For loading and storing to memory, MIPS32® Central Processing Units (CPUs) implement only one addressing mode: Base Address Plus Displacement
For example, any load or store machine instruction can be written:
You can use any of the CPU GPRs for the destination and source. The offset is a signed, 16-bit number (-32768 to +32767). In this example, the program address used for the load is the sum of $2 and the offset.
This addressing mode is sufficient to pick out a particular member of a C structure, or of a particular array element. It is also enough to provide a reasonable-sized global area around the gp value for access to static and extern variables.
Other, more complex memory addressing modes are implemented by the assembler as a sequence of instructions.
For register loading/storing, MIPS32® ALU instructions implement Register-Direct Addressing whereby the register numbers are encoded directly in the instruction.
Many ALU operations are 3-operand, with the result register and operands shown in the same order you'd use to write the operation in C or any other algebraic language, so:
$1 = $2 - $3