To put hexadecimal numbers into the assemble you can do something like 3Eh or 0x3E. While this works for hex numbers that start with a numerical digit like 3, it doesn't work for ones that start with a letter digit like E. For example, while 0xE3 works, E3h does NOT work in FASM. The reason is it says (in its popup error message box) it is an unrecognized label. It seems to think that just because the string started with an E that it's a non-numerical string, and it ignores the h at the end.

To fix this bug, you need to make FASM look at the end of the string for an h as one of the first things it does when analyzing a string that otherwise contains only valid hexadecimal digits so that it can automatically detect it as a hexadecimal number. This is important, because in ASM code, the standard designator for hex number is NOT "0x" at the beginning of the number (as it is in C or C++), but rather is "h" at the end of the number.

F3h:
        ; Implementation of Function 3h goes here    

Very early versions of fasm were universally case-sensitive and allowed numbers like E3h. From this followed that Ah was a valid hexadecimal number while ah was a register name. This was a very BAD idea and I quickly abandoned the experiment. I proceeded to implement the standard rule held by assemblers that a number must always start with a digit in 0-9 range, so you need to write 0Ah.

Again, it's important to stick with the industry standard for assemblers, that hex strings always use uppercase letters, and that the last symbol in a hex string is a lowercase h.

Again, it's important to stick with the industry standard for assemblers, that hex strings always use uppercase letters, and that the last symbol in a hex string is a lowercase h.

From this followed that Ah was a valid hexadecimal number while ah was a register name. This was a very BAD idea and I quickly abandoned the experiment.