Data Movement

Here are three instructions that move data between registers inside the processor:

Instruction	Operation
mov.s fd, fs	copy 32 bits from float register $fs to float register $fd
mtc1 rs, fd	move to coprocessor 1 copy 32 bits from general register $rs to float register $fd. No data conversion is done. Note: the source register is $rs and the destination register is $fd, the reverse of the usual order.
mfc1 rd, fs	move from coprocessor 1 copy 32 bits from float register $fs to general register $rd. No data conversion is done.

These instructions merely copy bit patterns between registers. The pattern is not altered. With the mfc1 instruction, the contents of a floating point register is copied "as is" to a general purpose register. So a complicated calculation with integers can use float registers for intermediate results. And a complicated calculation with floats can use general purpose resisters the same way.