80286: Which is the fastest way to multiply by 10?

Question

To multiply a number by any any multiple of 2, I'll shift it those many times.

Is there any such technique to multiply a number by 10 in less cycles?

Answer 1

The 80286 did not have a barrel shifter, that was introduced with the 80386. According to the timing tables in the Microsoft Macro Assembler 5.0 documentation (1987), SHL reg, immed8 takes 5+n cycles, whereas SHL reg, 1 takes 2 cycles. ADD reg, reg takes 2 cycles, as does MOV reg, reg. IMUL reg16, immed takes 21 cycles. Therefore, the fastest way to multiply by ten would appear to be:

           ;       // cycles
shl ax, 1  ; *2    // 2
mov bx, ax ; *2    // 4
shl ax, 1  ; *4    // 6
shl ax, 1  ; *8    // 8
add ax, bx ; *10   // 10

or, alternatively:

           ;      // cycles
mov bx, ax ; *1   // 2
shl ax, 1  ; *2   // 4
shl ax, 1  ; *4   // 6
add ax, bx ; *5   // 8
shl ax, 1  ; *10  // 10

Ten cycles either way.