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Chapter: Microprocessor and Microcontroller : 8086 System Bus Structure

80286 Microprocessor

Salient Features of 80286, Need for Memory Management, Concept of Virtual Memory, Concept of Virtual Memory, Internal Architecture of 80286 , Internal Block Diagram of 80286 , Interrupts of 80286 , Signal Description of 80286 , Real Address Mode , Protected Virtual Address Mode (PVAM).

Advanced processors: 80286 Microprocessor


Salient Features of 80286


ü The 80286 is the first member of the family of advanced microprocessors with memory management and protection abilities. The 80286 CPU, with its 24-bit address bus is able to address 16 Mbytes of physical memory. Various versions of 80286 are available that runs on 12.5 MHz, 10 MHz and 8 MHz clock frequencies. 80286 is upwardly compatible with 8086 in terms of instruction set.


ü 80286 has two operating modes namely real address mode and virtual address mode. In real address mode, the 80286 can address upto 1Mb of physical memory address like 8086. In virtual address mode, it can address up to 16 Mb of physical memory address space and 1 GB of virtual memory address space.


ü The instruction set of 80286 includes the instructions of 8086 and 80186. 80286 has some extra instructions to support operating system and memory management. In real address mode, the 80286 is object code compatible with 8086. In protected virtual address mode, it is source code compatible with 8086. The performance of 80286 is five times faster than the standard 8086.


Need for Memory Management


The part of main memory in which the operating system and other system programs are stored is not accessible to the users. It is required to ensure the smooth execution of the running process and also to ensure their protection. The memory management which is an important task of the operating system is supported by a hardware unit called memory management unit.


Swapping in of the Program


Fetching of the application program from the secondary memory and placing it in the physical memory for execution by the CPU.


Swapping out of the executable Program


Saving a portion of the program or important results required for further execution back to the secondary memory to make the program memory free for further execution of another required portion of the program.


Concept of Virtual Memory


Large application programs requiring memory much more than the physically available 16 Mbytes of memory, may be executed by diving it into smaller segments. Thus for the user, there exists a very large logical memory space which is not actually available. Thus there exists a virtual memory which does not exist physically in a system. This complete process of virtual memory management is taken care of by the 80286 CPU and the supporting operating system.


 Internal Architecture of 80286

Register Organization of 80286

The 80286 CPU contains almost the same set of registers, as in 8086, namely


1. Eight 16-bit general purpose registers

2. Four 16-bit segment registers

3. 3. Status and control registers

4. 4.Instruction Pointer


D2, D4, D6, D7 and D11 are called as status flag bits. The bits D8 (TF) and D9 (IF) are used for controlling machine operation and thus they are called control flags. The additional fields available in 80286 flag registers are:

1. IOPL - I/O Privilege Field (bits D12 and D13)

2. NT - Nested Task flag (bit D14)

3. PE - Protection Enable (bit D16)

4. MP - Monitor Processor Extension (bit D17)

5. EM - Processor Extension Emulator (bit D18)

6. TS – Task Switch (bit D19)


Protection Enable flag places the 80286 in protected mode, if set. This can only be cleared by resetting the CPU. If the Monitor Processor Extension flag is set, allows WAIT instruction to generate a processor extension not present exception.


Processor Extension Emulator flag if set, causes a processor extension absent exception and permits


the emulation of processor extension by the CPU.


Task Switch flag if set, indicates the next instruction using extension will generate exception 7, permitting the CPU to test whether the current processor extension is for the current task.

Machine Status Word (MSW)

The machine status word consists of four flags – PE, MO, EM and TS of the four lower order bits D19 to D16 of the upper word of the flag register. The LMSW and SMSW instructions are available in the instruction set of 80286 to write and read the MSW in real address mode.


Internal Block Diagram of 80286



The CPU contain four functional blocks

1. Address Unit (AU)

2. Bus Init (BU)

3. Instruction Unit (IU)

4. Execution Unit (EU)


The address unit is responsible for calculating the physical address of instructions and data that the CPU wants to access. Also the address lines derived by this unit may be used to address different peripherals. The physical address computed by the address unit is handed over to the bus unit (BU) of the CPU. Major function of the bus unit is to fetch instruction bytes from the memory. Instructions are fetched in advance and stored in a queue to enable faster execution of the instructions. The bus unit also contains a bus control module that controls the prefetcher module. These prefetched instructions are arranged in a 6-byte instructions queue. The 6-byte prefetch queue forwards the instructions arranged in it to the instruction unit (IU). The instruction unit accepts instructions from the prefetch queue and an instruction decoder decodes


them one by one. The decoded instructions are latched onto a decoded instruction queue. The output of the decoding circuit drives a control circuit in the execution unit, which is responsible for executing the instructions received from decoded instruction queue. The decoded instruction queue sends the data part of the instruction over the data bus. The EU contains the register bank used for storing the data as scratch pad, or used as special purpose registers. The ALU, the heart of the EU, carries out all the arithmetic and logical operations and sends the results over the data bus or back to the register bank.


Interrupts of 80286

The Interrupts of 80286 may be divided into three categories,


1. External or hardware interrupts

2. INT instruction or software interrupts

3. Interrupts generated internally by exceptions


While executing an instruction, the CPU may sometimes be confronted with a special situation because of which further execution is not permitted. While trying to execute a divide by zero instruction, the CPU detects a major error and stops further execution. In this case, we say that an exception has been generated. In other words, an instruction exception is an unusual situation encountered during execution of an instruction that stops further execution. The return address from an exception, in most of the cases, points to the instruction that caused the exception.


As in the case of 8086, the interrupt vector table of 80286 requires 1Kbytes of space for storing 256, four-byte pointers to point to the corresponding 256 interrupt service routines (lSR). Each pointer contains a 16-bit offset followed by a 16-bit segment selector to point to a particular ISR. The calculation of vector pointer address in the interrupt vector table from the (8-bit) INT type is exactly similar to 8086.


Like 8086, the 80286 supports the software interrupts of type 0 (INT 00) to type FFH (INT FFH).


Maskable Interrupt INTR: This is a maskable interrupt input pin of which the INT type is to be provided by an external circuit like an interrupt controller. The other functional details of this interrupt pin are exactly similar to the INTR input of 8086.


Non-Maskable Interrupt NMI: It has higher priority than the INTR interrupt.


Whenever this interrupt is received, a vector value of 02 is supplied internally to calculate the pointer to the interrupt vector table. Once the CPU responds to a NMI request, it does not serve any other interrupt request (including NMI). Further it does not serve the processor extension (coprocessor) segment overrun interrupt, till either it executes IRET or it is reset. To start with, this clears the IF flag which is set again with the execution of IRET, i.e. return from interrupt.


Single Step Interrupt


As in 8086, this is an internal interrupt that comes into action, if trap flag (TF) of 80286 is set. The CPU stops the execution after each instruction cycle so that the register contents (including flag register), the program status word and memory, etc. may be examined at the end of each instruction execution. This interrupt is useful for troubleshooting the software. An interrupt vector type 01 is reserved for this interrupt.


Interrupt Priorities:


If more than one interrupt signals occur simultaneously, they are processed according to their priorities as shown below:


Signal Description of 80286

CLK: This is the system clock input pin. The clock frequency applied at this pin is divided by two internally and is used for deriving fundamental timings for basic operations of the circuit. The clock is generated using 8284 clock generator.


D15-D0: These are sixteen bidirectional data bus lines.

A23-A0: These are the physical address output lines used to address memory or I/O devices. The address lines A23 - A16 are zero during I/O transfers


BHE: This output signal, as in 8086, indicates that there is a transfer on the higher byte of the data bus (D15 – D8) .


S1 , S0: These are the active-low status output signals which indicate initiation of a bus cycle and with M/IO and COD/INTA, they define the type of the bus cycle.


M/ IO: This output line differentiates memory operations from I/O operations. If this signal is it “0” indicates that an I/O cycle or INTA cycle is in process and if it is “1” it indicates

that a memory or a HALT cycle is in progress.


COD/ INTA: This output signal, in combination with M/ IO signal and S1 , S0 distinguishes different memory, I/O and INTA cycles.


LOCK: This active-low output pin is used to prevent the other masters from gaining the control of the bus for the current and the following bus cycles. This pin is activated by a "LOCK" instruction prefix, or automatically by hardware during XCHG, interrupt acknowledge or descriptor table access

READY This active-low input pin is used to insert wait states in a bus cycle, for interfacing low speed peripherals. This signal is neglected during HLDA cycle.


HOLD and HLDA This pair of pins is used by external bus masters to request for the control of the system bus (HOLD) and to check whether the main processor has granted the control (HLDA) or not, in the same way as it was in 8086.


INTR: Through this active high input, an external device requests 80286 to suspend the current instruction execution and serve the interrupt request. Its function is exactly similar to that of INTR pin of 8086.


NMI: The Non-Maskable Interrupt request is an active-high, edge-triggered input that is equivalent to an INTR signal of type 2. No acknowledge cycles are needed to be carried out.


PEREG and PEACK (Processor Extension Request and Acknowledgement)


Processor extension refers to coprocessor (80287 in case of 80286 CPU). This pair of pins extends the memory management and protection capabilities of 80286 to the processor extension 80287. The PEREQ input requests the 80286 to perform a data operand transfer for a processor extension. The PEACK active-low output indicates to the processor extension that the requested operand is being transferred.


BUSY and ERROR: Processor extension BUSY and ERROR active-low input signals indicate the operating conditions of a processor extension to 80286. The BUSY goes low, indicating 80286 to suspend the execution and wait until the BUSY become inactive. In this duration, the processor extension is busy with its allotted job. Once the job is completed the processor extension drives the BUSY input high indicating 80286 to continue with the program execution. An active ERROR signal causes the 80286 to perform the processor extension interrupt while executing the WAIT and ESC instructions. The active ERROR signal indicates to 80286 that the processor extension has committed a mistake and hence it

is reactivating the processor extension interrupt.


CAP: A 0.047 μf, 12V capacitor must be connected between this input pin and ground to filter the output of the internal substrate bias generator. For correct operation of 80286 the capacitor must be charged to its operating voltage. Till this capacitor charges to its full capacity, the 80286 may be kept stuck to reset to avoid any spurious activity.


Vss: This pin is a system ground pin of 80286.

Vcc: This pin is used to apply +5V power supply voltage to the internal circuit of 80286. RESET The active-high RESET input clears the internal logic of 80286, and reinitializes it RESET The active-high reset input pulse width should be at least 16 clock cycles. The 80286 requires at least 38 clock cycles after the trailing edge of the RESET input signal, before it makes the first opcode fetch cycle.


Real Address Mode

Act as a fast 8086

Instruction set is upwardly compatible

It address only 1 M byte of physical memory using A0-A19.

   In real addressing mode of operation of 80286, it just acts as a fast 8086. The


instruction set is upward compatible with that of 8086.


The 80286 addresses only 1Mbytes of physical memory using A0- A19. The lines A20-A23 are not used by the internal circuit of 80286 in this mode. In real address mode, while addressing the physical memory, the 80286 uses BHE along with A0- A19. The 20-bit physical address is again formed in the same way as that in 8086.


The contents of segment registers are used as segment base addresses. The other registers, depending upon the addressing mode, contain the offset addresses. Because of extra pipelining and other circuit level improvements, in real address mode also, the 80286 operates at a much faster rate than 8086, although functionally they work in an identical fashion. As in 8086, the physical memory is organized in terms of segments of 64Kbyte maximum size.


An exception is generated, if the segment size limit is exceeded by the instruction or the data. The overlapping of physical memory segments is allowed to minimize the memory requirements for a task. The 80286 reserves two fixed areas of physical memory for system initialization and interrupt vector table. In the real mode the first 1Kbyte of memory starting from address 0000H to 003FFH is reserved for interrupt vector table. Also the addresses from FFFF0H to FFFFFH are reserved for system initialization.


The program execution starts from FFFFH after reset and initialization. The interrupt vector table of 80286 is organized in the same way as that of 8086. Some of the interrupt types are reserved for exceptions, single-stepping and processor extension segment overrun, etc.

When the 80286 is reset, it always starts the execution in real address mode. In real address mode, it performs the following functions: it initializes the IP and other registers of 80286, it prepares for entering the protected virtual address mode.


Protected Virtual Address Mode (PVAM)


80286 is the first processor to support the concepts of virtual memory and memory management. The virtual memory does not exist physically it still appears to be available within the system. The concept of VM is implemented using Physical memory that the CPU can directly access and secondary memory that is used as a storage for data and program, which are stored in secondary memory initially.


The Segment of the program or data required for actual execution at that instant is fetched from the secondary memory into physical memory. After the execution of this fetched segment, the next segment required for further execution is again fetched from the secondary memory, while the results of the executed segment are stored back into the secondary memory for further references. This continues till the complete program is executed


During the execution the partial results of the previously executed portions are again fetched into the physical memory, if required for further execution. The procedure of fetching the chosen program segments or data from the secondary storage into physical memory is called swapping. The procedure of storing back the partial results or data back on the secondary storage is called unswapping. The virtual memory is allotted per task.


The 80286 is able to address 1 G byte (230 bytes) of virtual memory per task. The complete virtual memory is mapped on to the 16Mbyte physical memory. If a program larger than 16Mbyte is stored on the hard disk and is to be executed, if it is fetched in terms of data or program segments of less than 16Mbyte in size into the program memory by swapping sequentially as per sequence of execution.


Whenever the portion of a program is required for execution by the CPU, it is fetched from the secondary memory and placed in the physical memory is called swapping in of the program. A portion of the program or important partial results required for further execution, may be saved back on secondary storage to make the PM free for further execution of another required portion of the program is called swapping out of the executable program.


80286 uses the 16-bit content of a segment register as a selector to address a descriptor stored in the physical memory. The descriptor is a block of contiguous memory locations containing information of a segment, like segment base address, segment limit, segment type, privilege level, segment availability in physical memory, descriptor type and segment use another task.

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