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Riyadh Philanthropic Society For Science Prince Sultan College For Woman Dept. of Computer & Information Sciences CS 251 Introduction to Computer Organization.

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Presentation on theme: "Riyadh Philanthropic Society For Science Prince Sultan College For Woman Dept. of Computer & Information Sciences CS 251 Introduction to Computer Organization."— Presentation transcript:

1 Riyadh Philanthropic Society For Science Prince Sultan College For Woman Dept. of Computer & Information Sciences CS 251 Introduction to Computer Organization & Assembly Language Lecture 10 8086 Register Set

2  From Lecture notes only  Organization of the 8086/8088 Microprocessors  Registers  Data Registers: AX, BX, CX, DX  Address Registers  Segment Registers: CS, DS, SS, ES  Pointer and Index Registers: SP, BP, SL, DL  Instruction Pointer: IP  Flag Register 8086 Register Set2

3  The IBM personal family are all based on the Intel 8086 family of microprocessors.  The 8086/8088 processors have the simplest structure.  Most of the instructions we will study are 8086/8088 instructions.  Because the 8086 and 8088 have essentially the same internal structurethe name 8086 will be applied to both 8086 and 8088.  Registers in 8086 are 16 bits in size ( means 16 bits can be saved in them) 3

4 8086 Register Set4  Information inside microprocessors are stored in registers  The classification of the registers is done according to the function they perform  In general there are fourteen 16-bit registers  Data Registers  There are 4 general purpose data registers  They hold data for the operation  Address Registers  They are divided into segment, pointer, and index registers  They hold the address of the data or instruction in memory  Status Registers  It is called the FLAGS register  It keeps the current status of the processor

5 8086 Register Set5 AHAL BHBL CHCL DHDL AX BX CX DX CS DS SS ES SI DI SP BP IP Data Registers Segment Registers Pointer and Index Registers FLAGS Register

6 8086 Register Set  These four registers are available to the programmer for general data manipulation.  Even thought the processor can operate on data stored in memory, the same instruction is faster if data are stored in registers.  This is why modern processors tend to have a lot of registers.  The high and low bytes of these registers can be accessed separately.  Ex. The high byte of AX is called AH, and the low byte is called AL.  This arrangement gives us more registers to use when dealing with byte-size data.  In addition to being general purpose, each of these registers have a specific operation as well. 6

7 8086 Register Set  AX is the preferred register to use in the logic, arithmetic, and data transfer instructions (its use will generate the shortest machine code)  In multiplication and division operations, one of the numbers should be in AX or AL  Input and Output operations also require the use of AL and AX 7

8 8086 Register Set  Also serves as an address register  Ex. Table look-up instruction XLAT (used for translation) 8

9 8086 Register Set  Program Loop constructions are facilitated by the use of CX, which acts as a loop counter  CL is used as a count in instructions that shift and rotate bits 9

10 8086 Register Set  DX is used in multiplication and Division  It is also used in I/O operations 10

11 8086 Register Set  Address registers store addresses of instructions and data in memory  These values are used by the processor to access memory locations  In the 8086 processor (16-bit processor)  Memory is composed of bytes, each memory byte has an address starting with 0.  The processor assigns a 20-bit physical address to its memory locations  It is possible to address 2 20 =1,048,576 bytes (1 megabyte of memory)  The bytes in memory have addresses from 00000h to FFFFFh 11

12 8086 Register Set  It is a direct consequence of using a 20-bit address in a 16-bit processor  The addresses are too big to fit in a 16-bit register or memory word  8086 gets around this by partitioning its memory into segments 12

13 8086 Register Set  A memory segment is a block of 2 16 (64 K) consecutive memory bytes.  Each segment is identified by a segment number (16 bits)  The range for the segment number is 0000h to FFFF h  Within the segment, the memory location is specified by giving an offset  Offset: number of bytes from the beginning of a segment (16 bits)  Also the offset range is 0000h to FFFF h 13

14 8086 Register Set  A memory location can be specified by providing a segment number and an offset, written in the form Segment:Offset  The Segment: Offset is known as the logical Address  Ex. A4FB:4872h segment offset  To obtain a 20-bit physical address, the 8086 microprocessor first shifts the segment number by 4 bits to the left and then adds the offset  The physical address of A4FB:4872h is: A4FB0h + 4872h 20-bit physical address A9822h 14

15 8086 Register Set  For memory location whose physical address is specified by 1256Ah, give the address is segment:offset form for segments 1256h and 1240h  Physical address = segment x 10h + offset offset = physical address – segment x 10h  For segment 1256: offset = 1256Ah – (1256h x 10h) = Ah 1256:000Ah  For segment 1240: offset = 1256Ah – (1240h x 10h) = 16Ah 1240: 016Ah  Thus 1256Ah = 1256:000Ah = 1240:016Ah 15

16 8086 Register Set  A memory location has physical address 80FD2h. In what segment does it have offset BFD2h?  Physical address = segment x 10h + offset segment = (physical address - offset) / 10h segment = (80FD2h - BFD2h) / 10h = 7500h 16

17 8086 Register Set  A machine language program consists of instructions (code) and data  The stack is a data structure used by the processor to make procedure calls  The data, instructions, & stack parts of the program are loaded into different memory segments  They are called: data segment, code segment, and stack segment 17

18 8086 Register Set  There are four segment registers that the 8086 processor uses to keep track of the segment numbers in the program.  CS: Code Segment: holds the code (or instruction) segment number  DS: Data Segment: holds the data segment number  SS: Stack Segment: contains the stack segment number  ES: Extra Segment: it is used in case the program needs to access another data segment 18

19 8086 Register Set19 8086 Processor Address Memory CS 0F8Ah 0F8A:0000 code segment begins DS 0F89h 0F89:0000 data segment begins SS 0F69h 0F69:0000 stack segment begins ES

20 8086 Register Set  The memory locations addressed by the segment registers are accessible  i.e. only four segments are active at a time  However, the content of the segment register could be modified in the program to address different segments 20

21 8086 Register Set  These registers usually point to (contains the offset addresses) of memory locations  Unlike the Segment registers, the pointer and index registers can be used for arithmetic and other operations as well.  Each of these registers, in addition to the general purpose functionality, has a specific task 21

22 8086 Register Set  SP ( Stack Pointer) register: used in conjunction with SS to access the stack segment  BP (Base Pointer) register: used primarily to access data on the stack. However, unlike SP, the BP can be used to access data on other segments  SI ( Source Index) register: used to point to memory locations in the data segment addressed by DS. By incrementing the contents of SI, we could easily access consecutive memory locations  DI (Destination Index) register: performs the same operation as SI. There is also a class of instructions, called string operations, that use DI to access memory locations addressed by ES. 22

23 8086 Register Set  I nstruction P ointer  To access instructions, the processor 8086 uses the registers CS and IP.  The CS register holds the segment number of the next instruction  The IP register holds the offset  IP is updated each time an instruction is executed so that it will point to the next instruction  Unlike other registers, the IP can’t be directly manipulated by an instruction (i.e. the instruction can’t have IP an operand) 23

24 8086 Register Set  Its function is to indicate the status of the processor  It sets individual bits called FLAGS  There are two kinds of flags:  Status Flags : Reflects the result of an instruction executed by the processor (ex. When a subtraction results in a 0, the ZF “Zero Flag” is set to 1, a subsequent instruction can examine the ZF and branch to some code that handles a zero result)  Control Flags: enable or disable certain operations of the processor (ex. If the IF “Interrupt Flag” is set to 0, inputs from the keyboard are ignored by the processor). 24

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