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I was confused about this part while I study MIPS.
The textbook written by Professor John L. Hennessy say if we get some big constant to load, we should
lui $s0, upper(big)
ori $s0, $s0, lower(big)
But why don’t we just do
addi $s0, $zero, big
Since the registers are 32-bit, this is more strightforward, isn’t it?
The immediate argument passed to
addiis only 16 bits. To load a 32-bit immediate value that is outside the range of a 16-bit value you need to do it in two goes, as in the example from your text book.(Anticipating a further question, the reason there is no load immediate or add immediate instruction which takes a 32-bit immediate value is because the MIPS ISA uses fixed size 32-bit instructions, so there are always < 32 bits available for any instruction arguments – this is very common in RISC architectures.)