MIPS, ADDIU out of range in PCSPIM

Question

I have the follow instruction, and I don't understand why the PCSPIM is giving me the following warning:

spim:(parser) immediate value (61440) out of rainge (-32768..32767)
on line 88 of file
   addiu $a, $zero, 0xF000
                    ^

I according to the ISA, I should have

Description: Adds a register and a sign-extended immediate value and
stores the result in a register

Operation: $t = $s + imm; advance_pc(4);

Syntax: addiu $t, $s, imm 

Encoding: 0010 01ss ssst tttt iiii iiii iiii iiii

If I convert addiu $a0, $zero, 0xF000 into machine code I would have

opcode | rs  | rt   | imm val
-------+-----+------+--------------------
0010 01|00 00|0 0100| 1111 0000 0000 0000

which should fit the instruction

Answer 1

I assume you meant something like this:

    addiu    $a0,$zero,0xF000

This is a quirk of spim because it is not treating this as a pseudo-op. But, given this, it is correct to flag this.

Regardless of whether you use addi or addiu they both have the same signed range of -32768 to 32767. The only difference is whether the instruction will generate an exception on integer overflow. That is, both instructions will do sign extend on the immediate value and not do sign extend for addi and zero extend for addiu.

Your value 0xF000 is 61440 [decimal] which is outside the allowable range for the instructions. In both cases, you'll get a sign extended value of 0xFFFFF000

spim saw the 0xF000 and recognized that you wanted to add 0x0000F000 and not 0xFFFFF000 but wouldn't automatically generate the code because it treated the addiu as a low level literal instruction.

On the other hand, mars does treat this as a pseudo-op and generates:

    lui    $at,0
    ori    $at,$at,0xF000
    addu   $a0,$zero,$at

It recognizes that you want an unsigned operation, so it has to generate instructions that produce a true unsigned addition using addu

However, if you rewrite your original as:

    addu    $a0,$zero,0xF000

Now, spim will treat this as a pseudo-op and generate:

    ori     $a0,$zero,-4096

And, mars is happy with that rewrite as well