npnum.asm

# npnum.asm — Nth prime (RV32I, RARS).
# In: unsigned N in a0/x10. Out: a0 = Nth prime, or -1 if N == 0.
# Trial division; runtime grows quickly with N (e.g. ~tens of thousands is slow).

.globl npnum

npnum:
	addi sp, sp, -4
	sw ra, 0(sp)

	beqz x10, valuezero

	li t1, 1
	li t2, 1
loop:
	ble t1, x10, label1
	j print
label1:
	li t3, 2
	li t4, 3
	beq t2, t3, true
	beq t2, t4, true
	rem t5, t2, t3
	beqz t5, false
	blt t2, t3, false
	div t3, t2, t3
	addi t3, t3, 1
loop_head:
	bge t4, t3, loop_end
	rem t5, t2, t4
	beqz t5, false
	addi t4, t4, 2
	j loop_head
loop_end:
	j true

true:
	mv a2, t2
	addi t1, t1, 1
	addi t2, t2, 1
	j loop
false:
	addi t2, t2, 1
	j loop
print:
	mv a0, a2
	j exit
valuezero:
	li a0, -1
exit:
	lw ra, 0(sp)
	addi sp, sp, 4
	ret