MIPS - Why doesn't it print the right integer?

I hope you can help me and sorry for bad english.

I have a test program which loads the address of a string "5B" in $a0 and the number 16 in $a1. and wants me to print the number "90" (hexadecimal to decimal). (after that it moves the content of register $v0 in $a0)

but before that I wanted to simply try and print the number "90", so the test will be positive. that should work:

li $v0 1
li $a0 90
syscall

but with that the number "901" is shown although I save 90. Do you have a idea of what I did wrong? I am bothered for days by this.