Programming with 8085 Microprocessor

Explain briefly the programmer's model of 8085 microprocessor.

The programming model consists of some segments of the ALU and the registers. This model doesnot reflect the physical structure of 8085 but includes the information that is critical in writing the assembly language progra

Registers

8085 has six registers to store 8 bit data. They are identified as B, C, D, E, H & L. They can be combined as register pair BC, DE an HL - to perform some 16-bit operations. These registers are used to store or copy data into it by using data copy instructions.

Accumulator

The accumulator is an 8 bit register that is used to store 8 bit data and perform arithmetic and logical operation. The result of an operation is stored in accumulator.

ALU (Arithmetic Logic Unit)

This is the area of microprocessor where various computations are performed on data.

Flags

The ALU includes five flipflops, which are set or reset after an operation according to data conditions of results in the accumulator. They are Zero (Z), Carry (CY), Sign (S), Parity (P) and Auxiliary Carry (AC) flags. The microprocessor uses these flags to test data conditions. These flags have critical importance in decision making process in the microprocessor.

Memory Pointers

It is also called as stack pointer. It points to a memory location in R/W memory called as stack

Program Counter register

The microprocessor uses the PC register to sequence the execution of the instruction. i.e. it points to the memory address from which the next byte is to be fetched.

Timing and Control Unit

It provides necessary clock pulses to all the components and controls in the operation.

16-bit Address Bus

It is used to carry 16-bit address of pheripheral or memory locations.

8 bit Data Bus

It is used to transfer binary informations.

Draw the internal block diagram of the 8085 microprocessor? Explain each part in brief.

Accumulator

The accumulator is an 8 bit register that is used to store 8 bit data and perform arithmetic and logical operations. When the data is read from input, it is first moved to accumulator. When the data is output, it is first placed in the accumulator.

Temporary Register

The temporary register is an 8 bit register not accessible to the programmer. While executing the instruction the 8085 A often places the data into temporary register

Instruction Register

The instruction register receives the operation code of instructions from the internal data bus and passes it to the instruction decoder and machine cycle encoder circuit.

Instruction decoder and Machine cycle encoder

The instruction register is not directly accessible to the programmer. The instruction decoder decodes the op-code so that the microprocessor knows what type of operation is to be performed before executing it.

Register Array

8085 has six registers to store 8 bit data. They are identified as B, C, D, E, H & L. They can be combined as registers pairs BC, DE and HL - to perform some 16-bit operations. These registers are used to store or copy data into it by using data copy instructions. In addition, registers H & L are utilized to in indirect addressing mode. In this mode, the memory location whose address is specified by the content of the register.

Stack Pointer

A stack is an area of memory set aside for the purpose of storing data, by an operation known as stacking. As data is stored and retrieve, its location is specified by a 16-bit register known as stack pointer.

Program Counter

As the 8085 A begins to execute an instruction the memory address of the first instruction to be executed is placed in the register called program counter.

ALU (Arithmetic Logic Unit)

This is the area of microprocessor where various computations are performed on data.

Flags

The ALU includes five flipflops, which are set or reset after an operation, according to data conditions of result in accumulator. They are zero (Z), Carry (CY), Sign (S), Parity (P) and Auxiliary Carry (AC).

Timing and Control

It provides necessary clock pulses to all the components and controls in the operation.

Interrupt Control

An interrupt is an asynchronous process of communication with the microprocessor, initiated by an external pheripheral. It is controlled by interrupt control.

In how many ways Intel 8085 Instruction set can be classified? Explain with examples.

The Intel 8085 Instruction set can be classified into 5 categories

1. Data Transfer Group
2. Arithmetic Group
3. Logical Group
4. Branching Group
5. Miscellaneous Group.

Data Transfer Group

These are used to transfer data to register, memory or accumulator.

No flags are affected in data transfer group

Arithmetic Group

These are used to perform arithmetic operations on the accumulator.

All flags are affected in arithmetic group

Logical Group

These are used to perform logical operations on the accumulator.

All flags are affected in logical group

Miscellaneous Group.

What is the use of Program Counter and Stack pointer registers of 8085 microprocessor? How these registers get affected during CALL, RET, PUSH and POP instructions explain with suitable examples.

The program Counter stores the 16 bit Address data. The program counter stores the address of the memory that is to be executed next. So the microprocessor uses this register to sequences the execution of instructions. Depending upon the instruction type the program counter is incremented by one, two or three to point to the next memory location. Its value cannot be changed by the user

The Stack Pointer is the 16 bit register that stores the location of the bottom top of the stack. Its value decreases as the Push operation is done.

When CALL instruction is used, the Program counter is increased by 3 as the CALL instruction is 3 byte instruction.

And the address of next instruction is stored on the stack i.e. the stack pointer register is decreased by 2.

When RET instruction is used

Program Counter is increased by 1 as it is 1 byte instruction.

And the 2 byte from top of stack is transfered to program counter and program counter is changed to the data of stack.

And SP is decreased by 2 increased by 2

When PUSH or POP instruction is used, the PC is increased by 1 as it is 1 byte instruction

And, the stack pointer increases by 2 when POP is done and decreases by 2 when PUSH is done.

What is flag? Discuss about 8085 associated flags. Along with suitable examples show how these flags are affected by arithmetic and logical group of instructions.

Flags.

Registers containing of five flipflops, each holding status of different states separately is known as Flag Registers and each flip flops are called flags.

In the process of performing arithmetic and logic operations, the 8085 A can set or reset one or more of the flag.

The 8085 associated flags are:

Carry flag

Sets when Accumulator exceeds 8 bit

Zero

Sets when Accumulator is 00H

Sign

Sets when MSB of Accumulator (i.e D₇) is 1

Parity

Sets when the content of Accumulator contain even no of 1.

Auxiliary Carry

Set when D₃ generates carry.

For the program below:

Here the addition between CC & D9 is performed i.e.

Carry flag is set because The sum exceeds 8 bit.

Sign becomes set as MSB is 1.

Parity is also set because the sum has four 1.

Auxiliary Carry is also set as D₃ generates a carry.

Zero is reset

For program below:

Here the data in Accumulator is 35H. So when CPI 35H is performed, the carry flag becomes reset and zero flag becomes set.

When CPI 37H is performed, the carry flag becomes set and zero flag becomes reset.

When ANI 00H is performed the Auxiliary Carry flag is set Carry flag is reset and Zero flag is set.

What do you mean by instructions? Explain how instruction and opcodes are designed in microprocessor. Describe about data format used in microprocessor.

Instructions.

An instruction is a command to the microprocessor to perform a given task on specified data.

Each instructions have two parts

• Operation code
• Operands.

The memory of the microprocessor consists of various memory locations called words of 40 binary digits each. Both data and instructions are stored there. Here numbers must be represented in binary form, each instructions also has to be in binary code. A word may also contain two 20-bit instruction with each instructions consisting of an 8 bit op code, specifying the operation to be performed and a 12 bit address designating one of the word in the memory. So the instructions consists of mainly two fields op-code and the address. The opcond determines which operation is to be performed.

The data format used in microprocessor are explained as:-

i) ASCII code

7 bit alphanumeric code that represents decimal numbers, alphabets and non-pointable characters

ii) BCD code

Decimal bit from 0 to 9 are represented by 4 binary bits and (A-F) are invalid.

iii) Signed integers

Either positive or negative

D₇ of 8 bit number is specified as sign.

iv) Unsigned integer

All integer without a sign can be represented by the 8 bit in a microprocessor register.

What do you mean by Addressing modes in microprocessor? Explain the different addressing modes of 8085 microprocessor along with examples.

Addressing mode in microprocessor.

An instruction consists of op code and operands. The operands may be source only, destination only or both. The source can be a register, an input port or a memory. The destination can be a register, an output port or a memory. This various format of specifying the operand are called addressing mode.

8085 microprocessor instruction set have 5 different addressing modes.

i) Direct Addressing mode

Instructions using this mode may contain 2 or 3 bytes, with 1st byte as the upcode followed by 1 or 2 bytes of address of data

ii) Register indirect addressing mode

In this mode, the address part of the instruction specifies the memory location whose content is the address of operand.

iii) Register direct Addressing mode

In this mode, of addressing, specifies the register or register pair that contains the data

iv) Immediate Addressing mode

For a 8 bit data, this mode uses 2 bytes with the 1st byte as the opcode followed by 1 byte of data

For 16 bit data, this mode uses 3 bytes with the 1st byte as the opcode followed by 2 byte of data

v) Implied/Inherent addressing mode

The instruction of these mode doesnot have operands

What do you mean by stack and subroutine? What is the purpose of stack in subroutine call? Explain with suitable example.

Stack

Stack is the memory address pointed by stack pointer which the data Last entered are pushed are poped out i.e. LIFO type.

Subroutine

A subroutine is a group of instructions that performs a subtask (arithmetic operation or time delay) of repeated occurance.

The CALL instruction is used in main program to call a subroutine and RET instruction is used at the end of the subroutine to return to the main program.

When a subroutine is called, the content of the program counter, which is the address of instruction following the CALL instruction is stored on the stack and the program execution is transferred to the subroutine address. When RET instruction is executed at the end of the subroutine the memory address stored on the stack is retrieved and the sequence is resumed in the main program.

What do you mean by instruction sheet? What is the purpose of branching instructions? Explain all of them with examples. State why branching instructions are termed as most powerful instructions.

Instruction sheet

Each microprocessor can handle predefined number of instructions. The sheet, which contains all these instructions with their hex code, mnemonics, descriptions and functions is called instruction sheet.

Instruction sheet can be available in different formats with different information

The purpose of branching instruction is to goto a label or a address when called.

I Unconditional jump

JMP 16 bit address/LABEL

Used to jump when called.

II Conditional jump

Here, A is 00H. So Z = 1 and JZ jumps to NEXT skipping other lines.

The branching instruction are termed as the most powerful instruction because these allows the microprocessor to change the program. The microprocessor is a sequential machine and it executes machine codes from one memory location to the next but branch instructions instructs the microprocessor to go to the different memory location and microprocessor continues executing machine codes from that location.

Compare CALL and RET instruction with PUSH and POP instruction for 8085. Illustrate their usages with appropriate examples.

CALL instruction calls a subroutine unconditionally. With the instruction is called, it saves the content of the program counter (i.e. the address of next instruction) on the stack and decrements the stack pointer register by 2.

PUSH instruction also decreases the stack pointer by 2, but it saves the content of the Rₚ when called i.e the data we want to save is saved in the stack top.

The RET instruction returns the subroutine unconditionally. With this instruction, it inserts the two byte from the top of the stack into the program counter and increments stack pointer by 2.

The POP instruction when called, it also increases the stack pointer by 2 but it inserts the two byte of the top of the stack into the register pair as the data to be executed.

And for POP and PUSH

Here for PUSH and POP instruction, the PC goes in the sequential way as the microprocessor works and when PUSH instruction is called the SP decreases by 2 and the data of the Rₚ is stored in the stack. When POP instruction is called SP increases by 2 and the data of the stack is transferred to the register pair.