Jump instructions

Instruction

Semantics

j     offset

#

PC <-- PC31-28 :: IR25-0 :: 00

jump to target address

jr    $rs

#

PC <-- R[s]

jump to address in $rs

jal   offset

#

R[31] = PC + 4

save return address in $ra

#

PC <-- PC31-28 :: IR25-0 :: 00

jump to target address

jalr  $rs

#

R[31] = PC + 4

save return address in $ra

#

PC <-- R[s]

jump to address in $rs

Formats

What's different about j, jal? No regs specified.

offset: usually 16-bit 2C immediate, except j and jal 26-bit UB (J-type)

Note that type refers to FORMAT, not function, so branch instrs are not J-type

What type are jr and jalr?

Since registers have to be specified, jr and jalr are R-type

Addressing: pseudo-direct

doesn't specify 32-bit address directly

allows you to access how much of all possible word-aligned addresses?

Jumping to arbitrary word-aligned addresses

j and jal still don't allow you to access all possible word-aligned addresses

jr and jalr refer to a register, which specifies the entire 32 bit instruction