la t0, trap_entry
csrw mtvec, t0
csrwi mstatus, 0
+
+ // make sure these registers exist by seeing if either S or U bits
+ // are set before attempting to zero them out.
+ csrr t1, misa
+ addi t2, x0, 1
+ slli t2, t2, 20 // U_EXTENSION
+ and t2, t1, t2
+ bne x0, t2, 1f
+ addi t2, x0, 1
+ slli t2, t2, 18 // S_EXTENSION
+ and t2, t1, t2
+ bne x0, t2, 1f
+ j 2f
+1:
csrwi mideleg, 0
csrwi medeleg, 0
+2:
csrwi mie, 0
# initialize global pointer