Question

Describe mechanism for generation of physical address in 8086 microprocessor with example

Answer 1

Generation of 20 bit physical address:

• 8086 has 20bit address line. So the maximum value of address that can be addressed by 8086 is 2^20 = 1MB. So 8086 can address the locations ranging between 00000 H to FFFFF H. This 1MB memory is divided into 16 logical segments, each with a memory of 64KB.

• To provide flexible base addressing and indexing, a data address may be formed by adding together a combination of the contents of BX or BP register and displacement. The result of such address computation is called 'effect address' ( EA ) or 'offset address'. The 20 bit physical address is determined by combining the 'EA' and the contents of the respective segment register.

Calculation of physical address:

physical address = segment address×10H + offset Address.

Example:

If cøde segment is 1234H and offset address is 0002H then find physical address.

physical address = CS×10H+OA

=1234H×10H+ 0002H

= 12340H + 0002H

= 12342H

Therefore 20 bit physical address is '12342H'.