具有真实指令助记符的伪指令

Question

str9 is a label for a byte allocated in a .data section, and the address of this byte is 0x1001_58e4 . Determine the machine code that would be generated as a result of this pseudo instruction:

lbu $t1, str9

I don't understand why this is a pseudo instruction at all and why 0x1001_58e4 can not just simply be loaded in to the $t1 GPR with the given instruction. Can someone please explain to me why this does not work in this case. What real instructions does the assembler do when dealing with this case? I know how to translate the instruction to machine code once I find the real instructions but just don't understand the need for real instructions as my professor has said it needs to be broken down into 2.

Answer 1

Have you tried to turn this into a single instruction? Have you succeeded? :)

Hint: the address itself is a 32 bit value ( 0x1001_58e4 ) and mips instructions are 32 bit, so if you wanted to encode this there would be no place left for the opcode and the destination register.

So, you need to do it in 2 instructions. First, you load the top 16 bits of the address into $t1 , then use a true lbu giving the low 16 bits as offset to the top 16:

lui $t1, 0x1001
lbu $t1, 0x58e4($t1)

What real instructions does the assembler do when dealing with this case?

You can always just assemble and disassemble:

Disassembly of section .text:

00000000 <.text>:
   0:   3c091001        lui     t1,0x1001
   4:   912958e4        lbu     t1,22756(t1)