. the controlling code from the microprogram determines the circuitry/hardware that will perform the operation.
• The results are written to memory.

In an ideal CISC machine, each complete instruction would require only one clock cycle (which means that each stage would complete in a fraction of a cycle.) In fact, this is the maximum possible speed for a machine that executes 1 instruction at a time.

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A realistic CISC machine
In reality, some instructions may require more than one clock per stage, as the animation shows. However, a CISC design can tolerate this slowdown since the idea behind CISC is to keep the total number of cycles small by having complicated things happen within each cycle.

CISC and the Classic Performance Equation
The usual equation for determining performance is the sum for all instructions of (the number of cycles per instruction * instruction cycle time) = execution time.
This allows you to speed up a processor in 3 different ways --- use fewer instructions for a given task, reduce the number of cycles for some instructions, or speed up the clock (decrease the cycle time.)

CISC tries to reduce the number of instructions for a program, and (as we will see) RISC tries to reduce the cycles per instruction.

CISC Pros and Cons

The advantages of CISC
At the time of their initial development, CISC machines used available technologies to optimize computer performance.

• Microprogramming is as easy as assembly language to implement, and much less expensive than hardwiring a control unit.
• The ease of microcoding new instructions allowed designers to make CISC machines upwardly compatible: a new computer could run the same programs as earlier computers because the new computer would contain a superset of the instructions of the earlier computers.
• As each instruction became more capable, fewer instructions could be used to implement a given task. This made more efficient use of the relatively slow main memory.
• Because microprogram instruction sets can be written to match the constructs of high-level languages, the compiler does not have to be as complicated.

The disadvantages of CISC
Still, designers soon realized that the CISC philosophy had its own problems, including:

• Earlier generations of a processor family generally were contained as a subset in every new version --- so instruction set & chip hardware become more complex with each generation of computers.
• So that as many instructions as possible could be stored in memory with the least possible wasted space, individual instructions could be of almost any length---this means that different instructions will take different amounts of clock time to execute, slowing down the overall performance of the machine.
• Many specialized instructions aren't used frequently enough to justify their existence --- approximately 20% of the available instructions are used in a typical program.
• CISC instructions typically set the condition codes as a side effect of the instruction. Not only does setting the condition codes take time, but programmers have to remember to examine the condition code bits before a subsequent instruction changes them.