Microprocessor
Programming and Interface
Dr. Saad Qasim Khan
Assistant Professor
CIS Department NEDUET
saadqasimkhan@neduet.edu.pk
Special Addressing Mode
• There is a special addressing mode that does not appear in Tables 4–
2, 4–3, or 4–4. It occurs whenever memory data are referenced by
only the displacement mode of addressing for 16-bit instructions.
• Examples are the MOV [1000H],DL and MOV NUMB,DL instructions.
• The first instruction moves the contents of register DL into data
segment memory location 1000H.
• The second instruction moves register DL into symbolic data segment
memory location NUMB.
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Special Addressing Mode
• Whenever an instruction has only a displacement, the MOD field is
always a 00 and the R/M field is always 110.
• As shown in the tables, the instruction contains no displacement and
uses addressing mode [BP].
• You cannot actually use addressing mode [BP] without a displacement
in machine language.
• The assembler takes care of this by using an 8-bit displacement
(MOD=01) of 00H whenever the [BP] addressing mode appears in an
instruction. This means that the [BP] addressing mode assembles as a
[BP+0], even though a [BP] is used in the instruction.
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Segment MOV Instructions
• If the contents of a segment register are moved by the MOV, PUSH, or
POP instructions, a special set of register bits (REG field) selects the
segment register.
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PUSH/POP
• The PUSH and POP instructions are important instructions that store
and retrieve data from the LIFO (last-in, first-out) stack memory.
• The microprocessor has six forms of the PUSH and POP instructions:
register, memory, immediate, segment register, flags, and all registers.
• The PUSH and POP immediate and the PUSHA and POPA (all registers)
forms are not available in the earlier 8086/8088 microprocessors, but
are available to the 80286 through the Core2.
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PUSH Instruction
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POP Instruction
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LOAD-EFFECTIVE ADDRESS
• There are several load-effective address instructions in the
microprocessor instruction set.
• The LEA instruction loads any 16-bit register with the offset address,
as determined by the addressing mode selected for the instruction.
• The LDS and LES variations load any 16-bit register with the offset
address retrieved from a memory location, and then load either DS or
ES with a segment address retrieved from memory.
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LEA
• The LEA instruction loads a 16- or 32-bit register with the offset
address of the data specified by the operand.
• By comparing LEA with MOV, we observe that LEA BX,[DI] loads the
offset address specified by [DI] (contents of DI) into the BX register;
MOV BX,[DI] loads the data stored at the memory location addressed
by [DI] into register BX.
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Example
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LEA / LDS / LES
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The End
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