Trouble sign extending a register in MIPS

Question

Currently I have

lh $t1, 0($t1)

for sign extending $t1 from 16 bit to 32 bits, but this doesn't work. Is there a simple way to sign extend registers in MIPS?

Answer 1

The way I understand your question, even though you had attempted to use lh you actually wanted to sign-extend the halfword value of $t1 rather than the halfword $t1 is pointing to.

You can accomplish this with two shifts:

sll $t1, $t1, 16
sra $t1, $t1, 16

If you started out with 0x8000, you'd get 0x80000000 after the first shift, and 0xffff8000 after the second.
If you started out with 0x7fff, you'd get 0x7fff0000 after the first shift, and 0x7fff after the second.