Assembly Language Data Movement Instructions

1. Assembly LanguageData Movement Instructions

2. MOV Instruction • Move source operand to destination mov destination, source • The source and destination are often called operands • MOV AX, BX instruction • transfers the word contents of the source register (BX) into the destination register (AX) • the leftmost 16 bits of register EAX does not affected • the source register’s contents BX do not change • Source and destination operands can vary (Register, Memory, Immediate, …etc)

3. MOV Instruction..Rules • Rules • Both operands must be of same size • No memory to memory moves • No segment to segment moves • EAX, EBX, ECX, EDX, ESP, EBP, EDI,ESI, CS, ES, DS, SS, FS, and GS registers can be used • Destination cannot be CS, EIP, or IP • No immediate to segment moves • MOV instructions not affect the flag bits.

4. Addressing modes • Define how machine language instructions identify the operand(s) of each instruction • Addressing Mode Type: • Register  MOV AX,BX • Immediate  MOV CH,3AH • Direct  MOV [1234H],AX • Register indirect  MOV [BX],CL • Base-plus-index  MOV [BX+SI],BP • Register relative  MOV CL,[BX+4] • Scaled index  MOV [EBX+2 × ESI],AX

5. Examples of register-addressedinstructions

6. Examples of immediate addressing

7. Direct addressed instructions

8. Examples of register indirect addressing

9. Examples of base-plus-index addressing

10. Examples of register relative addressing

11. XCHG Instruction • XCHG exchanges the values of two operands xchg reg, reg xchg reg, mem xchg mem, reg • xchg ah, al ; exchange 8-bit regs • xchg ax, bx ; exchange 16-bit regs • Rules • Operands must be of the same size • At least one operand must be a register • No immediate operands are permitted

12. LAHF and SAHF Instructions • LAHF (load status flags into AH) instruction copies the low byte of the EFLAGS register into AH. The following flags are copied: Sign, Zero, Auxiliary Carry, Parity, and Carry • These instructions do not require any operands .data saveflags BYTE ? .code lahf ; load flags into AH mov saveflags, ah ; save them in a variable • SAHF (store status flags into AH) instruction copies AH register into the low byte of the EFLAGS. The following flags are copied: Sign, Zero, Auxiliary Carry, Parity, and Carry. mov ah, saveflags ; load saved flags into AH sahf ; copy into Flags register • These instructions have little application today

13. Load Effective Address Instructions • There are several load-effective address instructions in the microprocessor instruction set (LEF, LDS, LES,LFS,LSS,LGS) • LEF : loads a 16- or 32-bit register with the offset address of the data specified by the operand • LEA BX,[DI] loads the offset address specified by [DI] (contents of DI) into the BX register BUT • MOV BX,[DI] loads the data stored at the memory location addressed by [DI] into register BX • LEA BX, 3[BX] BX=BX+3 ; ADD Plus Move • LEA AX,NUMB Loads AX with the offset address of NUMB • The OFFSET directive performs the same function as an LEA instruction • MOV BX,OFFSET LIST performs the same function as LEA BX,LIST

14. LDS, LES, LFS, LGS, and LSS • The LDS, LES, LFS, LGS, and LSS instructions let you load a 16 bit general purpose register and segment register pair with a single instruction • LxS dest, source • lds reg16, mem32 • Reg16 :is any general purpose 16 bit register and mem32: is a double word memory location • reg16 = [mem32], ds = [mem32 + 2] • The LDS and LES instructions are the only instructions that directly process values larger than 32 bits.

15. LDS Example

16. Assembly Example

17. The PUSH and POP instructions • 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 • push reg16 pop reg16 • push reg32 pop reg32 • push segreg pop segreg (except CS) • push memory pop memory • push immediate_data • Pusha popa • Pushad popad • Pushf popf • Pushfd popfd • The stack memory it is always in the stack segment (wherever ss points) • The stack grows down in memory • Stack pointer (ss:sp) always contains the address of the value on the top of the stack

18. Push instruction • Push instructions (16 bits) • sp=sp-2 • store 16-bit register operand at location SS:SP • The first (most-significant) data byte moves to the stack segment memory location addressed by sp-1 • The second (least-significant) data byte moves into the stack segment memory location addressed by sp-2 • push instructions (32 bits) • SP := SP - 4 • [SS:SP] = 32 bit operand • pop instructions (16 bits): • 16-bit operand = [SS:SP] • SP = SP + 2 • pop instructions (32 bits): • 32 bit operand = [SS:SP] • SP = SP + 4

19. Push example Push AX Before execution the instruction

20. Push examples • The word ptr operator force 16 bit operation or we can use PushD • The dword ptr operator force 32 bit operation • When push 32-bit immediate can use: • PUSH 0045H • PUSHD 0045H

21. POP Examples • Suppose that a POP BX instruction executes • The first byte of data removed from the stack (the memory location addressed by SP in the stack segment) moves into register BL • The second byte is removed from stack segment memory location SP + 1and is placed into register BH • After both bytes are removed from the stack, the SP register is incremented by 2.

22. PUSHA Instruction • PUSHA (push all) instruction copies the 16-bit registers to the stack in the following order: AX,CX, DX, BX, SP, BP, SI, and DI • The value for SP that is pushed onto the stack is whatever it was before the PUSHA instruction executed • The POPA (pop all) instruction removes 16 bytes of data from the stack and places them into the following registers, in the order shown: DI, SI, BP, SP, BX, DX, CX, and AX • PUSHA instruction requires 16 bytes of stack memory space to store all eight 16-bit registers • Ex.: PUSHA (no operands) • After PUSHA instruction the sp=sp-16 • The PUSHAD instruction places the 32-bit register set on the stack : EAX,ECX, EDX, EBX, ESP, EBP, ESI, and EDI • PUSHAD requires 32 bytes of stack storage space.

23. PUSHF instruction • The PUSHF and POPF instructions allow you to push/pop the processor status register (the flags register) into stack • The POPF (pop flags) instruction removes a 16-bit number from the stack and places it into the flag register • The PUSHFD and POPFD instructions allow you to push/pop the EFLAG register into stack • The POPFD removes a 32-bit number from the stack and places it into the extended flag register

24. IN and OUT instructions • IN/OUT instructions perform I/O operations • In IN/OUT instructions the contents of AL, AX, or EAX are transferred only between the I/O device and the microprocessor

25. IN/OUT Example-1 • IN AL,6AH instruction • executes, data from I/O address 6AH are input to AL. • The address appears as a 16-bit 006AH on pins A0–A15 of the address bus • OUT 19H,AX instruction • Transfers the contents of AX to I/O port 19H. Notice that the I/O port number appears as a 0019H on the 16-bit address bus • and that the data from AX appears on the data bus of the microprocessor

26. IN/OUTExample-2 • A short program that clicks the speaker in the personal computer appears

27. HLTInstruction • The halt instruction (HLT) stops the execution of software. • There are three ways to exit a halt: • by an interrupt, by a hardware reset, or during a DMA operation. • This instruction normally appears in a program to wait for an interrupt. • It often synchronizes external hardware interrupts with the software system. • Note that DOS and Windows both use interrupts extensively, so HLT will not halt the computer when operated under these operating systems.