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Assembly Language

Assembly Language. Symbol Table. Variables .DATA var DW 0 sum DD 0 array TIMES 10 DW 0 message DB ’ Welcome ’ ,0 char1 DB ?. Symbol Table Name Offset var 0 sum 2 array 6 message 26 char1 35. Addressing Modes. Register Addressing Mode

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Assembly Language

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  1. Assembly Language

  2. Symbol Table Variables .DATA var DW 0 sum DD 0 array TIMES 10 DW 0 message DB ’Welcome ’,0 char1 DB ? Symbol Table Name Offset var 0 sum 2 array 6 message 26 char1 35

  3. Addressing Modes • Register Addressing Mode • Internal registers contain the data to be manipulated by the instruction • Register-addressing mode is the most efficient way of specifying data. • Data are processed within the processor with memory access. • Examples mov EAX,EBXmov BX,CXmov AL,CL

  4. Addressing Modes (Cont.) • Immediate Addressing Mode • Data are specified as part of the instruction itself. • The data are in memory, but is located in the code segment, not in the data segment. • The immediate data are always a constant, given either directly or via the EQU directive • Example mov AH, 10 mov AX, 322

  5. Memory Addressing Modes • Direct Addressing Mode • This mode requires access to the main memory • They tend to be slower than the immediate and register addressing modes. • Locate a datum in the data segment, require specifying a segment address and an offset value. Data Segment char1 DB ‘Y’ array TIMES 20 DD 0 name DB ’David’ Code Segment mov AL, [char1] mov [char1], ‘X’ mov [array], 46 mov [name], ‘W’

  6. Memory Addressing Modes • Indirect Addressing Mode • The offset or effective address of the data are stored in general registers. • It is not required for single element variables having. • It is intended to be used for lists of variables (arrays). Data Segment char1 DB ‘Y’ array TIMES 20 DD 0 name DB ’David’ Code Segment mov EBX, array mov [EBX], 13 add EBX, 4 mov [EBX], 17

  7. Indirect Addressing Mode • The effective address can be loaded into a register by • mov EBX, array Or • lea register, source lea EBX, [array]

  8. Data Transfer Instructions The MOV Instruction • mov destination, source Possible forms • mov register, register • mov register, immediate • mov memory, immediate • mov register, memory • mov memory, register There is no mov instruction to transfer data from memory to memory (except for strings)

  9. Data Transfer Instructions The MOV Instruction • Examples • mov [var1], BH • mov EDX, [array] • mov [name+4], ‘W’ • Ambiguous Moves movWORD [EBX], 100 movBYTE [ESI], 100 mov EBX, array mov ESI, name mov [EBX], 100 mov [ESI], 100 mov [EBX], WORD 100 mov [ESI], BYTE 100

  10. Data Transfer Instructions The XCHG Instruction • The xchg instruction exchanges 8-, 16-, or 32-bit source and destination operands. • Examples xchg EAX, EDX xchg [answer], CL xchg [total], DX • It cannot be used to exchange two variables located in memory. • xchg [var1], [name1] ; is illegal mov ECX, EAX mov EAX, EDX mov EDX, ECX xchg EAX, EDX

  11. Arithmetic Instructions The inc and dec • The inc (INCrement) instruction adds one to its operand and the dec (DECrement) instruction subtracts one from its operand. • It can be either in a register or in memory. Examples inc EBX ; increment 32-bit register dec DL ; decrement 8-bit register • Type Specifiers are used to resolve ambiguity inc WORD [EBX] dec BYTE [ESI] .DATA count DW 0 value DB 25 .CODE inc [count] ;unambiguous dec [value] ;unambiguous mov EBX, count inc [EBX] ;ambiguous mov ESI, value dec [ESI] ;ambiguous

  12. Arithmetic Instructions The ADD Instruction • Is used to add two 8-, 16- or 32-bit operands. The syntax is • add destination, source ↔ destination = destination + source • Similar to mov instruction, it takes the five basic forms of operands. • Note inc EAX vs. add EAX,1 The first improves readability and requires less memory space to store the instruction, both instructions execute at the same speed.

  13. Arithmetic Instructions • The sub instruction • It is used to subtract two 8-, 16- or 32-bit numbers. sub destination, source ↔ destination = destination – source Examples: • The cmp instruction • The cmp instruction is used to compare two operands. • It is similar to sub instruction except that the result of subtraction is not saved. • The operation sets the zero flag

  14. Conditional Execution jmp- Unconditional Jump • It tells the processor that the next instruction to be executed is located at the label. • jmp label; label identifies the next instruction to be executed. Example mov EAX,1 inc_again: inc EAX jmp inc_again mov EBX,EAX

  15. Conditional Execution Conditional Jump The program execution is transferred to the target instruction only if the specified condition is satisfied. j<cond> label; <cond> identifies the condition of the branch (jump) Usually, the condition being tested is the result of the last arithmetic or logic operation. Example: read_char: mov DL,0 . . . (code for reading a character into AL) . . . cmp AL,0DH ;compare the character to CR je CR_received inc CL jmp read_char ;go to to read another CR_received mov DL,AL …

  16. Conditional Execution Conditional Jumps je jump if equal jg jump if greater jl jump if less jge jump if greater than or equal jle jump if less than or equal jne jump if not equal Conditional jumps that test flags. jz jump if zero (i.e., if ZF = 1) jnz jump if not zero (i.e., if ZF = 0) jc jump if carry (i.e., if CF = 1) jnc jump if not carry (i.e., if CF = 0)

  17. Conditional Execution Example: go_back: inc AL . . . cmp AL,BL statement_1 mov BL,77H

  18. Iteration Instruction The <loop body> is executed 50 times. Iteration using jump instructions. mov CL,50 repeat1: <loop body> dec CL jnz repeat1 Using the loop instruction mov ECX,50 repeat1: <loop body> loop repeat1

  19. Logical Instructions and destination, source and AL, 01H or destination, source or AX, FAH xor destination, source xor EAX, DDFFH not destination not EBX

  20. Logical Instructions and AL,01H je bit_is_zero <code to be executed when the bit is one> jmp skip1 bit_is_zero: <code to be executed when the bit is zero> skip1: <rest of the code>

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