Moving from DX:AX register to single 32 bit register

Question

I\'m having a problem adding to a product of a 16 bit multiplication. I want to multiply a year such as 2015, by 365 to do so I

mov dx, 0    ; to clear the regis         


        

Answer 1

The usual method is to use a 32 bit multiply to start with. It's especially easy if your factor is a constant:

movzx ecx, cx      ; zero extend to 32 bits
                   ; you can omit if it's already 32 bits
                   ; use movsx for signed
imul ecx, ecx, 365 ; 32 bit multiply, ecx = ecx * 365

You can of course also combine 16 bit registers, but that's not recommended. Here it is anyway:

shl edx, 16 ; move top 16 bits into place
mov dx, ax  ; move bottom 16 bits into place

(There are other possibilities too, obviously.)