Draw block diagram of 80286 microprocessor and explain its main four functional sub-units. Differentiate between Real Address Mode and Protected Virtual address mode.

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The functional block diagram of 80286 microprocessor is as given below.

Block diagram of 80286 and explain its functional units

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.

Difference between Real and Protected Virtual Addressing Mode:

The difference between real mode and protected virtual address mode are the following:

Real Address mode:
If the 80286 is operating in the real address mode, the address unit computer address using a segment base and an offset. The maximum physical address space n this mode is 1 Mbyte, Just as it is for the 8086.

After the 80286 is reset, it starts executing in its real address mode. Due to the extenments, the 80286 will execute most programs several times faster than an 8086 with the same frequency clock signal. When operating in real address mode, the interrupt vector table of the 80286 is located in the first 1 kbyte of memory.

Protected virtual address mode(PVAM):-

If the 80286 is operating in its protected virtual address mode (Protected mode) the address unit functions as a complete MMu. In this address mode the 80286 uses all 24 address lines to access up to 16 Mbytes of physical memory. If an 80286- based system is running an operating system such as Microsoft’s Os/2, which uses the protected mode. Switching an 80286 to protected mode enables the integrated MMu to provide virtual memory and protection.

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