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PC IBM Organization

PC IBM Organization. 16-Bit Intel Processor Architecture. A-16 bit microprocessor can operate on 16 bits of data at a time. 8086/8088 have the simplest structure 8086/8088 have the same instruction set, it forms the basic set of instructions for other Intel families.

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PC IBM Organization

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  1. PC IBM Organization 241 Computer Organization

  2. 16-Bit Intel Processor Architecture • A-16 bit microprocessor can operate on 16 bits of data at a time. • 8086/8088 have the simplest structure • 8086/8088 have the same instruction set, it forms the basic set of instructions for other Intel families. 241 Computer Organization

  3. Example 8086 microprocessor 241 CAP Computer Organization

  4. Example 8086 microprocessor 2 main components: . Execution Unit (EU). . Bus Interface Unit (BIU). EU: ALU + Registers (AX, BX, CX, DX, SI, DI, BP, and SP) + FLAGS register. ALU: performs arithmetic & logic operations. Registers: store data FLAGS register: Individual bits reflect the result of a computation. 241 CAP Computer Organization

  5. Example 8086 microprocessor BIU: facilitates communication between the EU & the memory or I/O circuits. Responsible for transmitting addresses, data, and control signals on the buses. Registers (CS, DS, ES, SS, and IP) hold addresses of memory locations. IP (instruction pointer) contain the address of the next instruction to be executed by the EU. 241 CAP Computer Organization

  6. Organization of the 8088/8086 16-bit registers, 1M Bytes Memory Registers: Information is stored in registers Registers are classified according to the functions they perform: 241 Computer Organization

  7. Registers • Data registers: 4 general data registers hold data for an operation. • Address registers: (segment, pointer and index registers) hold the address of an instruction or data. • Status register: FLAG register keeps the current states of the processor. • 14 16-bit registers 241 Computer Organization

  8. 16-Bit Intel Processor ArchitectureRegister 241 Computer Organization

  9. General Data Register: Used for general data manipulation. • They are 16-bit registers that can also be used as two 8 bit registers: low and high bytes can be accessed separately → more registers to use when dealing with byte-size data. • In addition to being general-purpose registers, they perform special functions: 241 Computer Organization

  10. AX (Accumulator) • Most efficient register for arithmetic, logic operations and data transfer: the use of AX generates the shortest machine code. 241 Computer Organization

  11. BX (Base) • Can hold addresses (offset), ex. a table look-up table instruction XLAT (translation EBCDIC to ASCII) 241 Computer Organization

  12. CX (Counter) • Counter for looping operations: loop counter, in REP instruction, and in the shift and rotate bits 241 Computer Organization

  13. DX (Data) • Used in multiply and divide, also used in I/O operations 241 Computer Organization

  14. Address registers • Store addresses of instructions and data in memory. 241 Computer Organization

  15. The 8086 processor The 8086 processor assign a 20-bit physical address to its memory locations. 2 → 1 Mbytes 20 bits → 5 hex digits first addresses: 00000h, 00001h,…,0000Ah,… last address: FFFFFh The addresses are too big to fit in a 16-bit register. registers are 16-bits → can address only 2 = 64 K Bytes. → Partition the memory into segments 20 16 241 Computer Organization

  16. Memory Segment 16 • Is a block of 2 (64) K Bytes consecutive memory bytes. • Each segment is identified by a 16-bit number called segment number, starting with 0000 up to FFFFh . Segment registers hold segment number. • Within a segment, a memory location is specified by giving an offset (16-bit) = It is the number of bytes from the beginning of the segment (0→ FFFFh). 241 Computer Organization

  17. F0000 E0000 D0000 C0000 B0000 A0000 8000:FFFF 90000 One Segment 80000 8000:0000 70000 60000 segment offset 50000 40000 30000 20000 10000 00000 Memory Segment 241 Computer Organization

  18. Segment : Offset Address • A memory location may be specified by a segment number and offset ( logical address ). Example : A4FB : 4872 h Segment Offset 241 Computer Organization

  19. Offset : is the distance from the beginning to a particular location in the segment. • Segment number : defines the starting of the segment within the memory space. 241 Computer Organization

  20. Start location of the segment must be 20 bits  the absolute address is obtained by appending a hexadecimal zero to the segment number (shift 4-bit to the left) , i.e. multiplying by 16(10h). 241 Computer Organization

  21. Physical Address Physical Address :is equal to segment number X 10 + Offset h 241 Computer Organization

  22. Physical AddressforA4FB : 4872 A4FB0 + 4872 A9822 (20 bits ) 241 Computer Organization

  23. Location of Segments Segment 0 starts at address 0000:0000  00000 h ends at address 0000:FFFF  0FFFF h 241 Computer Organization

  24. Location of Segments Segment 1 starts at address 0001:0000  00010 h ends at address 0001:FFFF  1000F h Overlapping between segments 241 Computer Organization

  25. Segment Physical Address (hex) … 10021 10020 End of Segment 2 1001F 1001E … 10010 End of Segment 1 1000F 1000E … 10000 End of Segment 0 0FFFF 0FFFE … 00021 Start of Segment 2 00020 0001F … 00011 Start of Segment 1 00010 0000F … 00003 00002 00001 Start of Segment 0 00000 Seg.0002 Seg.0001 Seg. 0000 241 Computer Organization

  26. Location of Segments • The segments start every 10 = 16 bytes ( called Paragraph ) and the starting address of a segment always ends with a hex digit 0. • Paragraph boundary is an address divisible by 16(ends with a hex digit 0). • Because of overlapping, the segment: offset form of an address is not unique. 16 241 Computer Organization

  27. Example For the memory location whose physical address is specified by 1256A (hex) , give the address in “ segment : offset “ form for the segments 1256 & 1240 . Solution : Physical address = segment X 10 + offset offset = physical address – segment X 10 241 Computer Organization

  28. Solution • Segment 1256 : offset = 1256A - 12560 = A Address 1256 : 000A • Segment 1240 : offset = 1256A - 12400 = 0016A Address 1240 : 016A 241 Computer Organization

  29. Program Segments • A typical machine language program consists of: • instructions ( CODES ) • data • stack  is a data structure used by the processor to implement procedure calls. 241 Computer Organization

  30. Codes , data , and stack are loaded into different memory segments : • Code segment. CS : holds segment number of the code segment. • Data Segment DS : holds segment number of the data segment. ES extra segment: holds alternate segment number of the data segment. • Stack Segment SS : holds segment number of the stack segment. 241 Computer Organization

  31. Program Segment • A program segment can occupy less than 64 Kbytes. • Overlapping permits program segments that are less than 64 KB to be placed close together. 241 Computer Organization

  32. - At any time, only those memory locations addressed by the 4 segment registers are accessible; → only 4 memory segments are active. However, the contents of a segment register can be modified by a program to address different segments. 241 Computer Organization

  33. Pointer and Index Registers • Used for offset of data, often used as pointers. Unlike segment registers, they can be used in arithmetic and other operations. 241 Computer Organization

  34. Pointer Registers • SP (Stack Pointer): Used with SS for accessing the stack segment. • BP ( Base Pointer): Used with SS to access data on the stack. However, unlike SP, BP can be used to access data in other segments. 241 Computer Organization

  35. Index Registers • SI (Source Index): Source of string operations. Used with DS (or ES) to point to memory locations in the data segment. • DI (Destination Index): Destination of string operation. the same function as SI. Used with ES (or DS). 241 Computer Organization

  36. Instructionpointer To access instructions, the 8086 uses the registers CS and IP. CS contains the segment number of the next instruction. IP (Instruction pointer):Points to the next instruction (contains the offset). Used with CS. An instruction may not contain IP as its operand. 241 Computer Organization

  37. Flags register Flags:Bits specify status of CPU and information about the results of the arithmetic operations. Status flags: reflect the result of an instruction executed by the processor. Control flags: enable or disable certain operations of the processor. 241 Computer Organization

  38. Organization of the PC • A computer is made of: Hardware & software. Software controls the H/W operations. • The purpose of the OS is to coordinate the operations of all the devices that make up the computer systems. 241 Computer Organization

  39. Some of the OS functions 1) reading and executing the commands typed by the user. 2) performing I/O operations 3) generating error messages 4) managing memory and other resources. 241 Computer Organization

  40. Very popular O.S. for IBM PC is DOS. • DOS manage only 1 M byte memory, does not support multitasking. • DOS is a collection of routines that coordinates the operations of the computer. The routine that executes user command is COMMAND.COM. • Information stored on disk is organized into files. A file has a name and an optional extension. 241 Computer Organization

  41. The BIOS routines are used to perform I/O operations. • DOS routines operate over the entire PC family. • BIOS routines are machine specific. • Each PC model has its own H/W configuration and its own BIOS routines. • The compatibility of PC clones with the IBM PC depends on how well their BIOS routines match those of the IBM PC • The addresses of BIOS routines (interrupt vectors) are placed in memory starting at 00000h. 241 Computer Organization

  42. Memory Organization • 8086/8088 used 20-bit memory addresses (physical addresses go from 0 to 2^{n} – 1) • That allowed 1MB = 1,048,576 bytes of memory • IBM PC’s allowed up to 640 K RAM for the operating system and application • 360 K was reserved for system hardware (e.g. video and hard drives) and ROM 241 Computer Organization

  43. Memory Organization • Interrupt Vector Table: Lowest 1024 bytes. - each entry of this table contains the segment:offset address of an interrupt handler - this is the routine invoked when an interrupt has occurred (more later) • Bios Data Area: - serial and parallel port addresses, time and date, keyboard buffer pointers … • Software BIOS: Basic I/O routines loaded from IO.SYS. • DOS Kernel, Device Drivers, Etc: Basic operating systems routines from MSDOS.SYS. 241 Computer Organization

  44. MemoryOrganization • Resident part of COMMAND.COM: The permanently loaded code needed to interpret commands typed at the DOS prompt. • Available RAM for transient programs: Space available for applications. • Transient part of COMMAND.COM: Code from COMMAND.COM that is loaded as needed. It may disappear when applications are running. • Video Graphics Buffer: Used by EGA and VGA for graphics 241 Computer Organization

  45. MemoryOrganization • MDA Text Buffer: Storage for information displayed on a Monochrome Display Adapter. • CGA/EGA/VGA Text Buffer: Area for text. • Reserved: Examples: Code for special cards such as SCSI cards. • ROM BIOS: Provided by machine manufacturer. Code needed to boot and run the machine's hardware. It is hardware dependent. 241 Computer Organization

  46. I/O Ports Addresses • I/O devices are connected to the computer through i/o circuits. Each of them contains several registers called ports. • I/O ports have addresses I/O addresses . • 8086/8088 supports 64 KB of I/O ports. Example: keyboard controller: 60 - 63 h h 241 Computer Organization

  47. Start-up operation • When PC is powered on CS is set to FFFFh & IP is set to 0000h. PC executes the instruction with the address FFFF0h. This instruction transfers the control to the BIOS routines. 241 Computer Organization

  48. Start-up operation BIOS: • Tests the microprocessor (µp) , and the system's RAM memory. • Looks for a master boot record to which to turn over control of the computer; it checks the floppy disk drive first, then the hard disk. 241 Computer Organization

  49. Start-up operation • BIOS loads the boot program. • Boot program loads the OS and COMMAND.COM is given control 241 Computer Organization

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