MIPS(R-Format/ I-Format/ J-Format) by CHEOK LI LI B031310127

e. Instruction Type : Store

Example : sw rt , imm ( rs )

Type : I

Instruction Coding :

C.2. Immediate Addressing

Definition : A numeric value embedded in the instruction is the control operand .

·         the operand is a constant within the encoded instruction

·         instructions will be executed faster in comparison with other addressing modes

reason : does not involves with memory access

Ø  However , the size of operand is limited to 16 bits .

·         Example :

Consider the following instruction :

addi $t2 , $t2 , 1

where ; $t2 = rd , $t2 = r1 , 1 = immediate value

Thus ; $t2 = $t2 + $1

The figure below is the implementation of Immediate Addressing :

C.3. PC-Relative Addressing

Definition : A data or instruction memory location is specified as an offset relative to the incremented PC .

·         usually used in conditional branches

Ø  PC ( Program Counter ) – stores the address of the next instruction to be fetched .

·         the offset value can be an immediate value / an interpreted label value

·         I-Type instruction

·         effective address = PC + offsets value

Ø  The effective address determines the branch target .

·         implements position-independent codes

Ø  Only a small offset is adequate for shorter loops .

·         Example :

Consider the following instruction :

beqz $t0 , strEnd

where ; $t0 = rs , 100 = offset

Thus ; if ( $t1 == 0 ) goto PC + 4 + ( 4 * 2 )

Ø  beqz ( conditional instruction ) – branches to label in the code if the content of $t0 = zero .

Ø  if the current address = 0x4000000C , then the effective address = 40000018 .

Ø  the table below show the details of the example and the following figure show the implementation of PC-Relative Addressing :

C.4. Base Addressing / Indirect Addressing

Definition : A data / instruction memory location is specified as a signed offset from a register .

·         where a register act as a pointer to an operand located at the memory location whose address is in the register .

·         operand address = offset value + base value ( rs )

Ø  However , the size of operand is limited to 16 bits because each MIPS instruction fits into a word .

·         offset value

Ø  signed number ( two’s complement format )

Ø  can be a negative value

·         Consider the following instruction :

lw $rt , offset ( $rs ) 

where $rs and $rt are any two registers .

Example :

lw $t1 , 4 ( $t2 )

Thus ; $t1 = Memory [ $t1 + 4 ]