Load/Store and Data Movement Instructions
These instructions are with the I-type format and include:
- the load instructions move a value (
w—word, 32 bits, h—halfword, 16 bits, b—byte, 8 bits) from memory into a register,
- the store instructions move a value from the register to memory, and
- the
move instruction moves a value between two registers.
| Loads |
Stores |
Data Movement |
Immediate |
lw rd, mem
lh rd, mem
lhu rd, mem
lb rd, mem
lbu rd, mem
|
sw rs, mem
sh rs, mem
sb rs, mem
|
move rd, rs
|
la rd, mem
li rd, imm
lui rd, imm
|
The effective address (EA) of the memory location (i.e., the
mem field) is specified by any of the three addressing modes, which will be detailed later:
- Relative such as
EA = relocatable address ± expression,
- Register indirect such as
EA = [$n], and
- Base such as
EA = [$n]+offset.
where
[ ] means “contents of.”
The
lh and
lb are signed loads; i.e., the sign is extended to fill the upper bits of the register.
lhu and
lbu are unsigned—the upper bits of the register are filled with zeros.
The
la,
li,
lui, and
move instructions are not really load instructions since they do not access memory.
- The
la computes the effective address in the instruction and places the address (not the contents of the memory location) in the register.
- The
li places the value of an expression that can be
computed at assembly time into the register, while lui places the value into the upper half of the register and fills the lower half with zeros.
- The
move instruction copies a value from one register to another.