MIPS assembly loading and storing memory address

Question

I am learning MIPS assembly and was wondering how I load and store a memory address in MIPS.

For example lets say I have this C code:

int i;

i = 0xFFFF0000;

And then how would I store an address? For example I want to store the value of i to address 0x2000A000.

Answer 1

LUI V0, 0xFFFF         << load 0xffff0000 into v0
LUI A0, 0x2000
ORI A0, A0, 0xA000     << load address to A0
SW V0, (A0)            << Stores V0 at address held in A0

Alternatively if your assembler supports macro opcodes (I think almost all do these days)

LUI V0, 0xFFFF
LI A0, 0x2000A0000
SW V0, (A0) 

Be careful that A0 is WORD/32bit aligned.

Answer 2

Let the compiler show you:

#define SOME_ADDRESS (*((volatile unsigned int *) 0x2000A000  ))
void fun ( void )
{
    SOME_ADDRESS = 0xFFFF0000;
}

compile and disassemble

Disassembly of section .text:

00000000 <fun>:
   0:   3c022000    lui $2,0x2000
   4:   3442a000    ori $2,$2,0xa000
   8:   3c03ffff    lui $3,0xffff
   c:   ac430000    sw  $3,0($2)
  10:   03e00008    jr  $31
  14:   00000000    nop

As pointed out in the comments there is a missed optimization, but that is not the point of this answer. 1) Learn asm 2) learn to optimize the asm...down the road. Using the compiler you will get to see some of those optimizations. But not in this case.

#define SOME_ADDRESS (*((volatile unsigned int *) 0x20004000  ))
void fun ( void )
{
    SOME_ADDRESS = 0xFFFF0000;
}

00000000 <fun>:
   0:   3c022000    lui $2,0x2000
   4:   3c03ffff    lui $3,0xffff
   8:   ac434000    sw  $3,16384($2)
   c:   03e00008    jr  $31
  10:   00000000    nop

Yes it was built for mips32 to get this -march=mips1 and -march=mips32r6 produce the same code. as commented.