B18

.data
    indice: .word 0
    numero: .string "3"
    mil: .word 0,1000,2000,3000,4000,5000,6000,7000,8000,9000
    centena: .word 0,100,200,300,400,500,600,700,800,900
    decena: .word 0,10 ,20, 30, 40, 50, 60, 70, 80, 90
    unidad: .word 0,1 ,2 ,3, 4, 5, 6, 7, 8, 9
.text
#Lectura
    la x5 numero
    la x6 indice
    lw x6 0(x6)
loop:
    add x7 x5 x6
    lb x8 0(x7)
    beq x8 x0 carga
    addi x6 x6 1
    beq x0 x0 loop
carga:
#Unidad
    beq x6 x0 fin
    addi x6 x6 -1
    add x7 x5 x6
    lb x8 0(x7)
    addi x8 x8 -48
    slli x8 x8 2
    la x9 unidad
    add x15 x9 x8
    lb x9 0(x15)
    add x11 x11 x9
#Decena
    beq x6 x0 fin
    addi x6 x6 -1
    add x7 x5 x6
    lb x8 0(x7)
    addi x8 x8 -48
    slli x8 x8 2
    la x9 decena
    add x15 x9 x8
    lb x9 0(x15)
    add x11 x11 x9
#Centena
    beq x6 x0 fin
    addi x6 x6 -1
    add x7 x5 x6
    lb x8 0(x7)
    addi x8 x8 -48
    slli x8 x8 2
    la x9 centena
    add x15 x9 x8
    lh x9 0(x15)
    add x11 x11 x9
#Mil
    beq x6 x0 fin
    addi x6 x6 -1
    add x7 x5 x6
    lb x8 0(x7)
    addi x8 x8 -48
    slli x8 x8 2
    la x9 mil
    add x15 x9 x8
    lh x9 0(x15)
    add x11 x11 x9
fin:
    nop