MULS instruction gives error A1858E in Keil

Question

    MOV R0, #20
    MOV R1, #1
    MULS R0, R0,R1

error: A1858E: Flag setting form of this instruction not available

When I use the mul command, there is no problem, but when I type MULS, it gives the above error.

My Board: STM32F4 Discovery

Answer 1

I solved the problem by swapping the registers.

MULS R0, R1,R0; MULS Rd, Rn, Rd