MIPS Assembler Instruction parameters

why does such a instruction as

mul   $t1, $v0 , 4 

evaluates as expected. But

mul   $t1, 4 , $v0 

results in a syntax error!

I wonder why the first one works, because mul only works with registers per default, so I expect that only a solution like this will be workin

li   $t1, 4             # set $t1 = 4
mul   $t1,开发者_JAVA技巧 $v0 , $t1      # set $t1 = 4 * n

I'm using the SPIM simulator.

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Ok, I would suggest looking at the opcode output. Perhaps the multiply by 4 is being swapped for adds or perhaps sll immediate for speed purposes. The 4,reg version may not be caught via this 'optimization' and that is why it is throwing an error.

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This results in a syntax error simply because MIPS only supports the pseudo-instruction mul [rs] [rt] [imm], and not mul [rs] [imm] [rt]. I'm assuming it does this for consistency because there is no real instruction in MIPS that supports an immediate value in the middle of an instruction.