1. Micro-Computer Applications: Addressing Modes Dr. Eng. Amr T. Abdel-Hamid ELECT 707 Fall 2011

2. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Organization of 8088 AHAL BHBL CHCL DHDL SP BP SI DI ALU Flag register Execution Unit (EU) EU control  CS DS SS ESALU Data bus (16 bits) Address bus (20 bits) Instruction Queue Bus control External bus IP Data bus (16 bits) Bus Interface Unit (BIU) General purpose register Segment register

3. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Instruction Machine Codes  Instruction machine codes are binary numbers  For Example: 1 0 0 0 1 0 0 0 1 1 0 0 0 0 1 1 MOV AL, BL MOV  Machine code structure OpcodeOperand1  Opcode tells what operation is to be performed.  EU control logic generates ALU control signals accordi ng to Opcode)  Some instructions do not have operands, or have only one operand  Operands tell what data should be used in the operation. Operands can be addresses telling where to get data (or where to store results) Register mode ModeOperand2  Mode indicates the type of a instruction: Register type, or Memory type

4. Dr. Amr Talaat ELECT 707 Micro-Computer Applications EU Operation 1. Fetch an instruction from instruction queue 2. According to the instruction, EU control logic generates control signals. ( This process is also referred to as instruction decoding) 3. Depending on the control signal, EU performs one of the following operations:  An arithmetic operation  A logic operation  Storing a datum into a register  Moving a datum from a register  Changing flag register

5. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Registers  A register is a storage element inside a m icroprocessor.  Almost all operations would involve using registers.  Some registers are general purpose regis ters, while others have special purposes.  General purpose registers can hold various da ta sizes and used for almost any purpose as d ictated by the program.  However, each general purpose register does have its own special purposes.

6. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Registers

7. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Registers Since the x86 instr uction set is design ed to be compatible with previous micro processors, the sam e register can be ac cessed using differe nt names. Different names are given for 64-bit, 32 -bit, 16-bit and 8-b it version of the sam e register.

8. Dr. Amr Talaat ELECT 707 Micro-Computer Applications General Purpose Registers  RAX, EAX, AX (AH & AL)  A general purpose register  Also an accumulator – stores intermediate res ults after arithmetic and logic operations  Can also hold the offset address of a location in memory (80386 and above)  RBX, EBX, BX (BH & BL)  A general purpose register  Also a base index register – holds the offset a ddress of a location in memory  Can also address memory data (80386 and a bove)

9. Dr. Amr Talaat ELECT 707 Micro-Computer Applications General Purpose Registers  RCX, ECX, CX (CH & CL)  A general purpose register  Also a count register – holds the count for var ious instructions  Can also hold the offset address of a location in memory (80386 and above)  RDX, EDX, DX (DH & DL)  A general purpose register  Also a data register – stores data related to a ccumulator’s calculation (multiply and divide)  Can also address memory data (80386 and a bove)

10. Dr. Amr Talaat ELECT 707 Micro-Computer Applications General Purpose Registers  RBP, EBP, BP  A general purpose register  Also a base pointer register – points to a me mory location for memory data transfer  RDI, EDI, DI  A general purpose register  Also a destination index register – holds the memory address for the destination data of a string instruction

11. Dr. Amr Talaat ELECT 707 Micro-Computer Applications General Purpose Registers  RSI, ESI, SI  A general purpose register  Also a source index register - holds the memo ry address for the source data of a string instr uction  R8 through R15  General purpose registers  Found only in 64-bit microprocessors  Data are addressed in 64-, 32-, 16-, or 8-bit sizes

12. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Special Purpose Registers  RIP, EIP, IP  Instruction pointer – points to the next instru ction in the memory to be executed  RSP, ESP, SP  Stack pointer – points to an area in memory c alled the stack  RFLAGS, EFLAGS, FLAGS  Indicates the condition of the microprocessor and control its operation

13. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Flags

14. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Flags  The C, P, A, Z, S and O flags are changed by mo st arithmetic and logic operations.  Flags never change for any data transfer or prog ram control operations.  Some flags are also used to control features fou nd in the microprocessor.

15. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Flags  Descriptions for some of the flag bits:  C (carry): holds the carry after addition or b orrow after subtraction.  P (parity): the count of 1s in a number expr essed as even or odd.  0 for odd, 1 for even.  A (auxiliary carry): holds the half-carry afte r addition or the borrow after subtraction bet ween bit positions 3 and 4 of the result.

16. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Flags  Z (zero): shows that the result of an arithmetic or logi c operation is zero.  0 result is not zero, 1 result is zero.  S (sign): holds the arithmetic sign of the result after a n arithmetic or logic instruction executes.  0 for positive, 1 for negative.  T (trap): enables trapping through an on-chip debuggi ng feature.  0 disable trapping, 1 enable trapping.  If enabled, allows the microprocessor to interrupt the flow of the program on conditions indicated by the debug regis ters and control registers.  Microsoft Visual Studio debugging tool uses this feature.

17. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Flags  I (interrupt): control the operation of the INTR (interrupt request) input pin.  0 disables INTR pin, 1 enables INTR pin.  D (direction): selects increment or decrement mode for SI/DI registers.  0 increment, 1 decrement.  O (overflow): indicates that the result of an addition/sub traction of signed numbers exceeded the capacity of the machine.

18. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Memory Segmentation  A segment is a 64KB block of memory starting from any 16-byte boundary  For example: 00000, 00010, 00020, 20000, 8CE90, and E0840 are all valid segment addresses  The requirement of starting from 16-byte boundary is due to the 4-bit left shifting  Segment registers in BIU CS SS DS ES Code Segment Data Segment Stack Segment Extra Segment 015

19. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Generating Memory Addresses  How can a 16-bit microprocessor generate 20-bit memory addresses? Segment (64K) 0000 + 16-bit register 20-bit memory address 00000 FFFFF Left shift 4 bits Intel 80x86 memory address generation 1M memory space Offset Segment address Offset Addr1 Addr1 + 0FFFF

20. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Fetching Instructions  Where to fetch the next instruction? CS IP 1 2 3 9 0 0 1 2 1 2 3 A 2 MOV AL, 0 8088Memory  Update IP — After an instruction is fetched, Register IP is updated as follows: IP = IP + Length of the fetched instruction — For Example: the length of MOV AL, 0 is 2 bytes. After fetching this instruction, the IP is updated to 0014

21. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Accessing Data Memory  There is a number of methods to generate the memory address when accessing data memory. These methods are referred to as Addressing Modes  Examples: — Direct addressing: MOV AL, [0300H] 12340 0300 26401 DS Memory address (assume DS=1234H) — Register indirect addressing: MOV AL, [SI] 12340 0310 26501 DS Memory address (assume DS=1234H) (assume SI=0310H)

22. Dr. Amr Talaat ELECT 707 Micro-Computer Applications A memory system showing the placement of four memory segments. – a memory segment can touch or overlap if 64K bytes of me mory are not required for a se gment – think of segments as window s that can be moved over any area of memory to access dat a or code – a program can have more tha n four or six segments, but only access four or six segments at a time

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25. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Accessing Data Memory  There is a number of methods to generate the memory address when accessing data memory. These methods are referred to as Addressing Modes  Examples: — Direct addressing: MOV AL, [0300H] 12340 0300 26401 DS Memory address (assume DS=1234H) — Register indirect addressing: MOV AL, [SI] 12340 0310 26501 DS Memory address (assume DS=1234H) (assume SI=0310H)

26. Dr. Amr Talaat ELECT 707 Micro-Computer Applications MOV instruction provides a basis for explanation of data-addressing modes opcode an opcode, or operation code, tells the microprocessor which operation to perform DATA ADDRESSING MODES

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28. Dr. Amr Talaat ELECT 707 Micro-Computer Applications MOV BX, CX The source register’s contents do not change. the destination register’s contents do change The contents of the destination register or destina tion memory location change for all instructions e xcept the CMP and TEST instructions. Note that only the rightmost 16 bits of register EB X change. The MOV BX, CX instruction does not affect the leftmost 16 bits of register EBX.

29. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Register Addressing 8028680386 +80486 + Register8-bit16-bit32-bit64-bit AccumulatorAH, ALAXEAXRAX IndexBH, BLBXEBXRBX CountCH, CLCXECXRCX DataDH, DLDXEDXRDX Stack PointerSPESPRSP Base PointerBPEBPRBP Src. IndexSIESIRSI Dest. IndDIEDIRDI R8 – R15

30. Dr. Amr Talaat ELECT 707 Micro-Computer Applications  Important for instructions to use registers that are the same size.  never mix an 8-bit \with a 16-bit register, an 8- or a 16-bit register with a 32-bit register  this is not allowed by the microprocessor and results in an error when assembled

31. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Immediate Addressing  Term immediate implies that data immediately follo w the hexadecimal opcode in the memory.  immediate data are constant data  data transferred from a register or memory locat ion are variable data  Immediate addressing operates upon a byte or wor d of data.

32. Dr. Amr Talaat ELECT 707 Micro-Computer Applications MOV EAX,3456H the source data overwrites the destination data. After the instruction executed Before the instruction executed

33. Dr. Amr Talaat ELECT 707 Micro-Computer Applications  In symbolic assembly language, the symbol # precedes imme diate data in some assemblers.  MOV AX,#3456H instruction is an example  Most assemblers do not use the # symbol, but represent immediate data as in the MOV AX,3456H instruc tion.  an older assembler used with some Hewlett-Packard logic d evelopment does, as may others  in this text, the # is not used for immediate data

34. Dr. Amr Talaat ELECT 707 Micro-Computer Applications  The symbolic assembler portrays immediate data i n many ways.  The letter H appends hexadecimal data.  If hexadecimal data begin with a letter, the assem bler requires the data start with a 0.  to represent a hexadecimal F2, 0F2H is used in assembly language  Decimal data are represented as is and require no special codes or adjustments.  an example is the 100 decimal in the MOV AL,100 instruction

35. Dr. Amr Talaat ELECT 707 Micro-Computer Applications  An ASCII-coded character or characters may be de picted in the immediate form if the ASCII data are enclosed in apostrophes.  Binary data are represented if the binary number i s followed by the letter B.  in some assemblers, the letter Y

36. Dr. Amr Talaat ELECT 707 Micro-Computer Applications  Each statement in an assembly language program consists of four parts or fields.  The leftmost field is called the label.  used to store a symbolic name for the memory l ocation it represents  All labels must begin with a letter or one of the foll owing special characters: @, $, -, or ?.  a label may any length from 1 to 35 characters  The label appears in a program to identify the na me of a memory location for storing data and for o ther purposes. NEXT:MOV AX, [BX]; next element Labels

37. Dr. Amr Talaat ELECT 707 Micro-Computer Applications  The next field to the right is the opcode field.  designed to hold the instruction, or opcode  the MOV part of the move data instruction is an example of an opcode  Right of the opcode field is the operand field.  contains information used by the opcode  the MOV AL,BL instruction has the opcode MOV and operands AL and BL  The comment field, the final field, contains a com ment about the instruction(s).  comments always begin with a semicolon (;) NEXT:MOV AX, [BX]; next element

38. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Direct Data Addressing  Applied to many instructions in a typical program.  Two basic forms of direct data addressing:  direct addressing, which applies to a MOV betwee n a memory location and AL, AX, or EAX  displacement addressing, which applies to almost any instruction in the instruction set  Address is formed by adding the displacement to th e default data segment address or an alternate seg ment address.

39. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Direct Addressing  Direct addressing with a MOV instruction transfers data between a memory location, located within th e data segment, and a register.  usually a 3-byte long instruction MOV CX,DATA  loads CX from the data segment memory location DATA (1234H).  DATA is a symbolic memory location, while 1234H is the actual hexadecimal location

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41. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Example DS=1512H MOV AL, 99H MOV [3518], AL After execution memory location Logical address DS:3518 Physical address18638 will have 99H Direct Data Addressing

42. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Register Indirect Addressing  Allows data to be addressed at any memory locati on through an offset address held in any of the foll owing registers: BP, BX, DI, and SI.  In addition, 80386 and above allow register indire ct addressing with any extended register except E SP.  In the 64-bit mode, the segment registers serve n o purpose in addressing a location in the flat model.

43. Dr. Amr Talaat ELECT 707 Micro-Computer Applications MOV AX,[BX] BX = 1000H and DS = 0100H. Register Indirect Addressing

44. Dr. Amr Talaat ELECT 707 Micro-Computer Applications  The data segment is used by default with register indirect addressing or any other mode that uses BX, DI, or SI to address memory.  If the BP register addresses memory, the stack se gment is used by default.  these settings are considered the default for these four index and base registers  For the 80386 and above,  EBP addresses memory in the stack segment by default.  EAX, EBX, ECX, EDX, EDI, and ESI address mem ory in the data segment by fault.

45. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Base-Plus-Index Addressing  Similar to indirect addressing because it indirect ly addresses memory data.  The base register often holds the beginning loca tion of a memory array.  the index register holds the relative position of an element in the array  whenever BP addresses memory data, both t he stack segment register and BP generate th e effective address

46. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Locating Data with Base-Plus-Index Addressing MOV DX,[BX + DI]  The Intel assembler requires this addressing mod e appear as [BX][DI] instead of [BX + DI].  The MOV DX,[BX + DI] instruction is MOV DX,[BX ][DI] for a program written for the Intel ASM ass embler.

47. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Base-plus-index addressing mode MOV DX,[BX + DI] DS=0100H, BX=1000H and DI=0010H memory address 02010H is accessed because

48. Dr. Amr Talaat ELECT 707 Micro-Computer Applications An example of the base-plus-index addressing mode. Here an element (DI) of an ARRAY (BX) is addressed.

49. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Register Relative Addressing  Similar to base-plus-index addressing and displaceme nt addressing.  data in a segment of memory are addressed by add ing the displacement to the contents of a base or a n index register (BP, BX, DI, or SI)

50. Dr. Amr Talaat ELECT 707 Micro-Computer Applications MOV AX, [BX+1000H] BX=1000H and DS=0200H.

51. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Addressing Data with Base Relative-Plus-Index  Least-used addressing mode.  Figure 3–12 shows how data are referenced if the i nstruction executed by the microprocessor is MOV AX,[BX + SI + 100H].  displacement of 100H adds to BX and SI to form the offset address within the data segment  This addressing mode is too complex for frequent u se in programming.

52. Dr. Amr Talaat ELECT 707 Micro-Computer Applications MOV AX,[BX + SI + 100H] DS=1000H

53. Dr. Amr Talaat ELECT 707 Micro-Computer Applications WHY Addressing Arrays with Base Relative-Plus-Index ?  Suppose a file of many records exists in memory, each record with many elements.  displacement addresses the file, base register a ddresses a record, the index register addresses an element of a record  Figure 3–13 illustrates this very complex form of a ddressing.

54. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Base relative-plus-index addressing used to access a FILE that contains multiple records (REC).

55. Dr. Amr Talaat ELECT 707 Micro-Computer Applications Scaled-Index Addressing  Unique to 80386 - Core2 microprocessors.  uses two 32-bit registers (a base register and an index register) to access the memory  The second register (index) is multiplied by a scali ng factor.  the scaling factor can be 1x, 2x, 4x, 8x Examples of scaled-index instructions.  MOV AX,[ EBX + 2*ECX]  MOV [4 * ECX ],EDX

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57. Dr. Amr Talaat ELECT 707 Micro-Computer Applications References: Based on slides from B. Brey, The Intel Microprocessor: Archit ecture, Programming, and Interfacing, 8 th Edition, 2009 & other s