NetNews Usenet Archive 1992 #30/NN_1992_30.iso/spool/comp/sys/amiga/programm/17539

Internet Message Format | 1992-12-21 | 3.0 KB

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From: [e-mail] (Wolfgang Meisl)
Subject: Re: CISC and RISC
Message-ID: <[e-mail]>
Sender: [e-mail] (News-Operator)
Organization: University of Passau, Germany
References: <[e-mail]> <[e-mail]> <[e-mail]> <[e-mail]> <[e-mail]>
Date: Fri, 18 Dec 1992 18:00:13 GMT
Lines: 64

In article <[e-mail]>, [e-mail] (Trent Bills) writes:
|> In article <[e-mail]>, [e-mail] (Andrew L. Hansford) writes:
|> |> >>>>> On 18 Dec 92 00:56:48 GMT, [e-mail] (Ian D Romanick) said:
|> |>
|> |> idr> Article-I.D.: pdxgate.6587
|> |>
|> |> idr> In article <[e-mail]> [e-mail]
|> |> idr> (Eric Dewald) writes:
|> |> >What are the advantages of CISC and RISC?
|> |>
|> |> idr> Well, since RISC is a more basic instruction set, it is MUCH
|> |> idr> easier for a compiler to generate good code for it.
|> |>
|> |> You are joking right?  The compiler and even assemblers are the
|> |> elements that got much more complicated.  Dependency avoidance, delay
|> |> slot scheduling...easy?
|> |>
|>
|> No.  He is not joking.  Have you ever written a compiler?  Compiler
|> problems such as register allocation are MUCH more difficult that
|> avoiding data hazzards and filling delay slots.

Then, why produce many compilers on SPARC processsors only intermediate
c-code? It may be simplier to write a compiler on RISC architecture,
but it is really hard to write an efficient one.


|> |> idr> It also makes it so
|> |> idr> that the instructions themselves are smaller and can be fetched
|> |> idr> and decoded quicker.
|> |>
|> |> The instructions are generally the same format for all instructions
|> |> (or the processor supports very few formats) that makes them easier to
|> |> decode.  THEY ARE NOT SMALLER.
|> |>
|> |> -- Andrew Hansford
|> |>    [e-mail]
|>
|> In many cases, RISC instructions are smaller than CISC instructions.
|> Consider an add instruction in a CISC chip.  The source operands could
|> both be some place in memory.  Depending on the addressing mode used,
|> both source operands could take up 4 bytes or more each.  RISC
|> instructions are usually a fixed size (32 or 64 bits) which is obviously
|> smaller than the above CISC example.
|>
|>  - Trent Bills

Indeed, there is not so much difference in total code size. But RISC
programs are bigger:

GNU Emacs  18.57.1 on different platforms:

DEC,  MIPS          983040 Bytes    (RISC)
Sun4, SPARC         937984 Bytes    (RISC)

Sun3, 68020         860160 Bytes    (CISC)
DEC,  Vax           720896 Bytes    (complex CISC)

- Wolfgang Meisl