8086 Architecture BIU (Bus Interface Unit):- It sends out tasks. It fetches instructions from memory. It reads data from memory and ports. It also writes data from memory and ports. So BIU takes care of all the address and data transfers on the buses.
Execution Unit:- It tells the BIU, from where to fetch the instruction or data. It decodes the fetched instructions. It executes the fetched instructions. EU takes care of performing operations on data. The Bus Interface unit (BIU):- This unit handles all transfer of data and addresses on the buses for the EU(execution unit). This unit sends out addresses, fetches instructions from memory, reads data from ports and memory and writes data to ports and memory. Different Parts of BIU: a. Segment Register b. Instruction Pointer c. The Queue
1.)Segment Register:- BIU contains four 16-bit segment registers as follows: Code segment (CS) register Stack segment (SS) register Extra segment (ES) register Data segment (DS) register Function of Segment Register:- In 8086 complete 1MB memory is divided into 16 logical segments. Each segment thus contains 64 KB of memory. While addressing any location in the memory bank, the Physical address is calculated from two parts, the first part is Segment address, and the second is Offset. The segment registers contain 16-bit segment base addresses related to different segments. Thus the CS, DS, ES, SS segment registers, respectively contain the segment addresses for the Code, Data, Extra and Stack segments. They may or may not be physical separated.
Each segment register contains a 16-bit base address that points to the lowest-addressed byte of that particular segment in memory. Generation of physical address:- Segment address- 1005H Offset address - 5555H Segment address-1005h- 0001 0000 0000 0101 Shifted by 4-bit positions-0001 0000 0000 0101 0000 + Offset address - 0101 0101 0101 0101 Physical address -0001 0101 0101 1010 0101 1 5 5 A 5 Instruction Pointer:- It is 16-bit register, which identifies the location of the next word of instruction code that is to be fetched in the current code segment. IP contains an offset instead of the actual address of the next instruction.
The 20-bit address produced after addition of the offset stored in IP to segment base address in the CS is called the Physical address of the code byte. The Queue:- The last section of BIU is the FIFO group of registers called a queue. It is basically a group of registers. This arrangement makes possible for the BIU to fetch the instruction byte while EU is decoding an instruction or executing an instruction which does not require use of buses. This arrangement is called pipelining. This is done to speed up the program execution. Execution Unit (EU) : Execution unit receives program instruction codes and data from the BIU, executes them and stores the results in the general registers. It can also store the data in a memory location or send them to an I/O device by passing the data back to the BIU. This unit, EU, has no connection with the system Buses. It receives and outputs all its data through BIU.
ALU (Arithmetic and Logic Unit) : The EU unit contains a circuit board called the Arithmetic and Logic Unit. The ALU can perform arithmetic, such as, +,-,,/ and logic such as OR, AND, NOT operations. Register : A register is like a memory location where the exception is that these are denoted by name rather than numbers. It has 4 data registers, AX, BX, CX, DX and 2 pointer registers SP, BP and 2 index registers SI, DI and 1 temporary register and 1 status register FLAGS. AX, BX, CX and DX registers has two 8-bit registers to access the high and low byte data registers. The high byte of AX is called AH and the low byte is AL. Similarly, the high and low bytes of BX, CX, DX are BH and BL, CH and Cl, DH and DL respectively. All the data, pointer, index and status registers are of 16 bits. Else these, the temporary register holds the operands for the ALU and the individual bits of the FLAGS register reflect the result of a computation. Memory Segmentation The total memory size is divided into segments of various sizes. A segment is just an area in memory. The process of dividing memory this way is called Segmentation.
In memory, data is stored as bytes. Each byte has a specific address. Intel 8086 has 20 lines address bus. With 20 address lines, the memory that can be addressed is 2^20bytes. 2^20= 1,048,576 bytes (1 MB). 8086 can access memory with address ranging from 00000 H to FFFFF H. In 8086, memory has four different types of segments. These are: Each of these segments are addressed by an address stored in corresponding segment register. -bit in size. corresponding segment.
the upper 16 bits nnot store 20 bits, they only store. IP - the instruction pointer: Always points to next instruction to be executed Offset address relative to CS IP register always works together with CS segment register and it points to currently executing instruction. FLAGS REGISTER Flags Register - determines the current state of the processor. They are modified automatically by CPU after mathematical operations, this allows to determine the type of the result, and to determine conditions to transfer control to other parts of the program. Generally you cannot access these registers directly. Carry Flag (CF) - this flag is set to 1 when there is an unsigned overflow. For example when you add bytes 255 + 1 (result is not in range 0...255). When there is no overflow this flag is set to 0. Parity Flag (PF) - this flag is set to 1 when there is even number of one bits in result, and to 0 when there is odd number of one bits. Auxiliary Flag (AF) - set to 1 when there is an unsigned overflow for low nibble (4 bits).
Zero Flag (ZF) - set to 1 when result is zero. For non-zero result this flag is set to 0. Sign Flag (SF) - set to 1 when result is negative. When result is positive it is set to 0. (This flag takes the value of the most significant bit.) Trap Flag (TF) - Used for on-chip debugging. Interrupt enable Flag (IF) - when this flag is set to 1 CPU reacts to interrupts from external devices. Direction Flag (DF) - this flag is used by some instructions to process data chains, when this flag is set to 0 - the processing is done forward, when this flag is set to 1 the processing is done backward. Overflow Flag (OF) - set to 1 when there is a signed overflow. For example, when you add bytes 100 + 50 (result is not in range - 128...127).