Talk:Complex instruction set computer: Difference between revisions

Revision as of 20:09, 25 July 2011

Revision as of 18:55, 25 August 2011

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Not all CISCs are microcoded or have "complex" instructions (compared to a Z80, the MIPS's 32-bit divide or any RISC floating-point instructions are extremely complex) and it's not the number of instructions nor the complexity of the implementation or of the instructions themselves that distinguish a CISC from a RISC, but the addressing modes and memory access. CISC is a catch-all term meaning anything that's not a load-store (RISC) architecture. A PDP-10, a PDP-8, an Intel 386, an Intel 4004, a Motorola 68000, a System z mainframe, a Burroughs B5000, a VAX, a Zilog Z80000, and a 6502 all vary wildly in the number, sizes, and formats of instructions, the number, types, and sizes of registers, and the available data types. Some have hardware support for operations like scanning for a substring, arbitrary-precision BCD arithmetic, or computing an arctangent, while others have only 8-bit addition and subtraction. But they are all CISC because they have "load-operate" instructions that read from memory and perform a calculation at the same time. The PDP-8, having only 8 fixed-length instructions and no microcode at all, is a CISC because of ''how'' the instructions work (for example, fetching from memory and computing an addition at once), but PowerPC, which has over 230 instructions (more than some VAXes) and complex internals like register renaming and a reorder buffer is a RISC. This [http://www.cs.uiowa.edu/~jones/arch/cisc/ Minimal CISC] has 8 instructions, but is clearly a CISC because it combines memory access and computation in the same instructions. [[Special:Contributions/76.205.121.173|76.205.121.173]] ([[User talk:76.205.121.173|talk]]) 08:51, 30 May 2011 (UTC)

: I agree fully, but read the "definition" in the intro (last major edit by myself). I cannot see that it contradicts your points in any way really. However, I find your text well put, almost fit for inclusion in the article already as it stands here. [[Special:Contributions/83.255.33.7|83.255.33.7]] ([[User talk:83.255.33.7|talk]]) 00:04, 3 June 2011 (UTC)

== instructions or operations? ==

Seems to me that the article should generally be using the term "operation" or "opcode"

in place of "instruction".

Not all machines have one operation per instruction, and

I don't think it would make sense to label a load-store architecture as CISC

simply because each instruction can carry multiple opcodes.

: a computer in which a single operation (dictated by a single opcode) can execute several low-level operations (such as a load from memory, an arithmetic operation, and a memory store) and/or are capable of multi-step operations or addressing modes within single operations.

or, how about this more concise statement:

: a computer in which a single operation (dictated by a single opcode) can execute a load from memory, an operation on the loaded data, and a store of the result.

[[User:Encyclopedant|Encyclopedant]] ([[User talk:Encyclopedant|talk]]) 18:55, 25 August 2011 (UTC)