I am having a problem with NASM running on Linux amd64.
Running nasm on a single instruction mov eax, 0x12345678 produces 6 bytes:
00000000 <.data>:
0: 66 b8 78 56 mov $0x5678,%ax
4: 34 12 xor $0x12,%al
However, using as on the same instruction in AT&T format mov $0x12345678, %eax produces the correct result with 5 bytes:
0000000000000000 <.text>:
0: b8 78 56 34 12 mov $0x12345678,%eax
What am I missing with NASM?
NASM thinks you're assembling 16-bit code, sees a 32-bit instruction, and slaps a size override prefix on it. Then the disassembler thinks you're disassembling 32-bit code, gets confused by the size override prefix, and assumes it's 2 instructions.
GAS thinks you're assembling 32-bit code, so it doesn't generate a size override prefix.
You can fix NASM by figuring out why it thinks you're assembling 16-bit code. It's the default for the "flat binary" output file format, and if that's the case you can fix it by putting a bits 32 directive somewhere before the 32-bit instruction/s.
Alternatively, maybe you are trying to assemble 16-bit code and GAS is wrong. In that case maybe you need a .code16 directive somewhere before the 16-bit code.
Note that the size override prefix works like a toggle - if the default size is 16 bits then the prefix makes the instruction 32 bit; and if the default size is 32 bits then the same prefix makes the instruction 16 bit.