Intel Assembly

• 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), SI and DI are implicitly used.
• D = 0 : Autoincrement DI and SI .
• Use CLD instruction to clear this flag.
• D = 1 : Autodecrement DI and SI .
• Use STD instruction to set it.

 

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

Intel Assembly

• String Operations.
• LODS :
• Loads AL , AX or EAX with data stored at the data segment (or extra segment) + offset given by SI .
• SI is incremented or decremented afterwards:

• STOSB :
• Stores AL , AX or EAX to the extra segment (ES) + offset given by DI . ES cannot be overridden.
• DI is incremented or decremented afterwards:

Intel Assembly

• String Operations:

Intel Assembly

• String Operations.
• REP prefix:
• Executes the instruction CX times.

• NOTE: REP does not make sense with the LODS instruction.

Intel Assembly

• String Operations.
• MOVS :
• Moves a byte, word or doubleword from data segment and offset SI to extra segment and offset DI .
• Increments/decrements both DI and SI :

 

• INS/OUTS (not available on the 8086):
• Transfers a byte, word or doubleword of data from/to an I/O device into/outof the extra/data segment + offset DI / SI , respectively.
• The I/O address is stored in the DX register.

Intel Assembly

• String Operations.
• INS Example:

• NOTE:
• This assumes the device is able to make data available at the speed of the program, and in sync with the program.
• Since this is almost never true, the code above needs to have a "device status check" instruction(s).
• Of course, the REP prefix can not be used in this case.

 

• The same holds true for OUTS instructions.

Intel Assembly

• 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).

 

• XLAT :
• Performs a table lookup operation by converting the value in AL from an index to a value stored in the cell of a table in memory.

Intel Assembly

• Miscellaneous Data Transfer Operations.
• IN and OUT :
• Transfers a byte, word or doubleword of data from/to an I/O device into/outof 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 number encoded in instruction.
• 8-bit value is sign-extended into address bits 8-15 .
• Address bits 16-31 are undefined.
• Variable-port addressing:
• 16-bit port number stored in DX .

Intel Assembly

• Miscellaneous Data Transfer Operations.
• IN and OUT Example:

• Note that the old contents of the higher-order bit positions of the port are preserved.

Intel Assembly

• 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:

Intel Assembly

• Miscellaneous Data Transfer Operations.
• 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.

Intel Assembly

• Segment Override Prefix:
• Allows the programmer to override the default segment.
• Appends an additional byte to the front of an instruction.
• Jump and Call instructions cannot be prefixed, must use CS.

 

• Assembler Directives:

Intel Assembly

• Assembler Directives:

 

• Full Segment Definitions: