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Basic Architecture

  • Basic components

Basic Architecture

  • Bus Architecture:
  • The Pentium bus architecture is not this simple.
    • We will elaborate on this later.

Basic Bus Architecture

  • Bus Architecture: Three buses:
  • Address:
    • If I/O, a value between 0000H and FFFFH is issued.
    • If memory, it depends on the architecture:
      • 20 -bits (8086/8088)
      • 24 -bits (80286/80386SX)
      • 25 -bits (80386SL/SLC/EX)
      • 32 -bits (80386DX/80486/Pentium)
      • 36 -bits (Pentium Pro/II/III)
  • Data:
    • 8 -bits (8088)
    • 16 -bits (8086/80286/80386SX/SL/SLC/EX)
    • 32 -bits (80386DX/80486/Pentium)
    • 64 -bits (Pentium/Pro/II/III)
  • Control:
    • Most systems have at least 4 control bus connections (active low).
    • MRDC (Memory ReaD Control), MWRC , IORC (I/O Read Control), IOWC .

Basic Bus Architecture

  • Bus Standards:
  • ISA (Industry Standard Architecture): 8 MHz
    • 8-bit (8086/8088)
    • 16-bit (80286-Pentium)


  • EISA : 8 MHz
    • 32-bit (older 386 and 486 machines).


  • PCI (Peripheral Component Interconnect): 33 MHz
    • 32-bit or 64-bit (Pentiums)


  • VESA (Video Electronic Standards Association): Runs at processor speed.
    • 32-bit or 64-bit (Pentiums)
    • Only disk and video. Competes with the PCI but is not popular.

Basic Bus Architecture

  • Bus Standards:
  • USB (Universal Serial Bus): 10 Mbps (extensions to 100Mbps)
    • Newest systems.
    • Serial connection to microprocessor.
    • For keyboards, the mouse, modems and sound cards.
  • To reduce system cost through fewer wires.


  • AGP (Advanced Graphics Port): 66MHz
    • Newest systems.
    • Fast parallel connection: Across 64-bits for 533MB/sec.
    • For video cards.
  • To accommodate the new DVD (Digital Versatile Disk) players.

Basic Memory Architecture

  • Bank layout

Basic Memory Architecture

  • Bank layout

Basic Memory Architecture

  • Bank layout

Basic I/O Architecture

Interrupt Vectors (DOS PC)

I/O Space

  • It is important to notice that these I/O addresses are NOT memory-mapped addresses on the 80x86 machines.
  • Special instructions (IN/OUT) are used to communicate to the I/O devices.

Protected Mode Memory Addressing

    • Segments are interpreted differently in Protected Mode vs. Real Mode:
  • Segment register contains a selector that selects a descriptor from the descriptor table.
  • The descriptor contains information about the segment, e.g., it's base address, length and access rights.
  • The offset can be 32-bits.

Segment Descriptors in Protected Mode


  • Base address :
    • Starting location of the memory segment.


  • Limit :
    • Length of the segment minus 1.
    • 20-bits allows segments up to 1 MB.
    • This value is shifted by 12 bits to the left when the G (Granularity bit) is set to 1.

Segment Descriptors in Protected Mode

  • Segment Descriptors: Bits 52-55
  • G bit :
    • When G=0, segments can be 1 byte to 1MB in length.
    • When G=1, segments can be 4KB to 4GB in length.


  • U bit :
    • User (OS) defined bit.


  • D bit :
    • Indicates how the instructions (80386 and up) access register and memory data in protected mode.
  • When D=0, instructions are 16-bit instructions, with 16-bit offsets and 16-bit registers. Stacks are assumed 16-bit wide and SP is used.
  • When D=1, 32-bits are assumed.
    • Allows 8086-80286 programs to run.


  • X bit :
    • Reserved by Intel

Segment Descriptors in Protected Mode

  • Segment Descriptors: Access Rights (Byte 5):
    • The Access Rights (AR) byte controls access to a protected mode segment and how the segment functions in the system.

Segment Descriptors in Protected Mode

  • Details:
    • The A (accessed) bit is set automatically by the microprocessor and is never cleared.
      • This allows OS code to track frequency of usage.


    • The P (present) bit should be interpreted as "descriptor-is-valid".
      • If this bit is 0, the microprocessor will refuse any attempts to use this descriptor in an instruction.
      • Although the AR must always be valid, when P=0, the rest of the descriptor can be used in any way the OS likes.


    • The S (system) bit indicates how the descriptor is to be interpreted.
      • S=1 indicates a system descriptor (more on this later).
      • S=0 indicates a code, data or stack descriptor.

Segment Descriptors in Protected Mode

  • Details:
    • Non-system ( S =0) segments:
  • Type=0: The data segment is basically a ROM.
  • Type=1: Both read and write operations allowed.


    • Code can NOT be fetched and executed from either of these segment types.


  • Type=2 or 3: A stack segment is defined analogously to Types 0 and 1.


    • However, the interpretation of the limit field is different.
      • In this case, all offsets must be greater than the limit.
      • The upper limit is set to base address + FFFF (with D=0) or base address + FFFFFFFF (with D=1).


    • This means the stack segment ends 1 byte below the base address.


    • Expanding of the stack segment simply involves decreasing the limit.

Segment Descriptors in Protected Mode

  • Details:
  • Type=4: A code segment with no read permission.
      • This means no constants are allowed, since they cannot be read out.


  • Type=5: A code segment in which constants may be embedded.


    • In either case, no writing (self-modifying code) is permitted.


  • Type=6 and 7: Analogous to Types 4 and 5 without privilege protection.
      • We'll discuss the meaning of "conforming" soon.

Segment Registers in Protected Mode

  • Interpretation:
  • Descriptor Index and Table Index (TI) :
    • The 13 bit descriptor index selects one of up to 8K descriptors in either the GDT and LDT, as specified by the TI bit.
      • Therefore, these 14 bits allows access to 16K 8-byte descriptors.


  • RPL :
    • The desired privilege level of the program.
    • Access is granted if the RPL value is lower (higher in privilege) than the AR of the segment. Otherwise, a privilege violation is issued.

Segmentation Address Translation

  • So instead of left shifting by 4 bits in Real Mode to form the segment address, we right shift by 3 bits and use the value as a table index.

Segmentation Address Translation

  • There are actually three different descriptor tables, GDT , LDT and IDT .
    • Exactly one GDT and IDT must be defined for Protected Mode operation.


  • Global Descriptor Table ( GDT ).
      • The GDT is used by all programs.


  • Local Descriptor Table ( LDT ).
      • An LDT can optionally be defined on a per-task basis and is used to expand the addressable range of the task.


  • Interrupt Descriptor Table ( IDT ).
      • The IDT is a direct replacement to the interrupt vector table used in 8086 systems.


  • Note that references to IDT are done through the hardware interrupt mechanism , and not from a program via a selector.

Segmentation Address Translation

  • Programmer invisible registers:
    • The GDT and IDT (and LDT) are located in the memory system.
  • The addresses of the GDT and IDT and their limits (up to 64K bytes) are loaded in special registers, GDTR and IDTR, before switching to Protected Mode is possible.

Segmentation Address Translation

  • Programmer invisible registers:
    • The other registers enclosed by the red-dotted line are part of the descriptor cache.
      • The cache is used to reduce the number of actual memory references needed to construct the physical address.


    • There is one cache register for each of the 6 segment registers, CS, DS, etc. and the LDTR (Local Descriptor Table Register) and TR (Task Register) selectors.


      • The base address, limit and access rights of the descriptor are loaded from memory every time the corresponding selector changes.


    • The LDTR and TR selectors refer to special system descriptors in the GDT.
      • These registers provide hardware acceleration support for task switching.


    • Let's first consider how LDTs are used to extend the address space of individual tasks.

Local Descriptor Tables

  • The LDTR selector indexes a GDT system descriptor describing the segment containing the LDT while the cache stores the actual LDT descriptor.
    • The LDTR selector can be loaded with a new value when another task is run.

Local Descriptor Tables

  • LDT Segment Descriptor:
    • Bit 44: The S flag is clear to indicate an LDT descriptor.


    • Bit 40-43: The Type field is extended to 4 bits (no Accessed bit). Type 2 (0010) indicates a LDT descriptor.


    • Bit 47: If the Present bit is not set (e.g. there is no LDT defined), the 80x86 will not allow you to load the LDTR with its selector.


    • Bit 0-15, 16-19: Although the limit is still 20 bits (and the G bit is also valid), segments larger than 64KB don't make sense!
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  Здравствуйте, не подскажете, как считать информацию с isa-порта в языке python?
2007-07-18 21:37:38