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A comment on Intel x86 documentation has more pages than the 6502 has transistors. Parent comment

Re:Intel x86 documentation has more pages than the 6502 has ... by pcwalton on Wednesday December 07, 2016 @04:37PM (#13126470)
x86-64 has tons of cruft: complex instruction encoding (mod R/M and SIB bytes), bloated instruction encoding thanks to REX prefixes, real mode, virtual 8086 mode, odd SIMD limitations, pointless instructions like XLAT, binary coded decimal, a non-orthogonal instruction set with some 3-register instructions (LEA, IMUL, AVX2) mixed with a bunch of other 2-register instructions, individually addressable low and high bytes of certain registers but not others…
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Re:Intel x86 documentation has more pages than the 6502 has ... by h4nkoslo on Wednesday December 07, 2016 @06:04PM (#13127053)
Most of that cruft consumes minimal die area and results in absolutely minimal slowdown compared to an "optimized" minimal architecture. Fun exercise: do a histogram of instructions in some large program's binary. Not a ton of weirdness in practice.
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Re:Intel x86 documentation has more pages than the 6502 has ... by pcwalton on Wednesday December 07, 2016 @06:58PM (#13127345)
I'm not disputing that. I'm talking about complexity of the programming model (number of pages in the manual), not performance.
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Re:Intel x86 documentation has more pages than the 6502 has ... by flamedoge on Wednesday December 07, 2016 @11:38PM (#13128435)
What was the main reason to add REX prefix for 64bit? Why not create longer register bits to hold 16 registers? Was it for easier binary to binary transformation?
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Re:Intel x86 documentation has more pages than the 6502 has ... by pcwalton on Thursday December 08, 2016 @12:33AM (#13128622)
Because AMD was deathly afraid of AMD64 going the way of Itanium. So they went out of their way to make their architecture as similar to 32-bit x86 as possible, right down to reusing the encoding.

They also probably figured that they could reuse decoding logic.

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