8086 Architecture


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8086 Architecture

  1. 1. Microprocessor A microprocessor is an electronic device which computes on the given input similar to CPU of a computer. It is made by fabricating millions(or billions) of transistors on a single chip. History of Microprocessor Microprocessor journey started with a 4-bit processor called 4004, it was made by Intel corporation in 1971. It was 1st single chip processor. Then the idea was extended to 8-bit processors like 8008, 8080 and then 8085 (all are Intel products). 8085 was a very successful one among the 8-bit processors, however its application is very limited because of its slower computing speed and other quality factors. Some years later Intel came up with its 1st 16-bit processors 8086. Intel 8086: The 8086 is a 16-bit microprocessor chip designed by Intel corporation in between early 1976 and mid-1978. The release of Intel's 8086 microprocessor in 1978 was a watershed moment for personal computing.
  2. 2. 2 8086 Architecture Elements of the 8086 Microprocessor Architecture The 80x86 has: • 16-bit internal data bus • 20-bit address bus: 220 = 1,048,576 = 1 megabyte • Control bus • Execution Unit • Bus Interface Unit Among the on-chip peripherals are: • 2 direct memory access controllers (DMA) • Three 16-bit programmable timers • Clock generator • Chip select unit • Programmable Control Registers The 8086 Processor Model The simplified block diagram of the 80x86 processor model is organized as two separate processors : 1) Bus Interface Unit (BIU) 2) Execution Unit (EU).
  3. 3. 3 8086 Architecture Bus Interface Unit (BIU) The BIU provides hardware functions. Including generation of the memory and I/0 addresses for the transfer of data between itself and the outside world. Following functions are supported by BIU.  It provides a full 16 bit bidirectional data bus and 20 bit address bus.  It sends address of memory or I/O.  It fetches instruction from memory.  It reads data from port/memory.  It writes data into port/memory.  It supports instruction queuing .  It makes 8086’s interface to the outside world.
  4. 4. 4 8086 Architecture  The BIU uses a mechanism known as an instruction stream queue to implement a pipeline architecture.  If the BIU is already in the process of fetching an instruction when the EU request it to read or write operands from memory or I/O, the BIU first completes the instruction fetch bus cycle before initiating the operand read / write cycle.  The BIU also contains a dedicated adder which is used to generate the 20bit physical address. Execution Unit (EU) The EU receives program instruction codes and data from the BIU, executes these instructions, and stores the results in the general registers. Following functions are supported by BIU.  The Execution unit is responsible for decoding and executing all instructions.  The EU extracts instructions from the top of the queue in the BIU.  During the execution of the instruction, the EU tests the status and control flags and updates them based on the results of executing the instruction.  If the queue is empty, the EU waits for the next instruction byte to be fetched and shifted to top of the queue.  The EU accesses the queue from the output end. It reads one instruction byte after the other from the output of the queue.  It tells BIU from where to fetch instructions or data, decodes instructions & execute instructions.
  5. 5. 5 8086 Architecture The main linkage between the two functional blocks is the instruction queue, with the BIU looking ahead of the current instruction being executed in order to keep the queue filled with instructions for the EU to decode and operate on. Instruction Queue • It is of 6 Bytes. • To increase the execution speed, BIU fetches as many as six instruction bytes ahead to time from memory. • It operates on the principle first in first out (FIFO). • Then all bytes are given to EU one by one. • This pre-fetching operation of BIU may be in parallel with execution operation of EU. • It improves the execution speed of the instruction. The Fetch and Execute Cycle The organization of the processor into a separate BIU and EU allows the fetch and execute cycles to overlap. To see this, consider what happens when the 8086 is first started. Figure 3.2. 1. The BIU outputs the contents of the instruction pointer register (IP) onto the address bus, causing the selected byte or word in memory to be read into the BIU. 2. Register IP is incremented by one to prepare for the next instruction fetch.
  6. 6. 6 8086 Architecture 3. Once inside the BIU, the instruction is passed to the queue: a first- in/first-out storage register sometimes likened to a pipeline. 4. Assuming that the queue is initially empty, the EU immediately draws this instruction from the queue and begins execution. 5. While the EU is executing this instruction, the BIU proceeds to fetch a new instruction. Depending on the execution time of the first instruction, the BIU may fill the queue with several new instructions before the EU is ready to draw its next instruction. 6. The cycle continues, with the BIU filling the queue with instructions and the EU fetching and executing these instructions. The BIU is programmed to fetch a new instruction whenever the queue has room for two additional bytes. The advantage to this pipelined architecture is that the EU can execute instructions (almost) continually instead of having to wait for the BIU to fetch a new instruction. This is shown schematically in the following Figure
  7. 7. 7 8086 Architecture The “Wait” mode There are three conditions that will cause the EU to enter a "wait" mode. I. When an instruction requires access to a memory location. The BIU must suspend fetching instructions and output the address of this memory location. After waiting for the memory access, the EU can resume executing instruction codes from the queue, and the BIU can resume filling the queue. II. When the instruction to be executed is a jump instruction. In this case, control is to be transferred to a new address. The EU must wait while the instruction at the jump address is fetched. Any bytes presently in the queue must be discarded (they are overwritten). III. During the execution of slow-executing instructions. The 8086 Programming Model The programming model for a microprocessor shows the various internal registers that are accessible to the programmer. The Following Figure is a model for the 8086. In general, each register has a special function.
  8. 8. 8 8086 Architecture Registers of 8086 • General Purpose Registers • Pointer and Index Registers • Segment Registers • Instruction Pointer • Status Flags
  9. 9. 9 8086 Architecture General Purpose Registers There are four 16-bit general purpose registers:  AX  BX  CX  DX Each of these 16-bit registers are further subdivided into two 8-bit registers. AX BX CX DX 1. AX Register: AX register is also known as accumulator register that stores operands for arithmetic operation like divided, rotate. 2.BX Register: This register is mainly used as a base register. It holds the starting base location of a memory region within a data segment. 3.CX Register: It is defined as a counter. It is primarily used in loop instruction to store loop counter. 4.DX Register: DX register is used to contain I/O port address for I/O instruction. AH AL BH BL CH CL BH DL
  10. 10. 10 8086 Architecture Pointers and Index Registers Following four registers are under this category: 1. Stack Pointer (SP), 2. Base Pointer (BP), 3. Source Index (SI), 4. Destination Index (DI). Following Registers can also be used as a general Purpose Registers. 1. Stack Pointer (SP) is a 16-bit register pointing to program Stack, also contains 16-Bit offset address. 2. Base Pointer (BP) is a 16-bit register pointing to data in stack segment. BP register is usually used for based indexed or register indirect addressing. 3. Source Index (SI) is a 16-bit register. SI is used for indexed, based indexed and register indirect addressing, as well as a source data address in string manipulation Instructions 4. Destination Index (DI) is a 16-bit register. DI is used for indexed, based indexed and register indirect addressing, as well as a destination data address in string manipulation instructions. Segment Register There are four segment registers in Intel 8086: 1. Code Segment Register (CS), 2. Data Segment Register (DS), 3. Stack Segment Register (SS), 4. Extra Segment Register (ES).
  11. 11. 11 8086 Architecture A segment register points to the starting address of a memory segment. For Example: The code segment register points to the starting address of the code segment. The data segment register points to the starting address of the data segment, and so on. The maximum capacity of a segment may be up to 64 KB. Code segment (CS):- It is a 16-bit register containing address of 64 KB segment with processor instructions. The processor uses CS segment for all accesses to instructions referenced by instruction pointer (IP) register. CS register cannot be changed directly. The CS register is automatically updated during far jump, far call and far return instructions . Stack segment (SS):- It is a 16-bit register containing address of 64KB segment with program stack. By default, the processor assumes that all data referenced by the stack pointer (SP) and base pointer (BP) registers is located in the stack segment. SS register can be changed directly using POP instruction. Data segment (DS):- It is a 16-bit register containing address of 64KB segment with program data. By default, the processor assumes that all data referenced by general registers (AX, BX, CX, DX) and index register (SI, DI) is located in the data segment.
  12. 12. 12 8086 Architecture Extra segment (ES):- It is a 16-bit register containing address of 64KB segment, usually with program data. By default, the processor assumes that the DI register references the ES segment in string manipulation instructions. It is possible to change default segments used by general and index registers by prefixing instructions with a CS, SS,DS or ES prefix. Concept of Segmented Memory • It allows the memory addressing capacity to be 1 Mbytes. • It allows instruction code, data stack and portion of program to be more than 64KB long. • It facilitates use of separate memory areas for program, data and stack. • It permits a program or its data to be put in different areas of memory • In this program can be relocated which is very useful in multiprogramming i.e.multitasking becomes easy.
  13. 13. 13 8086 Architecture Concept of Segmented Memory 22 Bottom Of Data Segment Top Of Data Segment Top Of Code Segment Code Segment Base CS=348AH Stack Segment Base SS=5000H Top Of Stack Segment Extra Segment Base ES=7000H Top Of Extra Segment Highest Address 20000H 2FFFFH 348A0H 4489FH 50000H 5FFFFH 7FFFFH FFFFFH 64KB 64KB 64KB 64KB Instruction Pointer The Instruction Pointer (IP) in 8086 acts as a Program Counter. It points to the address of the next instruction to be executed. Its content is automatically incremented when the execution of a program proceeds further. The contents of the IP and Code Segment Register are used to compute the memory address of the instruction code to be fetched. This is done during the Fetch Cycle. Status Flags Status Flags determines the current state of the accumulator. They are modified automatically by CPU after mathematical operations. This allows to determine the type of the result. 8086 has 16-bit status
  14. 14. 14 8086 Architecture register. It is also called Flag Register or Program Status Word (PSW). There are nine status flags and seven bit positions remain unused. OF DF IF TF SF ZF AF PF CF 25 070809101112131415 04 03 02 01 0006 05 Undefined CarryFlag Parity Flag Auxiliary CarryFlag Zero Flag SignFlag Trap Flag InterruptFlag DirectionFlag OverflowFlag The following Figure shows the bit definitions for the 16-bit flag register. • Six of the flags are status indicators reflecting properties of the result of the last arithmetic or logical instruction • 8086 flag word. DF, IF, and TF can be set or reset to control the operation of the processor. • The remaining flags are status indicators. Bits marked X are undefined.