6.3: Accessing Memory

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All memory access in MIPS in done through some form of a load or store opertoar. These operators include loading/storing a: byte (lb, sb); half word (lh, sh); word (lw, sw); or double word (ld, sd)²⁰. In this chapter only words of memory will be considered, so only the lw and sw will be documented.

lw operator, which has a number of pseudo operator formats, but only two format will be documented here. The first format is the only real format of this operator. In this format, an address is stored in R_s, and an offset from that address is stored in the Immediate value. The value of memory at [R_s + Immediate] is stored into R_t.

format:	`lw R_t, Immediate(R_s)`
meaning:	`R_t <- Memory[R_s + Immediate]`

The second format of the lw is a pseudo operator that allows the address of a label to be stored in R_s and then the real lw operator is called to load the value.

format:

lw R_s, Label

meaning:

R_t <- Memory[Label]

translation:

lui R_s, 0x10010000

lw R_t, offset(R_s) # offset is displacement of value

# in the data segement

sw operator, which has a number of pseudo operator formats, but only two format will be documented here. The first format is the only real format of this operator. In this format, an address is stored in R_s, and an offset from that address is stored in the Immediate value. The value of Rt is stored in memory memory at [R_s + Immediate].

format:	`lw R_t, Immediate(R_s)`
meaning:	`Memory [R_s + Immediate] <- R_t`

The second format of the sw is a pseudo operator that allows the address of a label to be stored in R_s and then the real lw operator is called to load the value.

format:

lw R_s, Label

meaning:

Memory[label] <- R_t

translation:

lui R_s, 0x10010000

sw R_t, offset(R_s) # offset is displacement of value

# in the data segement

²⁰ The la operator is not included here because it does not load a value from memory into a register. It is a pseudo operator which is short hand for taking the address of a label, and putting it in a register. It does not load memory values into a register.