Author: Tom Kerrigan
Date: 17:25:18 07/03/03
Go up one level in this thread
On July 03, 2003 at 19:18:36, Robert Hyatt wrote: >On July 03, 2003 at 15:54:34, Tom Kerrigan wrote: > >>On July 03, 2003 at 09:35:41, Robert Hyatt wrote: >> >>>>>>What x86 problems? The x86 has variable length instructions anyway, so you can't >>>>>>say that n-bit-long instructions limit it somehow. >>>>> >>>>>Sure I can. It first limits the number of registers to 3 bits. I'd bet >>>>>that if Intel could "start over" the ISA would be greatly different with a >>>>>target of 32 bits from the beginning. Intel grew up from 8 bits. Other >>>>>vendors started at 32 and their instruction sets are _far_ better. Motorolla >>>>>is an example with the 680x0. The sparc has a nice instruction set, it's just >>>>>a dog for performance. >>>> >>>>I don't know what in the world you're talking about. Grew up from 8 bits? Target >>>>32 bits? Started at 32 bits? Do you know what "variable length instructions" >>>>means? x86/680x0 didn't start at, target, or grow up from ANY length. >>> >>>Sorry, but the X86 _started_ as an 8-bit cpu capable of doing 16 bit math. >>>It grew to 16 bits in the 80286 and 32 bits in the 80386. But it was >>>originally an 8 bit ISA. >> >>Wrong, the 8086 (the first x86) is a 16-bit processor. The 8088 used in the >>original PC was a variant of the 8086 with an 8-bit data bus, maybe that's why >>you're confused. > >The _first_ was the 8080 and it was _not_ a 16 bit cpu. The 8086 was the Uhhhhhhhhhhh, Bob? Does it make a lot of sense to call the 8080 an "x86"? Hint: there's a reason why the 8086, 80186, 80286, 80386, and 80486 are called "x86"s. Can you think of what that reason is? >second processor and it was compatible with the 8080. Each 80X86 processor No, the 8086 ISA is similar to but not compatible with the 8080 ISA. And the 8086 was not "the second processor." It was Intel's 5th processor. 4004, 8008, 8080, 8085, 8086. >ever since has maintained that compatibility. But it _started_ at 8 bits. Well, if you mean Intel started at 8 bits, no. The 4004 was a 4 bit processor. >The 8088 was a kludge, but it wasn't where I was talking about either. I was Depends on what you mean by a kludge. If you think the 8086 is a kludge, then yes, the 8088 is also a kludge, because they're the same except for the external bus width. But realize that having an 8 bit bus instead of a 16 bit bus resulted in SIGNIFICANT motherboard cost savings at the time. Saving a lot of money by sacrificing some performance is not a kludge, it's a trade-off. >talking about the _beginning_ of the product line which went back to the >original 8080 which the current processors will still execute assembly code >from. Right, the assembly is the same but the chips are not binary compatible. When people say backward compatible they usually mean binary compatibility. >>But now you're confusing instruction length with datapath width. Check the top >>of this post. We were talking about instruction width. Somehow you changed it to >>datapath width. > >Nope. 8080 was 8 bit everything, but with 16 bit registers. 8086 was >16 bits. 80186/286 were better. etc... but all are backwardly compatible >in terms of ISA. No, the 8080 was an 8 bit processor with instructions that were 8 to 32 bits. It had 8 bit general purpose registers. http://wombat.doc.ic.ac.uk/foldoc/foldoc.cgi?Intel+8086 -Tom
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