Instructions: Data Transfer

load and store architecture

MIPS: can access memory only to transfer data to or from registers

CISC: may allow, for example, add to memory location, store in register

load: copy the data from memory into a register.

store: copy the data from a register to memory.

lw  $rt, offset($rs)

Load word from memory location to register

sw  $rt, offset($rs)

Store word from register to memory location

Offset is a 16-bit 2C value (immediate); all are I-type

semantics of lw

Addr <-- R[s] + (IR15)16::IR15-0

R[t] <-- M4[ Addr ]

compute address

- add the contents of register s (base) to the sign-extended offset

- offset is immediate

- non-aligned address: hardware exception

get data

- copy 4 bytes located at memory address starting at Addr to register t.

- CPU fetches the four bytes based on the endianness of the machine

sw similar to lw but the 4 byte quantity is copied from the register to memory.

M4[ Addr ] <-- R[t]

stored in the endianness of the machine