Conditional Operations

Jump to the labeled instruction, if the given condition is == true==

JAL vs JALR

• JAL: Stores the address of the next instruction (PC + 4) in rd and jumps to PC + imm * 2.
  • Procedure call (jal ra, func)
• JALR: Stores the address of the next instruction (PC + 4) in rd and jumps to R[rs1] + imm.
  • Procedure return (jalr x0, 0(ra))

Why does only JALR not use *2?

JALR is an instruction in the I format. Applying a >> 1 shift to imm12 here would require additional hardware implementation, as I format instructions typically do not require this operation. Additionally, the jump distance can be longer compared to other branch instructions due to the use of a register in JALR.

Example

if (i == j) 
	f = g + h; 
else 
	f = g - h; 
	
// i in x22, j in x23 
// f in x19, g in x20, h in x21

while(A[i] == k) 
	i += 1; 

// i in x22, k in x24 
// address of A[] in x25

long max (long x, long y) { 
	if (x > y) 
		return x; 
	else 
		return y; 
} 
// x is in a0 
// y is in a1

long fact_do (long x) { 
	long result = 1; 
	do { 
		result *= x; 
		x = x - 1; 
	} while (x > 1); 
	return result; 
} 
// x is in a0

long fact_while (long x) { 
	long result = 1; 
	while (x > 1) { 
		result *= x; 
		x = x - 1; 
	} 
	return result; 
} 
// x is in a0