The 8085 Microprocessor

Architecture:

8085 microprocessor is an 8-bit microprocessor with a 40 pin dual in line package. 8085 consists of various units and each unit performs its own functions. The various units of a microprocessor are listed below

· Accumulator

· Arithmetic and logic Unit

· General purpose register

· Program counter

· Stack pointer

· Temporary register

· Flags

· Instruction register and Decoder

· Timing and Control unit

· Interrupt control

· Serial Input/output control

· Address buffer and Address-Data buffer

· Address bus and Data bus

 

Accumulator

Accumulator is a register which can hold 8-bit data. Accumulator aids in storing two quantities. The data to be processed by arithmetic and logic unit is stored in accumulator. It also stores the result of the operation carried out by the Arithmetic and Logic unit. The accumulator is also an 8-bit register. The accumulator is connected to Internal Data bus and ALU (arithmetic and logic unit). The accumulator can be used to send or receive data from the Internal Data bus.

Arithmetic and Logic Unit

There is always a need to perform arithmetic operations like +, -, *, / and to perform logical operations like AND, OR, NOT etc. So there is a necessity for creating a separate unit which can perform such types of operations. These operations are performed by the Arithmetic and Logic Unit (ALU). ALU performs these operations on 8-bit data. But these operations cannot be performed unless we have an input (or) data on which the desired operation is to be performed. ALU gets its Input from accumulator and temporary register. After processing the necessary operations, the result is stored back in accumulator.

General Purpose Registers

Apart from accumulator 8085 consists of six special types of registers called General Purpose Registers. These general purpose registers are used to hold data like any other registers. The general purpose registers in 8085 processors are B, C, D, E, H and L. Each register can hold 8-bit data. Apart from the above function these registers can also be used to work in pairs to hold 16-bit data. They can work in pairs such as B-C, D-E and H-L to store 16-bit data. The H-L pair works as a memory pointer. A memory pointer holds the address of a particular memory location. They can store 16-bit address as they work in pair.

Program Counter and Stack Pointer

Program counter is a special purpose register. A program counter stores the address of the next instruction to be executed. In other words the program counter keeps track of the memory address of the instructions that are being executed by the microprocessor and the memory address of the next instruction that is going to be executed. Microprocessor increments the program whenever an instruction is being executed, so that the program counter points to the memory address of the next instruction that is going to be executed. Program counter is a 16-bit register.

Stack pointer is also a 16-bit register which is used as a memory pointer. A stack is nothing but the portion of RAM (Random access memory). Stack pointer maintains the address of the last byte that is entered into stack. Each time when the data is loaded into stack, Stack pointer gets decremented. Conversely it is incremented when data is retrieved from stack.

Temporary Register:

This register acts as a temporary memory during the arithmetic and logical operations. Unlike other registers, this temporary register can only be accessed by the microprocessor and it is completely inaccessible to programmers. Temporary register is an 8-bit register.

Flags

Flags are a group of individual Flip-flops. The flags are mainly associated with arithmetic and logic operations. The flags will show either a logical (0 or 1) (i.e.) a set or reset depending on the data conditions in accumulator or various other registers. A flag is actually a latch which can hold some bits of information. It alerts the processor that some event has taken place. The 8085 has a set of 5 flags.

1.      Carry flag: The carry flag is set whenever a carry is generated and reset whenever there is no carry.

2.      Parity flag: Parity checks whether it’s even or add parity. This flag returns a 0 if it is odd parity and returns a 1 if it is an even parity. Sometimes they are also called as parity bit which is used to check errors while data transmission is carried out.

3.      Auxiliary carry flag: While an auxiliary carry flag is set only when a carry is generated in bits other than the most significant bit.

4.      Zero flag: Zero flag shows whether the output of the operation is 0 or not. If the value of Zero flag is 0 then the result of operation is not zero. If it is zero the flag returns value 1.

5.      Sign flag: Sign flag shows whether the output of operation has positive sign or negative sign. A value 0 is returned for positive sign and 1 is returned for negative sign.

 

Instruction Register and Decoder

Instruction register is 8-bit register just like every other register of microprocessor. Consider an instruction. The instruction may be anything like adding two data's, moving a data, copying a data etc. When such an instruction is fetched from memory, it is directed to Instruction register. So the instruction registers are specifically to store the instructions that are fetched from memory. There is an Instruction decoder which decodes the information present in the Instruction register for further processing.

Timing and Control Unit

Timing and control unit is a very important unit as it synchronizes the registers and flow of data through various registers and other units. This unit consists of an oscillator and controller sequencer which sends control signals needed for internal and external control of data and other units. The oscillator generates two-phase clock signals which aids in synchronizing all the registers of 8085 microprocessor. Signals that are associated with Timing and control unit are:

Control Signals: READY, RD’, WR’, ALE

Status Signals: S0, S1, IO/M’

DMA Signals: HOLD, HLDA

RESET Signals: RESET IN, RESET OUT

 

Interrupt Control

As the name suggests this control interrupts a process. Consider that a microprocessor is executing the main program. Now whenever the interrupt signal is enabled or requested the microprocessor shifts the control from main program to process the incoming request and after the completion of request, the control goes back to the main program. For example an Input/output device may send an interrupt signal to notify that the data is ready for input. The microprocessor temporarily stops the execution of main program and transfers control to I/O device. After collecting the input data the control is transferred back to main program. Interrupt signals present in 8085 are:

1.      INTR

2.      RST 7.5

3.      RST 6.5

4.      RST 5.5

5.      TRAP

 

Of the above four interrupts TRAP is a NON-MASKABLE interrupt control and other three are maskable interrupts. A non-maskable interrupt is an interrupt which is given the highest priority in the order of interrupts. Suppose you want an instruction to be processed immediately, then you can give the instruction as a non-maskable interrupt. Further the non-maskable interrupt cannot be disabled by programmer at any point of time. Whereas the maskable interrupts can be disabled and enabled using EI and DI instructions. Among the maskable interrupts RST 7.5 is given the highest priority above RST 6.5 and least priority is given to INTR.

Serial I/O control

The input and output of serial data can be carried out using 2 instructions in 8085.

SID-Serial Input Data

SOD-Serial Output Data

Two more instructions are used to perform serial-parallel conversion needed for serial I/O devices.

SIM

RIM

 

Address buffer and Address-Data buffer

The contents of the stack pointer and program counter are loaded into the address buffer and address-data buffer. These buffers are then used to drive the external address bus and address-data bus. As the memory and I/O chips are connected to these buses, the CPU can exchange desired data to the memory and I/O chips. The address-data buffer is not only connected to the external data bus but also to the internal data bus which consists of 8-bits. The address data buffer can both send and receive data from internal data bus.

Address bus and Data bus:

8085 is an 8-bit microprocessor. So the data bus present in the microprocessor is also 8-bits wide. So 8-bits of data can be transmitted from or to the microprocessor. But 8085 processor requires 16 bit address bus as the memory addresses are 16-bit wide. The 8 most significant bits of the address are transmitted with the help of address bus and the 8 least significant bits are transmitted with the help of multiplexed address/data bus. The eight bit data bus is multiplexed with the eight least significant bits of address bus. The address/data bus is time multiplexed. This means for few microseconds, the 8 least significant bits of address are generated, while for next few seconds the same pin generates the data. This is called Time multiplexing. But there are situations where there is a need to transmit both data and address simultaneously. For this purpose a signal called ALE (address latch enable) is used. ALE signal holds the obtained address in its latch for a long time until the data is obtained and so when the microprocessor sends the data next time the address is also available at the output latch. This technique is called Address/Data demultiplexing.