MiniISA has four registers: $0, $1, $2 and $r, represented as 00, 01, 10 and 11
respectively. The special register $01 is actually the 16-bit representation of
registers $0 and $1, with $0 as the high bits.

_'s in the bit representation of operations are ignored bits.

bits            name            function
----------------------------------------
0000aa__        load $a         load into register a from address in $01
0001aa__        store $a        store from register a into address in $01
0010____        jump            jump to address in $01
0011oooo        jump o          jump o instructions (signed)
0100iiii        high $0 i       high bits of register 0 set to iiii
0101iiii        low $0 i        low bits of 0
0110iiii        high $1 i       high bits of 1
0111iiii        low $1 i        low bits of 1
1000aabb        sub $a $b       $r = $a - $b
1001oooo        branch o        if ($r < 0) jump o instructions (signed)
1010aabb        swap $a $b      $a and $b are swapped
1100aabb        srl             $r = $a >> $b (logical)
1101aabb        sra             $r = $a >> $b (arithmetic)
111_aabb        sll             $r = $a << $b

pseudoinstructions:
imm $a 0xAB
        high $a 0xA
        low $a 0xB
addr 0xABCD
        high $0 0xA
        low $0 0xB
        high $1 0xC
        low $1 0xD