data movement instructions

Data Movement Instructions

Fundamental of Computer SystemDepartment of Electrical Engineering

University of Indonesia

© Prima d Purnamasari 2009 for Department of Electrical Engineering University of Indonesia 2

MOV Revisited

• The MOV instruction introduces the ma-chine language instructions available with various addressing modes and instructions– It is the native binary code that the micro-

processor understands and uses as its instructions to control its operation


MOV Revisited (cont’d)

• The Opcode– The opcode selects the operation (addition,

subtraction, move, etc) performed by the microprocessor

– The opcode is either one or two bytes long for most machine language instructions (Fig.4.2)



• The first six bits of the first byte are the binary op-code• The remaining two bits indicate the direction (D) of the data flow

and whether the data are byte or a word (W)


MOD field

• The MOD field specifies the addressing mode (MOD) or the type of addressing for the selected instruction, and whether the displacement is present with the selected type (table 4.1)

• Distinguish the MOV AL, [DI], MOV AL, [DI+2] and MOV Al, [DI+1000H]!


MOD Field for 16-bit instruction


REG and R/M when MOD=11


Example 8BEC = MOV BP,SP


R/M Memory Addressing

• If the MOD field contains a 00, 01, or 10, the R/M field takes on a new meaning (Table 4.4)

• Figure 4.5 illustrates the machine language version of the 16-bit instruction MOV DL,DI or instruction (8A15H)


R/M Memory Addressing


Example MOV DL,[DI]


Special Addressing Mode

• It occurs whenever memory data are referenced by only the displacement mode of addressing for 16-bit instructions MOV [1000H],DL

• Whenever an instruction has only a displacement, the MOD field is always a 00 and the R/M field is always a 110 (see Fig.4.6, Fig.4.7)


32-bit addressing modes



– The scaled-index byte (R/M=100) is mainly used when two registers are added to specify the memory address in an instruction (Fig.4.8)


Immediate Instruction

• An suppose the instruction MOV WORD PTR [BX +1000H], 1234H that moves 1234 into the word-sized memory location addressed by the sum of 1000H, BX, and DS x 10H

• The six byte instruction uses two bytes for the op-code, W, MOD, and R/M fields, two other bytes are the data of 1234H, and the last two are the displacement of 1000H See Fig.4.9



• 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 (see Table.4.6)


Example MOV BX,CS


PUSH/POP

• These are important instructions that store and retrieve data from the LIFO stack memory

• There are six forms of the PUSH and POP instructions: register, memory, immediate, segment register, flags, and all registers


PUSH

• It transfer two or four bytes of data to the stack

• PUSHA instruction copies the contents of the internal register set, except the segment registers to the stack– order: AX, BX, CX, DX, BX, SP, BP, SI, and DI

• The PUSHF (push flags) instruction copies the content of the flag register to the stack


PUSH Instructions


PUSH AX


POP

• It performs the inverse operation of PUSH, i.e., removes data from the stack and places it into the target 16-bit register, or a 16-bit memory location


POP Instructions


POP BX


POP Instructions

• The POPA (pop all) removes 16-bit data from the stack and places it into the following registers in order: DI, SI, BP, SP, BX, DX, CX, and AX; this is a reverse order from the way they are placed on the stack by the PUSHA

• POPF (pop flags) removes 16-bit number from the stack and places it into the flag register


Initializing the Stack

• If the stack area is initialized, load both the SS and SP registers; SS is normally designated with the bottom location of the stack segment– Fig.4.14 shows how the beginning of stack

segment is formed and used in PUSH CX– A stack segment is set up as illustrated in

example 4.1 and example 4.2


Initializing the Stack


Load-Effective Address (LEA)

• LEA– The LEA instruction loads a 16- or 32- bit

register with the offset address of the data specified by the operand

– E.g., LEA AX,NUMB– Compare: LEA BX,[DI] & MOV BX,[DI]– Understand Example 4.3


LEA Instructions


LDS, LES, LFS, LGS, LSS

• LDS, LES, LFS, LGS, LSS– load any 16- or 32-bit register with an offset

address and then load the DS, ES, FS, GS, or SS segment register with a segment address


LDS BX,[DI]


String Data Transfers

• There are 5 string data transfer instructions: LODS, STOS, MOVS, INS, and OUTS

• The Direction Flag– The direction flag (D) -located in the flag

register- selects the auto-increment (D=O) or the auto-decrement (D=1) operation for the DI and SI registers during string operations

– The CLD instruction clears the D flag (D=0), and the STD instruction sets it (D=1)


String Data Transfers (cont’d)

• DI and SI– The DI offset address accesses data in the extra

segment for all string instructions that use it– The SI offset address accesses data, by default,

in the data segment


LODS

• The LODS instruction loads AL, AX, or EAX with data stored at the data segment offset address indexed by the SI register


LODS


LODSW


STOS

• The STOS instruction stores AL, AX, or EAX at the extra segment memory location addressed by the DI register

• The STOSB (stores a byte), STOSW (stores a word) and STOSD (stores a doubleword)


STOS


REP prefix

• REP (repetition) is a prefix that can added to any string instruction except LODS

• REP causes CX to be incremented or decremented each time a string instruction is executed. When CX reaches 0, the repetition ends.


STOS


MOVS

• MOVS instruction transfers data (either byte, word or doubleword) from one memory location to another

• MOVS transfers data from the data segment location addressed by SI to the extra segment location addressed by DI


MOVS


INS

• The INS (input string) instruction transfers data from an I/O device into extra segment memory location addressed by the DI register

• The I/O address is contained in the DX register


INS


OUTS

• The OUTS (output string) transfers data from the data segment location addressed by SI to an I/O device (addressed by DX)


OUTS


Assembler Detail

• Directives– Directives indicate how an operand or section of

program is to be processed by the assembler:• Storing Data in a Memory Segment: DB, DW, DD,

SEGMENT, .DATA, ENDS, DUP, ALIGN -ex.4.12


– ASSUME, EQU, ORG: • the EQU equates a numeric, ASCII, or LABEL to another label -

ex.4.13; • the THIS refers the data as byte or word -ex.4.14; • the ORG (origin) changes the starting offset address of data in

the data segment; • the ASSUME tells the assembler what names have been chosen

for the code, data, extra, and stack segments; • the PROC and ENDP indicate the start and end of a procedure

(subroutine) -ex.4.15; • others are CALLF, CALLN, RETF, RETN, NEAR, FAR -ex.4.16


Assembler Detail

• Memory Organization– The assembler uses two basic formats for

developing software: models and full-segment definitions (available to the MASM assembler)

– Models (ex.4.17): The TINY model (64 KB) and the SMALL model (128 KB), and HUGE model (>128 KB)


Assembler Detail (cont’d)

• Full Segment Definitions– Ex.4.18 & Ex.4.19 illustrate the use of full-

segment definitions• Full-segment definitions are also used with the

Borland and Microsoft C/C++ environments for procedures developed in assembly language

– More structured form than the model method– STACK_SEG, DAT_SEG, CODE_SEG, END

MAIN


Assembler Detail

• A Sample Program– Study carefully Ex.4.20

data movement instructions

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