Question

Consider the following RISC-V assembly instructions:

slt x6, x0, x5

bne x0, x6, ELSE

j DONE

ELSE: addi x6, x6, 2

DONE:

a)For what value(s) of x5 is the addi instruction executed? b)Why?

Answer 1

First Statement:
slt x6, x0, x5
This instruction means if x0 < x5 then x6 = 1. When x0 >= x5 then x6 = 0

Second Statement:
bne x6, x0, ELSE
This instruction means if x6 is not equal to x0 then the code jump to method ELSE, otherwise the next statement will be executed.

Third Statement:
j DONE
This instruction means that the code jump to method DONE.

Fourth Statement:
ELSE:
addi x6, x6, 2
This instruction means that x6 = x6 + 2

Fifth Statement:
DONE:

This instruction indicates the completion of the code segment.

(a) and (b) From the first statement we can consider two scenarios where i) x6 = 0, and where ii) x6 = 1

i) From the first statement x6 = 0 or, x0 >= x5
From the second statement in order to execute addi x0 = 1(must condition) , therefore x5 < 1

ii) From the first statement x6 = 1 or, x0 < x5
From the second statement in order to execute addi x0 = 0(must condition) , therefore x5 < 0

Hope this helps.