This subroutine will multiply 16-bit value at r23:r22 with 16-bit value at r21:r20. Result is 16-bit at r17:r16.
(r23:r22)
(r21:r20)
-----------X
(r17:r16)
r17:r16 = r23:r22 * r21:r20
Register usage: r0, r1
mul16by16:
mul r22, r20 ; al * bl
movw r17:r16, r1:r0
mul r23, r20 ; ah * bl
add r17, r0
mul r21, r22 ; bh * al
add r17, r0
ret
(r23:r22)
(r21:r20)
-----------X
(r17:r16)
r17:r16 = r23:r22 * r21:r20
Register usage: r0, r1
mul16by16:
mul r22, r20 ; al * bl
movw r17:r16, r1:r0
mul r23, r20 ; ah * bl
add r17, r0
mul r21, r22 ; bh * al
add r17, r0
ret
