ColdFire/68K Microcontrollers and ProcessorsのトピックRe: ColdFire - CISC?

ColdFire - CISC?

admin — Thu, 29 Jan 2009 16:29:09 GMT

Hello All,

The Freescale ColdFire is a 68K architecture microprocessor manufactured for embedded systems development ..The Motorola 680x0/m68k/68k/68K is a family of 32-bit CISC microprocessor CPUchips . Is the ColdFire following a CISC architecture.?

Re: ColdFire - CISC?

J2MEJediMaster — Thu, 29 Jan 2009 21:32:49 GMT

ColdFire is derived from the 68K CISC processor, and is therefore CISC. Major difference between ColdFire and the 68K is that the 68K used a fixed-size instruction length, while ColdFire uses a variable-length instruction set to conserve memory.

---Tom

Re: ColdFire - CISC?

SimonMarsden_de — Thu, 29 Jan 2009 22:24:45 GMT

Sorry, but that's not right.

ColdFire and 68K use the same opcode encodings - and hence the same instruction widths - where the same instruction exists in both architectures.

Freescale describe the ColdFire architecture as a 'variable-length reduced instruction set (RISC) architecture'. Standard RISC implementations tend to use a fixed width of 2 or 4 bytes per instruction, (although both PowerPC and ARM slightly muddy that distinction) whereas ColdFire uses 2, 4 or 6 bytes depending on the instruction.

What Freescale actually did when designing ColdFire was to trawl through the 68K instruction set removing overly-complex instructions that were too difficult to implement efficiently in a redesigned RISC-like processor core.

One of the differences between ColdFire and more traditional RISC implementations is that ColdFire supports memory-based operands. You can add a register to a memory location in a single instruction, whereas RISC processors would normally need three (load from memory, add, store back to memory).

The word RISC has a debatable meaning nowadays anyway. The PowerPC is definitely considered as a RISC processor, but it has far more instructions that ColdFire.

Re: ColdFire - CISC?

J2MEJediMaster — Fri, 30 Jan 2009 01:12:31 GMT

I stand corrected. I should have looked into the references before relying on old memories.

Thanks for the info, and sorry to confuse things.

---Tom

Re: ColdFire - CISC?

admin — Fri, 30 Jan 2009 14:02:25 GMT

Hello All,

I am confused ,I got reply as CISC as well as RISC .How it can be concluded?

Re: ColdFire - CISC?

admin — Fri, 30 Jan 2009 14:30:27 GMT

I think it's the wrong question to ask. Asking "is it CISC or RISC?" is akin to asking "is it black or white?" It could be gray, you know.
CISC and RISC are processor design strategies. A particular design can draw from both of them, and most designs do. And there is no way to measure it in order to say, for example "it's 60% RISC and 40% CISC".