Why is MARS mips 32 bit immediate possible?

Question

So, according to a few sources I've looked at, it's not possible to use a 32-bit immediate in 32-bit mips, because machine instructions are 32-bit and immediate values are stored within the machine instructions. As far as I know, the largest an immediate can be is 16-bits, to leave room for the rest of the instruction. But in MARS 4.5 mips, this instruction works fine (using default settings):

.text
ori $t1, $0, 0xffffffff #load 32-bit pattern into $t1

This assembles and runs fine, which baffles me as I think it shouldn't for aforementioned reasons (how can 32-bit machine code hold a 32-bit immediate?). My best guess is ori in MARS actually runs a pseudo-instruction when the immediate field is above 0xffff that lui's and ori's to load regularly, but I'm not sure. Can someone shed some light on this?

Answer 1

If you notice when you assemble that instruction in mars 4.5 you get three instructions

lui $1,0xFFFFFFFF
ori $1,$1,0x0000FFFF
or $9,$0,$1

which is still a bit misleading, but basically it fixes it for you.