Architecture of 8085

Architecture of 8085

AU : May-04, 10, 11, 12, 16, 17, 18, Dec.-04, 07, 08, 09, 10, 13, 15, 16, 17, 18, 19, June-06, 12

Fig. 1.2.1 shows the architecture of 8085.

It consists of various functional blocks as listed below :

• Registers  • Incrementer/Decrementer address latch

• Arithmetic and Logic Unit • Interrupt control

• Instruction decoder and machine cycle encoder   • Serial I/O control

• Address buffer   • Timing and control circuitry.

• Address/Data buffer   

 

1. Register Structure

The Fig. 1.2.2 shows the register structure of 8085. The shaded portion of this register model is called programmer's model of 8085. It includes six 8-bit registers- (B, C, D, E, H and L) one accumulator, one flag register and two 16-bit registers (SP and PC). All these registers are accessible to programmer and hence they are included in the programmer's model. The remaining registers - temporary, W and Z are not accessible to the programmers; they are used by microprocessor for internal, intermediate operations.

1. General Purpose Registers :

C, D, E, H and L are 8-bit general purpose registers can be used as a separate 8-bit registers or as 16-bit register pairs, BC, DE and HL. HL pair also functions as a data pointer or memory pointer.

2. Special Purpose Registers :

a) Register A (Accumulator) : It is a tri-state eight bit register. It is extensively used in arithmetic, logic, load and store operations, as well as in, input/output (I/O) operations. Most of the times the result of arithmetic and logical operations is stored in the register A. Hence it is also identified as accumulator.

b) Flag Register : It is an 8-bit register, in which five of the bits carry significant information in the form of flags : S (Sign flag), Z (Zero flag), AC (Auxiliary Carry flag), P (Parity flag), and CY (Carry flag), as shown in Fig. 1.2.3.

S-Sign flag : After the execution of arithmetic or logical operations, if bit D₇ of the result is 1, the sign flag is set. In a given byte if D₇ is 1, the number will be viewed as negative number. If D₇ is 0, the number will be considered as positive number.

Z-Zero flag : The zero flag sets if the result of operation in ALU is zero and flag resets if result is non zero. The zero flag is also set if a certain register content becomes zero following an increment or decrement operation of that register.

AC-Auxiliary Carry flag : This flag is set if there is an overflow out of bit 3 i.e. , carry from lower nibble to higher nibble (D₃ bit to D₄ bit). This flag is used for BCD operations and it is not available for the programmer.

P-Parity flag : Parity is defined by the number of ones present in the accumulator. After an arithmetic or logical operation if the result has an even number of ones, i.e. even parity, the flag is set. If the parity is odd, flag is reset.

CY-Carry flag : This flag is set if there is an overflow out of bit 7. The carry flag also serves as a borrow flag for subtraction. In both the examples shown below, the carry flag is set.

c) Instruction Register : In a typical processor operation, the processor first fetches the opcode of instruction from memory (i.e. it places an address on the address bus and memory responds by placing the data stored at the specified address on the data bus). The CPU stores this opcode in a register called the instruction register. This opcode is further sent to the instruction decoder to select one of the 256 alternatives.

d) Program Counter (PC) : Program is a sequence of instructions. As mentioned earlier, microprocessor fetches these instructions from the memory and executes them sequentially. The program counter is a special purpose register which, at a given time, stores the address of the next instruction to be fetched. Program Counter acts as a pointer to the next instruction. How processor increments program counter depends on the nature of the instruction; for one byte instruction it increments program counter by one, for two byte instruction it increments program counter by two and for three byte instruction it increments program counter by three such that program counter always points to the address of the next instruction.

e) Stack Pointer (SP) : The stack is a reserved area of the memory in the RAM where temporary information may be stored. A 16-bit stack pointer is used to hold the address of the most recent stack entry.

 

2. Arithmetic Logic Unit (ALU)

The 8085's ALU performs arithmetic and logical functions on eight bit variables. The arithmetic unit performs bitwise fundamental arithmetic operations such as addition and subtraction. The logic unit performs logical operations such as complement, AND, OR and EX-OR, as well as rotate and clear. The ALU also looks after the branching decisions.

 

3. Instruction Decoder and Machine Cycle Encoder

The instruction decoder decodes the opcode and accordingly gives information to the timing and control circuit.

The 8085 executes seven different types of machine cycles. It gives the information about which machine cycle is currently executing in the encoded form on the S₀, S₁ and ppppppppp lines. This task is done by machine cycle encoder.

 

4. Address Buffer

This is an 8-bit unidirectional tristate buffer. It is used to drive external high order address bus (A₁₅-A₈).

 

5. Address/Data Buffer

This is an 8-bit bi-directional buffer. It is used to drive multiplexed address/data bus, i.e. low order address bus (A₇ – A₀) and data bus (D₇ – D₀).

 

6. Incrementer/Decrementer Address Latch

This 16-bit register is used to increment or decrement the contents of program counter or stack pointer.

 

7. interrupt control

 The interrupt control block has five interrupt inputs RST 5.5, RST 6.5, RST 7.5, TRAP and INTR and one acknowledge signal . It controls the interrupt activity of 8085 microprocessor.

 

8. serial I/O control

The 8085's serial I/O control provides two lines, SOD and SID for serial communication. The serial output data (SOD) line is used to send data serially and serial input data (SID) line is used to receive data serially.

 

9. Timing and Control Circuitry

The control circuitry in the processor 8085 is responsible for all the operations. The control circuitry and hence the operations in 8085 are synchronized with the help of clock signal. Along with the control of fetching and decoding operations and generating appropriate signals for instruction execution, control circuitry also generates signals required to interface external devices to the processor, 8085.

 

10. Instruction Execution and Data Flow in 8085

• In order to execute program, the starting address of the program is loaded into the Program Counter (PC).

• The 8085 sends the contents of PC as an address on the address bus and activates  control signal.

• Upon receiving the address and  signal memory puts the contents of addressed memory location on the data bus which is an opcode of an instruction. Meanwhile, PC is incremented to point the next memory location in the program sequence.

•  signal is deactivated and opcode is loaded into the instruction register via internal bus of microprocessor.

• The instruction decoding unit decodes the instruction and provides information to the timing and control unit to generate necessary signals for instruction execution.

Review Questions

1. With neat functional block diagram, explain the architecture of 8085 microprocessor.

AU : June-06, Dec.-04,07, 08,09,10,19, May-04,10,11,12, Marks 16

2. Draw and explain the architecture of 8085.

3. Give the format of flag register in 8085. Explain each flag.

4. Define the function of parity flag and zero flag in 8085.

5. Explain the architecture, data flow and instruction execution of 8085 microprocessor.

AU : May-11, Marks 8

6. Explain with a neat block diagram the architecture of 8085 microprocessor.

AU : May-12, 16, Dec.-15, 16, 17, Marks 12

7. Explain the registers of 8085 microprocessor.

8. Briefly describe stack pointer register.

9. With neat block diagram explain the various functional building blocks of 8085 processor

AU : May-18, Dec.-18, Marks 13