Address and Contents of an Address

The contents of a word of memory may be any 32-bit pattern. The address of a word of memory is also a 32-bit pattern. Both types of data fit into a 32-bit register.

Sometimes you want the contents of word of memory to be loaded into a register.

For example, you may wish to load the value 0x00000002. This is done with the lw instruction. Look at the picture. The value you wish to load is located in memory at address 0x10000008.

lw    d,exp    #  Load register $d with the value at 
               #  address "exp".  
               #  "exp" is often a symbolic address.
               #  (pseudoinstruction) 

Sometimes you want the address of a word in memory to be loaded into a register.

For example, you may wish to load the address of the word in memory that holds the 0x00000002, This is done with the la instruction.

la   d,exp     #  load register $d with the address
               #  described by the expression "exp".
               #  "exp" is often a symbolic address.
               #  (pseudoinstruction) 

Usually the programmer does not know the exact addresses the linker and loader will use for things, so symbolic addresses are used with the lw and la instructions. If symbolic address val2 corresponded to the word that contains 0x00000002 in the above picture, then

lw   $s0,val2

would load $s0 with 0x00000002, and

la   $s1,val2

would load $s1 with 0x10000008.