Branch instructions

Instruction

Condition for branch (2C values)

beq   $rs, $rt, offset

#  R[s] == R[t]

bne   $rs, $rt, offset

#  R[s] != R[t]

bgtz  $rs, offset

#  R[s] > 0

bgez  $rs, offset

#  R[s] >= 0

bltz  $rs, offset

#  R[s] < 0

blez  $rs, offset

#  R[s] <= 0

Semantics:

Addr = PC + (IR15)14 :: IR15-0 :: 00

compute branch address

if ( condition )

test condition

PC <-- Addr

true: go to branch address

else

PC <-- PC + 4

false:  increment PC

Don't use just 16 bits for address:

to next instruction

Can assume 00, since all addresses are word-aligned (like "hidden 1")

But, we still don't want to be limited to an 18-bit addresss

How do most branches occur?

To an if or else block

Back to top of loop

Usually not very far from current address (compared to entire 32-bit address range)

Add offset to current PC to get new address (PC relative addressing)

Range

PC - 217 to PC + 217 - 4

Why -4?