> An x86 CPU will start in 16-bit mode when it's powered-up. I don't think
> there's any way to bypass that via anything. The only thing that a
> 32/64-bit BIOS can do is to make the switch to 32/64-bit mode earlier
> in the bootup process.
>
> However, such a BIOS could theoretically allow for a x86 CPU to drop
> 16-bit support completely (if the OS can, thanks to such a BIOS, assume
> the CPU is already running in 32/64-bit mode when it boots.)

Let us hope this happens someday.

> There are some completely different CPU architectures where the
> distinction between 32-bit and 64-bit modes is much more transparent.
> The UltraSparc would be a perfect example. It's so transparent that it
> doesn't even need separate modes for 32-bit and 64-bit code: They both
> run just fine as-is, even if you mix them up.

The Motorola 68000 was a 16-bit CPU, but it had opcodes for 32-bit 
operations. It's just that they take twice as long. Then if you upgrade 
to an M68020 (?), it's 32-bit, and all your software goes 2x as fast. 
(Because ALMOST EVERYTHING uses mostly 32-bit arithmetic.)

Going to 64-bit took longer, because nobody really needs 64-bit integer 
arithmetic (and we already have 80-bit floating-point arithmetic), so 
the only real advantage is increased address space. [Which only matters 
once GB-sized RAM started to be manufactured.]

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