memory instructions

Complete

Summary

Load word/byte

  • I-type
mips
lw dest, n(addr) # dest <- *(addr + n)
lb dest, n(addr) # dest <- *(addr + n)

lw will error if (addr + n) % 4 != 0, due to word alignment

Store word/byte

  • I-type
mips
sw src, n(addr) # *(addr + n) <- src
sb src, n(addr) # *(addr + n) <- src

Unaligned load/store word

  • pseudo-instruction
mips
ulw dest, n(addr) # dest <- *(addr + n)
usw src, n(addr)  # *(addr + n) <- src

Concept

Memory

  • viewed as a large single-dimension array
  • byte addressing -> each address indexes one byte in memory

Word alignment

  • word -> usually 2ⁿ bytes, in MIPS a word is 4 bytes(32 bits)
  • aligned words would be at addresses that are multiples of 2ⁿ

each instruction occupies 32 bits

Offset

  • use the n(addr) to apply an offset to an address, the resulting address is n + addr

Application

Swapping two elements in an array

c
void swap(int v[], int k) {
	int tmp;
	tmp = v[k];
	v[k] = v[k+1];
	v[k+1] = tmp;
}
mips
# address v at $a0, int k at $a1
sll $v0, $a1, 2 # k * 4 since an int takes up 4 bytes
add $v0, $a0, $v0 # add to base address of v
lw $t0, 0($v0) # load the int at the base address
lw $s0, 4($v0) # load the int at the base address + 4
sw $s0, 0($v0) # store the int at the base address
sw $t0, 4($v0) # store the int at the base address + 4