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9. 64-Bit Computing and x86-64 > The 64-Bit Alternative: x86-64

The 64-Bit Alternative: x86-64

When AMD set out alter the x86 ISA in order to bring it into the world of 64-bit computing, they took the opportunity to do more than just widen the GPRs. x86-64 makes a number of improvements to x86, and this section looks at some of them.

Extended Registers

I don’t want to get into a historical discussion of the evolution of what eventually became the modern x86 ISA, as Intel’s hardware went from 4-bit to 8-bit to 16-bit to 32-bit. You can find such discussions elsewhere, if you’re interested. I’ll only point out that what we now consider to be the “x86 ISA” was first introduced in 1978 with the release of the 8086. The 8086 had four 16-bit integer registers and four 16-bit registers that were intended to hold memory addresses but also could be used as integer registers. (The four integer registers, though, could not be used to store memory addresses in 16-bit addressing mode.) This gave the 8086 a total of eight integer registers, four of which could also be used to store addresses.


  

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