chapter7

Integer Arithmetic

1. 1. 1. Shift and Rotate instructions – how do they work? What flags can they set? Look at code and be

able to tell what happens after one of these instructions runs.

a. Example: mov al,01101011b

shr al,2

1. 2. 2. The MUL/IMUL instruction/The DIV/IDIV instruction

SHIFT INSTRUCTION

UNSIGNED integer

SHL

destination,count

SHL reg,imm8

SHL mem,imm8

SHL reg,CL

SHL mem,CL

SHR

SIGNED INTEGER

SAL = SHL

SAR != SHR

to save the signed extension

ROL instruction

uses the Carry flag as an intermediate point for shifted bits.

UNSINGED INTEGER

ROL

example:

ROR

SINGED INTEGER

RCL

RCR

SHLD/SHRD instruction

Multiplication and Division Instructions

UNSINGED INTEGER

MUL

MUL sets the Carry and Overflow flags if the upper half of the product is not equal to zero.

EXAMPLE 2

IMUL (SINGED VARIATION)

• Preserves the sign of the product by sign extending the highest bit of the lower half of the product into the upper bits of the product.
• the Carry and Overflow flags are set if the upper half of the product is not a sign extension of the lower half.

Unsigned Integer Division (DIV)

8 bit division

16 bit division

32 bit division

Signed Integer Division (IDIV)

beware of divide overflow when the result cannot hold value bigger than it range.

The dividend must be sign-extended before the division takes place

example 2:

Sign Extension Instructions (CBW, CWD, CDQ)