Bitwise Shift Instructions

Question 1

lsl form

Answer 1

lsl xd, xn, xm
xn: bit pattern to be shifted
xm: shift count

Question 2

What does lsl do?

Answer 2

• Zero is shifted into the rightmost bit (shifted out bits lost)
• Quick way to do multiplication by a power of 2 (by 2^n) where n is the shift count
• 32 bit & immediate forms exist

Question 3

Multiply 5x8 using lsl

Answer 3

mov x20, 5 //0000 0101
mov x21, 3 //2^3 = 8
lsl x19, x20, x21 //0010 1000

Question 4

What does lsr do?

Answer 4

• Quick way to divide by power of 2
• Any remainder is lost
• Does not work for negative signed integers (use ASR to preserve the sign bit)
• 32-bit & immediate forms exist

Question 5

15/4 using lsr

Answer 5

mov x20, 15 //0000 1111
mov x21, 2 //2^2 = 4
lsr x19, x20, x21 //0000 0011

Question 6

If the original multiplier in binary multiplication is negative, what extra step is needed?

Answer 6

Subtract the multiplicand from the high-order part of the result (from the product register)

Question 7

Do 2x-5 (binary multiplication, 4 bits)

Answer 7

11110110, check notes for procedure