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It's quite probable that x86 processors are more complex than they need to be-- at least, the instruction set is more complicated than is necessary. CISC archi

by Goladus 17y ago

It's quite probable that x86 processors are more complex than they need to be-- at least, the instruction set is more complicated than is necessary. CISC architectures are meant to make life easier for assembly language programmers, letting you do things like load, multiply and store in a single command. In practice that hasn't been all that valuable, because programmers just use higher level languages like C when they need that much expressive power. From a compiler perspective, it's easier (or at worst about the same) to generate code for RISC than CISC, and RISC has much more flexibility with regards to automatic optimization, pipelining, etc.

rbanffy 17y ago
"quite probable" here is, perhaps, the biggest understatement ever presented on HN. Within every Core i7 is a Pentium M, within which is a Pentium, a 486, a 386, a 286, a 8086 and most of a 8085. The PC architecture is no better. I bet that, buried deep in the Centrino Duo notebook I use to work, there is a vestigial ISA bus that has to be primed before the text screen can be written to and the scan rate matched to an imaginary CGA monitor.
anamax 17y ago
> RISC has much more flexibility with regards to automatic optimization, pipelining, etc. Folks designing processors haven't believed that since before the Pentium was introduced. (I went to dinner with some of the MIPS principals right after the first Pentium tech talk. Their conclusion was that everyone would finally figure out that the RISC/CISC wars were over and they'd lost.) There are lots of things that go into designing a high performance processor. Instruction decoding and its consequences have little effect/cost compared to everything else.
- tyvkiuiyi 17y ago
  Tell that to the people at ARM who ship 10 RISC cpus for every Intel desktop chip out there.
  - anamax 17y ago
    If we're counting CPUs shipped, ARM is in the noise compared to 6502 and the like, let alone the 8051s. The claim was that RISC had certain advantages that would have significant performance or cost consequences. That hasn't happened. ARM wins where it wins for reasons that have nothing to do with RISC/CISC.
- Goladus 17y ago
  When pentium was introduced? Yeah, it looked like RISC had lost. But that was not because it wasn't a better design, but because Intel had a lot invested in the x86 and that's the processor that was used in the PC. Now that mobile devices are becoming as important or more important than PCs, the field has changed. Also, it's possible that RISC's parallel processing advantages will matter given the current trend toward multiple processors and multiple cores.
  - anamax 17y ago
    > RISC's parallel processing advantages will matter given the current trend toward multiple processors and multiple cores. What are these "parallel processing advantages"?
- Locke1689 17y ago
  RISC won. Intel continues to ship with CISC style artifacts, but that is only because Intel has almost never removed a feature from its microprocessors. However, the actual processor implementation is designed with CPU microcode, which is a RISC architecture. Even when you think you are doing something "CISCy" in an Intel proc, it's translated to RISC behind the scene.
  - anamax 17y ago
    > RISC won. Yup, MIPS and SUN are a thriving companies and Intel shut down. > However, the actual processor implementation is designed with CPU microcode, which is a RISC architecture. Even when you think you are doing something "CISCy" in an Intel proc, it's translated to RISC behind the scene. Do you really think that RISC machines don't have microcode? (They also have multi-cycle instructions and the like.) The claim was that RISC ISAs had inherent advantages that would cause CISC ISAs to be uncompetitive. That claiim was wrong.

by Goladus 17y ago

rbanffy 17y ago

"quite probable" here is, perhaps, the biggest understatement ever presented on HN. Within every Core i7 is a Pentium M, within which is a Pentium, a 486, a 386, a 286, a 8086 and most of a 8085. The PC architecture is no better. I bet that, buried deep in the Centrino Duo notebook I use to work, there is a vestigial ISA bus that has to be primed before the text screen can be written to and the scan rate matched to an imaginary CGA monitor.

anamax 17y ago

> RISC has much more flexibility with regards to automatic optimization, pipelining, etc. Folks designing processors haven't believed that since before the Pentium was introduced. (I went to dinner with some of the MIPS principals right after the first Pentium tech talk. Their conclusion was that everyone would finally figure out that the RISC/CISC wars were over and they'd lost.) There are lots of things that go into designing a high performance processor. Instruction decoding and its consequences have little effect/cost compared to everything else.

tyvkiuiyi 17y ago
Tell that to the people at ARM who ship 10 RISC cpus for every Intel desktop chip out there.
- anamax 17y ago
  If we're counting CPUs shipped, ARM is in the noise compared to 6502 and the like, let alone the 8051s. The claim was that RISC had certain advantages that would have significant performance or cost consequences. That hasn't happened. ARM wins where it wins for reasons that have nothing to do with RISC/CISC.
Goladus 17y ago
When pentium was introduced? Yeah, it looked like RISC had lost. But that was not because it wasn't a better design, but because Intel had a lot invested in the x86 and that's the processor that was used in the PC. Now that mobile devices are becoming as important or more important than PCs, the field has changed. Also, it's possible that RISC's parallel processing advantages will matter given the current trend toward multiple processors and multiple cores.
- anamax 17y ago
  > RISC's parallel processing advantages will matter given the current trend toward multiple processors and multiple cores. What are these "parallel processing advantages"?
Locke1689 17y ago
RISC won. Intel continues to ship with CISC style artifacts, but that is only because Intel has almost never removed a feature from its microprocessors. However, the actual processor implementation is designed with CPU microcode, which is a RISC architecture. Even when you think you are doing something "CISCy" in an Intel proc, it's translated to RISC behind the scene.
- anamax 17y ago
  > RISC won. Yup, MIPS and SUN are a thriving companies and Intel shut down. > However, the actual processor implementation is designed with CPU microcode, which is a RISC architecture. Even when you think you are doing something "CISCy" in an Intel proc, it's translated to RISC behind the scene. Do you really think that RISC machines don't have microcode? (They also have multi-cycle instructions and the like.) The claim was that RISC ISAs had inherent advantages that would cause CISC ISAs to be uncompetitive. That claiim was wrong.