The x in front of a vector

[Altera_Forum]

for count<=x"0"; 

 what does the x in front of "0" means?

[Altera_Forum]

Why not consult a VHDL reference or text book? It means hexadecimal, in this case just 4 bits width.

[Altera_Forum]

Also notice an expression like 16#1F4# means hex based number, while x"0B2" is hex based 12 bit string.

[Altera_Forum]

you can also use o and b to qualify octal and binary. You can also use underscore characters to help with readbility: 

 count <= b"0000_1101_0001"; count <= o"840"; count <= x"FF_00_08_1A";