My x86-64 Architectural Programmers Manuals are packed away somewhere so I'll
have to wing it here only stating what I know to be factual.
1. The original eight 32 bit x86 GPRs were increased in width to 64 bits
2. Eight additional 64 bit GPRs were added for a total of 16 GPRs
3. In 64 bit long mode, all registers are available
4. In 32 bit mode, only the first eight are available
5. In 32 bit mode, only the first 4 bytes of each register are available
New 64 bit mode instructions were created in essence duplicating most/all of the
previous 32 bit x86 instructions. I'm unsure of the actual instruction encoding
so I don't know the bit length of each/any/all the 64 bit instructions. I do
know that all of them are a few bits larger than the 32 bit cousins and some
many bits larger. I was being simplistic in my previous post for brevity. The
instruction word bit/byte length isn't doubled across the board, but it is
always larger on average, so 64 bit code is always going to have a larger
footprint, as you state, but it won't be twice the size in most cases. Wild ass
guess, I'd say it's probably 30-50% larger on average for user space code and
slightly greater for kernel code since it deals so heavily with programming the
hardware and managing memory.