why you need to size your calls to registers in assembly

[user@n0 poltut]$ nano test.s
[user@n0 poltut]$ nasm -f elf64 test.s -o test.o
[user@n0 poltut]$ objdump -d test.o

test.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <_start>:
   0:   b8 01 00 00 00          mov    $0x1,%eax
   5:   b0 01                   mov    $0x1,%al
[user@n0 poltut]$