Intel Assembly

• Data Movement Instruction:
• mov (covered already)
• push, pop
• lea (mov and offset)
• lds, les, lfs, lgs, lss
• movs, lods, stos
• ins, outs
• xchg, xlat
• lahf, sahf (not covered)
• in, out
• movsx, movzx
• bswap
• cmov

Stack Instructions

• There are six forms of the push and pop instructions.
• Register, memory (memory-to-memory copy), immediate, segment register, flags, and all registers

 

• push :
• The source of the data may be:
• Any 16- or 32-bit register, immediate data, any segment register, any word or doubleword of memory data

 

• pushad pushes eax , ecx , edx , ebx , esp , ebp , edi and esi where the value of esp saved on the stack is its value before the pushad .

 

• pop :
• The source of the data may be:
• Any 16- or 32-bit register, any segment register (except for cs ), any word or doubleword of memory data.

Stack Instructions

• push :

Address Loading Instructions

• Load-Effective Address.
• lea :
• Loads any 32-bit register with the address of the data, as determined by the instruction addressing mode.
• lds and les :
• Load a 32-bit offset address and then ds or es from a 48-bit memory location.
• lfs , lgs and lss (80386 and up):
• Load any 32-bit offset address and then fs , gs or ss from a 48-bit memory location.

• NOTE: lea calculates the ADDRESS given by the right arg and stores it in the left arg !

Address Loading Instructions

• Load-Effective Address.
• lea versus mov :

• 1 and 3 are equivalent.

 

• So what are the differences?
• 3 is faster than 1 and is preferred.
• However, mov only works with single args and cannot be used with LIST [ edi ].
• lea can take any address, e.g., lea esi , [ ebx + edi ].

String Operations

• movs , lods , stos , ins , outs
• Allow data transfers of a byte, a word or a double word, or if repeated, a block of each of these.

 

• The D flag-bit (direction), esi and edi are implicitly used.
• D = 0 : Autoincrement edi and esi .
• Use cld instruction to clear this flag.
• D = 1 : Autodecrement edi and esi .
• Use std instruction to set it.

 

• edi :
• Accesses data in the extra segment. Can NOT override.
• esi :
• Accesses data in the data segment. Can be overridden with segment override prefix.

String Operations

• lods :
• Loads al , ax or eax with data stored at the data segment (or extra segment) + offset given by esi .
• esi is incremented or decremented afterwards:

 

• stosb :
• Stores al , ax or eax to the extra segment (es) + offset given by edi . es cannot be overridden.
• edi is incremented or decremented afterwards:

String Operations

• An example

String Operations

• rep prefix:
• Executes the instruction ecx times.

• NOTE: rep does not make sense with the lodsb instruction.

 

• movs :
• Moves a byte, word or doubleword from data segment and offset esi to extra segment and offset edi .
• Increments/decrements both edi and esi :

String Operations

• ins/outs :
• Transfers a byte, word or doubleword of data from/to an I/O device into/out of the extra/data segment + offset edi / esi , respectively.
• The I/O address is stored in the edx register.

 

• Miscellaneous Data Transfer Operations:
• xchg :
• Exchanges the contents of a register with the contents of any other register or memory location.
• It can NOT exchange segment registers or memory-to-memory data.
• Byte, word and doublewords can be exchanged using any addressing mode (except immediate, of course).

Miscellaneous Data Transfer Operations

• in and out :
• Transfers a byte, word or doubleword of data from/to an I/O device into/out of al , ax and eax , respectively.
• Memory operations are not available (as they are in ins and outs ):

• Two forms: Fixed-port addressing ( 8-bit port value encoded in instruction) and variable-port addressing ( 16-bit port value stored in dx ).

 

• in and out example:

• The old contents of the higher-order bits of the port are preserved.

Miscellaneous Data Transfer Operations

• movsx and movzx (80386 and up only):
• Move-and-sign-extend and Move-and-zero-extend:

 

• bswap (80486 and up only):
• Swaps the first byte with the forth, and the second byte with the third.
• Used to convert between little endian and big endian:

 

• cmov (Pentium and up only):
• These instructions move data only if a condition is true.
• Conditions are set by a previous instruction and include Carry , Zero , Sign , Overflow and Parity :

• There are many variations of this instruction (see Appendix B of text).

Assembler Directives

• Segment Override Prefix:
• Allows the programmer to override the default segment.

 

• Code and Data Segment Declarations:

 

• Procedure Calls

 

• See nasm slides and documentation on other details.