Cisc...Again

CISC…AGAIN!!! (and a bit o’ RISC, too) by Javier Arboleda

 Agenda 

Brief History

 An example of “Closing the semantic semantic gap”

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Drawbacks of CISC

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Interesting RISC vs. CISC stuff 

CISC’s Roots

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Back in the 70’s memory & software = $$$ Hardware… not so much $

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Move burden of code from software & memory to hardware

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“Closing the semantic gap”

Department of Redundancy Department 

So, to reiterate, CISC architecture type was created to : Reduce software developing costs by making compilers easier to write and code easier to debug  Reduce calls to memory, thus making it possible to do more with less memory which at the time was the most expensive part of a computer system 

Exempli Gratia = E.G. != I.E. 

Let’s pretend that… 

H is the name for a high-level language. This language has a function Cube() which will cube an integer 

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H compiler translates code into assembly language for the A-1 computer, which only has two instructions

 A-1 Computer Instructions 

Move [destination register, integer or source register] This instruction takes a value, either an integer or the contents of  another register, and places it in the destination register. So Move [D, 5] would place the number 5 in register D. Move [D, E] would take whatever number is stored in E and place it in D

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Mult [destination register, integer or source register] This instruction takes the contents of the destination register and multiplies it by either an integer or the contents of the source register, and places the result in the destination register. So Mult [D, 70] would multiply the contents of D by 70 and place the results in D. Mult [D, E] would multiply the contents of D by the contents of E, and place the result in D

Pre-CISC example Statements in H

1.A = 20; 2.B = Cube(A);

Statements in Assembly for A-1 computer  1.Move [A, 20] 2.Mult [A, A] 3.Mult [A, A] 4.Move [B, A]

Here it takes four statements in the A-1 assembly to do the work of  two statements in H since the A-1 computer has no instruction for  taking the Cube of a number 

Three possible problems 1.

2.

3.

If the program H uses Cube() many times, then assembly code will be relatively larger, which is bad for the A-1 computer that has very little memory With computer speeds being so slow, compiler  takes a long time to translate all of the Cube() statements to multiple Mult[] instructions Programming in assembly language would be time consuming, tedious, and difficult to debug

How does CISC solve this problem? 

Include a Cube instruction in the instruction set of the next generation of computers, A-2 Cube[destination register, source register] This instruction takes the contents of the source register and cubes it. It then places the result in the destination register. So Cube [D, E] takes whatever value is in E, cubes it, and places the result in D

Post-CISC example Statements in H

1.A = 20; 2.B = Cube(A);  One-to-one

Statements in Assembly for A-2 computer  1.Move [A, 20] 2.Cube[B, A]

correspondence between H and assembly

code  “Semantic gap” is closed  Complexity has moved from the software level to the hardware level

Result 

Compiler does less work to translate

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Less memory needed

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Easier to debug

Drawbacks When using an instructions set with so many instructions, the decode function of the computer system must be able to recognize a wide variety of functions. As a result, the decode logic, while time critical for purposes of  performance, grows to be quite complex  Not every one of the complex instructions are used for  each software program, and thus much of the decode logic functionality is seldom used during operation   Another problem arises from the fact that the complex instructions are often of different lengths, i.e., each instruction could consist of any number of operands and takes any number of cycles to execute 

Here comes the 80’s and a….

Birth of RISC & CISC??? 

RISC = Reduced instruction set computer 

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Previous to RISC, CISC was not called

“CISC,” it was just the “really good way to do things computer” or RGWTDTC ( just kidding)  The term “complex instruction set computer” was forced upon anything else that was not RISC

Bloody hell, it’s obvious that RISC and pointy hats are the future, mate!!!

Pipelining… BRILLIANT!!

RISC = GOOD!!!! CISC = BAD!!!

RISC > CISC

 Are you ready to rumble?

CISC

RISC VS

Warning, Geeky Conspiracy Theory Ahead

CISC + Pipelining = i486 

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CISC chips started using pipelining with the Intel i486 processor. Now what, RISC?!? Several years later Apple starts using the G3 (third generation PowerPC processors) This was a RISC chip which actually had more

instructions than Intel’s Pentium II CISC processor!

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Hold up, Isn’t RISC suppose to have a reduced number of instructions? Isn’t that why RISC is so much better than CISC?

RISC’s people: “What we meant…” Proponents of RISC started to claim that the actual number of instructions was never  intended to be reduced; rather, only the individual instructions themselves were to be reduced in cycle time and complexity   All the hoopla was speculated to have been 

generated from Apple’s camp and users who must insist that the processors in their Macs are pure RISC chips, since

RISC = GOOD!!!! CISC = BAD!!!

RISC > CISC

10 Years The argument about RISC being so much better than CISC starts to quiet down, and why? What announcement did Apple make in 2005?

No more PowerPC for   Apple,

Now it’s all about Intel! Brilliant!!

Hilarious!!

In Conclusion… 

CISC chips dominate the personal computer market

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Line between RISC and CISC continues to blur 

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The RISC > CISC || CISC > RISC debate is unheard of 

Thank you!

Sources   

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http://arstechnica.com/cpu/4q99/risc-cisc/rvc-1.html http://www.amigau.com/aig/riscisc.html http://www.pcguide.com/ref/cpu/arch/int/instComplexityc.html http://en.wikipedia.org/wiki/Complex_instruction_set_co mputer  http://en.wikipedia.org/wiki/RISC http://en.wikipedia.org/wiki/X86 http://en.wikipedia.org/wiki/PowerPC