Intel Assembly

• Arithmetic Operations:
• Addition
• Subtraction
• Multiplication
• Division
• Comparison
• Negation
• Increment
• Decrement

 

• Logic Operations:
• AND
• OR
• XOR
• NOT
• shift
• rotate
• 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:
• Z (result zero?)
• C (carry out?)
• A (half carry out?)
• S (result positive?)
• P (result has even parity?)
• 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.

 

• AND: 0 AND anything is 0.
• Commonly used with a MASK to clear bits:

 

• OR: 1 OR anything is 1.
• Commonly used with a MASK to set bits:

Logic Operations

• XOR: Truth table: 0110.
• Commonly used with a MASK to complement bits:

 

• TEST: Operates like the AND but doesn't effect the destination.
• Sets the Z flag to the complement of the bit being tested:

 

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

 

• NOT (logical one's complement)
• NEG (arithmetic two's complement - sign of number inverted)

Logic Operations

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

• Double percision 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:
• Short jump : PC-relative using two bytes (+127/-128 bytes).
• (PC-relative: constant added to eip).

 

• Near jump :
• Within segment (max of +/- 2G).

 

• 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