06.5 Branches

In RISC-V, we can compare two registers and “branch” to a specific line of code depending on the result of the comparison.

We have:

beq  x0, x1, branch_label # branch if equal
bne  x0, x1, branch_label # branch if not equal
bge  x0, x1, branch_label # branch if greater equal
bgeu x0, x1, branch_label # branch if greater equal unsigned
blt  x0, x1, branch_label # branch if less than
bltu x0, x1, branch_label # branch if less than unsigned

We can use these to do if-else conditions, or loops.

Branches can use labels or immediates (which are relative to where the PC currently is)

It is structured as

imm	rs2	rs1	funct3	imm	opcode
7	5	5	3	5	7
7 bit immediate	second operand	first operand	3 bit function code	5 bit immediate	7bit operation code OP

Unconditional Jumps

We have jump instructions

j imm # jump to the immediate

Because there are no conditions, there is a lot of space for immediates in the jump instruction

imm	funct5	opcode
20	5	7
20 bit immediate	5 bit function code	7 bit operation code OP
06.6 Assembler directives