8086 Ahad

Simplified block diagram over Intel 8088 (a variant of 8086); 1=main registers; 2=segment registers and IP; 3=address adder; 4=internal address bus; 5=instruction queue; 6=control unit (very simplified!); 7=bus interface; 8=internal data bus; 9=ALU; 10/11/12=external address/data/control bus

Intel 8086 is a 16 bit processor. It has 16-bit data bus and 20-bit address bus. The lower 16-bit Address lines and 16-bit Data lines are multiplexed (AD0-AD15). Since 20-bit address lines are available, 8086 can access up to 220 or 1 Mega byte of physical memory.

Internal architecture of 8086 2 units - Bus Interface Unit (BIU) Execution Unit (EU)

BIU contains Instruction queue, Segment registers, Instruction pointer, Address adder. EU contains Control circuitry, Instruction decoder, ALU, Pointer and Index register, Flag register.

1. Bus Interface Unit (BIU)

The Bus Interface Unit (BIU) generates the 20-bit physical memory address & provides the interface with external memory (ROM/RAM). 8086 has a single memory interface. The BIU performs all bus operations such as instruction fetching, reading and writing operands for memory and calculating the addresses of the memory operands.

Instruction Queue To speed up the execution, 6-bytes of instruction are fetched in advance and kept in a 6-byte Instruction Queue while other instructions are being executed in the Execution Unit (EU). Hence after the execution of an instruction, the next instruction is directly fetched from the instruction queue without having to wait for the external memory to send the instruction. This is called pipe-lining and is helpful for speeding up the overall execution process.

These pre-fetching instructions are held in its FIFO queue. With its 16 bit data bus, the BIU fetches two instruction bytes in a single memory cycle. After a byte is loaded at the input end of the queue, it automatically shifts up through the FIFO to the empty location nearest the output.

Four 16-bit segment registers, 8086's BIU produces the 20-bit physical memory address by combining a 16-bit segment address with a 16-bit offset address. Four 16-bit segment registers, code segment (CS), stack segment (SS), extra segment (ES), and data segment (DS).

We know - These segment registers hold the corresponding 16-bit segment addresses. A segment address  the upper 16-bits of the starting address of that segment. [16] The lower 4-bits of the starting address of a segment is always zero. The offset address is held by another 16-bit register. The physical 20-bit address is calculated by shifting the segment address 4-bit left and then adding that to the offset address.

E.g., Code segment Register CS holds the segment address  4569h. Instruction pointer IP holds the offset address  10A0h The physical 20-bit address is calculated as: Segment address :  45690h  Offset address      : + 10A0h  Physical address  :  46730h

2. Execution Unit [EU]

The Execution unit is responsible for decoding and executing all instructions. The EU extracts instructions from the top of the queue in the BIU, decodes them, generates operands if necessary, passes them to the BIU and requests it to perform the read or write  to memory or I/O and perform the operation specified by the instruction on the operands.

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. When the EU executes a branch or jump instruction, it transfers control to a location corresponding to another set of sequential instructions. Whenever this happens, the BIU automatically resets the queue and then begins to fetch instructions from this new location to refill the queue.

Common issues Internal bus is a pathway for data transfer between registers and ALU in the µPs External bus is available externally to connect to RAM, ROM and I/O Int. and Ext. Bus width may be different For example In 8088 Int. Bus is 16-bit , Ext. bus is 8-bit In 8086 Int. Bus is 16-bit , Ext. bus is 16-bit