Data Movement Instructions

1. Data Movement Instructions Fundamental of Computer System 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

4. 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)

5. MOV Revisited (cont’d) • 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)

6. MOV Revisited (cont’d)

7. 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]!

8. MOD Field for 16-bit instruction

9. REG and R/M when MOD=11

10. Example 8BEC = MOV BP,SP

11. 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)

12. R/M Memory Addressing

13. Example MOV DL,[DI]

14. 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)

17. 32-bit addressing modes

18. MOV Revisited (cont’d) • 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)

19. 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

20. MOV Revisited (cont’d) • 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)

22. Example MOV BX,CS

24. 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

25. 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

26. PUSH Instructions

27. PUSH AX

29. 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

30. POP Instructions

31. POP BX

32. 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

33. 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

35. Initializing the Stack

36. 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

38. LEA Instructions

39. 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

40. LDS BX,[DI]

42. 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)

43. 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

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

45. LODS

46. LODSW

47. 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)

48. STOS

49. 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.

50. STOS