CSC263 Comp Org/Arch - Branch Instrs and Conditional Statements

Branch Instructions and Conditional Statements

MIPS does not have if statements, nor does it have for or while statements. To create conditional processing and loops it is necessary to build the control structure with branch statements. A branch statement changed the flow of control. Normally, the execution of a statement is following by the execution of the next statement in the code. A branch instruction causes execution to jump to a different statement. Some branches are conditional -- they only branch if a specified condition is met. Others are unconditional -- they always cause a branch to occur.

Branch Instructions

In the table below, Rs and Rt represent registers.

Name	Format	Meaning	Example
Branch If Equal	`beq Rs,Rt,label`	if Rs == Rt branch to label	`beq $t1,$t2,next`
Branch If Not Equal	`bne Rs,Rt,label`	if Rs != Rt branch to label	`bne $t1,$t2,else`
Branch If Greater Than	`bgt Rs,Rt,label`	if Rs > Rt branch to label	`bgt $t1,$t2,top`
Branch If Less Than	`blt Rs,Rt,label`	if Rs < Rt branch to label	`blt $t1,$t2,two`
Branch If Greater Than or Equal	`bge Rs,Rt,label`	if Rs >= Rt branch to label	`bge $t1,$t2,done`
Branch If Less Than or Equal	`ble Rs,Rt,label`	if Rs <= Rt branch to label	`ble $t1,$t2,three`
Branch If Equal To Zero	`beqz Rs,label`	if Rs == 0 branch to label	`beqz $t1,after`
Branch If Not Equal To Zero	`bnez Rs,label`	if Rs != 0 branch to label	`bnez $t1,second`
Branch If Greater Than Zero	`bgtz Rs,label`	if Rs > 0 branch to label	`bgtz $t1,high`
Branch If Less Than Zero	`bltz Rs,label`	if Rs < 0 branch to label	`bltz $t1,low`
Branch If Greater Than or Equal To Zero	`bgez Rs,label`	if Rs >= 0 branch to label	`bgez $t1,somehi`
Branch If Less Than or Equal To Zero	`blez Rs,label`	if Rs <= 0 branch to label	`blez $t1,somelo`
Branch	`b label`	branch to label	`b loop`

Conditional Statements

We use the branch statements shown above to construct conditional statements in MIPS. Here are some examples.

Pseudocode	MIPS
if $t1 > $t2 . . (1) . endif . . (2) .	bgt $t1,$t2,one b cont one: . . (1) . cont: . . (2) .
if $t1 > $t2 . . (1) . endif . . (2) .	ble $t1,$t2,cont . . (1) . cont: . . (2) .
if $t1 == $t2 . . (1) . else . . (2) . endif . . (3) .	beq $t1,$t2,equal . . (2) . b after equal: . . (1) . after: . . (3) .
if $t1 == $t2 . . (1) . else . . (2) . endif . . (3) .	bne $t1,$t2,noteq . . (1) . b after noteq: . . (2) . after: . . (3) .
if $t1 > $t2 \|\| $t3 <= $t4 . . (1) . else . . (2) . endif . . (3) .	bgt $t1,$t2,true ble $t3,$t4,true . . (2) . b endif true: . . (1) . endif: . . (3) .
if $t1 != $t2 . . (1) . else if $t3 > $t4 . . (2) . else . . (3) . endif . . (4) .	bne $t1,$t2,one bgt $t3,$t4,two . . (3) . b after one: . . (1) . b after two: . . (2) . after: . . (4) .