Intel Assembly

Arithmetic Operations:

n Addition

n Subtraction

n Multiplication

n Division

n Comparison

n Negation

n Increment

n Decrement

Logic Operations:

n AND

n OR

n XOR

n NOT

n shift

n rotate

n compare (test)

Arithmetic Operations

Addition, Increment, Add-with-carry and Exchange-and-add:

Contents of the rightmost 8 bits of the FLAGS register can change (+ Overflow) for arithmetic and logic instructions.

Flags include:

n Z (result zero)

n C (carry out)

n A (half carry out)

n S (result positive)

n P (result has even parity)

n O (overflow occurred)

Arithmetic Operations

Subtraction, Decrement and Subtract-with-borrow:

Comparison:

Changes only the flag bits.

Often followed with a conditional branch:

Arithmetic Operations

Multiplication and Division:

imul/idiv: Signed integer multiplication/division.

mul/div: Unsigned.

al always holds the multiplicand (or ax or eax).

Result is placed in ax (or dx and ax or edx or eax).

C and O bits are cleared if most significant 8 bits of the 16-bit product are zero (result of an 8-bit multiplication is an 8-bit result).

Division by zero and overflow generate errors.

Overflow occurs when a small number divides a large dividend.

Logic Operations

Allow bits to be set, cleared and complemented.

Commonly used to control I/O devices.

Logic operations always clear the carry and overflow flags.

m AND: 0 AND anything is 0.

Commonly used with a MASK to clear bits:

m OR: 1 OR anything is 1.

Commonly used with a MASK to set bits:

Logic Operations

m XOR: Truth table: 0110.

Commonly used with a MASK to complement bits:

m TEST: Operates like the AND but doesn't effect the destination.

Sets the Z flag to the complement of the bit being tested:

m BT: Test the bit, BTC: Tests and complements...

m NOT (logical one's complement)

m NEG (arithmetic two's complement - sign of number inverted)

Logic Operations

Shift: Logical shifts insert 0, arithmetic right shifts insert sign bit.

Double precision shifts (80386 and up):

Rotate: Rotates bits from one end to the other or through the carry flag.

Commonly used to operate on numbers wider than 32-bits:

Bit/String Scan

Bit Scan Instruction (80386 and up):

Scan through an operand searching for a 1 bit.

Zero flag is set if a 1 bit is found, position of bit is saved in destination register.

String Scan Instructions:

scasb/w/d compares the al/ax/eax register with a byte block of memory and sets the flags.

Often used with repe and repne

cmpsb/w/d compares 2 sections of memory data.

Program Control Instructions

Conditional and Unconditional Jumps, Calls, Returns, Interrupts

Unconditional Jumps

n Short jump: PC-relative using two bytes (+127/-128 bytes).

(PC-relative: constant added to eip).

n Near jump:

Within segment (max of +/- 2G).

n Far jump:

Four bytes give the offset and two bytes give a new segment address.

The segment value refers to a descriptor in protected mode.

Flow-of-Control Instructions

Conditional Jumps:

Test flag bits S, Z, C, P and O.

For unsigned numbers:

For signed numbers

For either signed or unsigned:

Test cx instead of flags:

Flow-of-Control Instructions

Conditional Set instructions:

Set a byte to either 01H or 00H, depending on the outcome of condition under test.

LOOP Instruction:

Combination of decrement ecx and jnz conditional jump.

Decrement ecx

If ecx != 0, jump to label

else fall through.

Example