The 8085 microprocessor is an 8 - bit microprocessor with a 40 pin dual-in-line package (DIP). The address and data bus are multiplexed in this microprocessor which helps provide more control signals.
Note that Intel 8085 has one non maskable interrupt and three maskable interrupt. It provides serial interfacing with serial input data and output data.
Classification of Intel 8085 Signals:
1. Power Supply and Frequency Signal
2. Address Signals: These signals are associated with lower order address bus and time multiplexed higher order address bus.
3. Data Signals
4. Control and Status Signals: These signals are used in the timing and controlling of microprocessors. For Example: HOLD, RD, WR
5. Interrupt Signals: Examples of interrupt signals are ==> TRAP, RST 5.5, RST 6.5
6. Serial I/O Signals: Example ==> S/O
7. Acknowledgement Signals: Examples ==> INTA, HLDA
Properties of the Control Unit
1. It generates signals in the microprocessor to enable it carry out decoded instructions.
2. It directs the flow of data in the microprocessor.
3. It directs arithmetic and logic unit operations.
Properties of the Arithmetic and Logic Unit
1. It carries out arithmetic instructions.
2. It carries out logical operations.
3. It fetches data from the memory and accumulator.
4. It stores result in the accumulator.
5. It processes data.
There are basically 8 types of 8 - bit registers which are:
1. Register A (Accumulator)
2. Register B
3. Register C
4. Register D
5. Register E
6. Register H
7. Register L
8. Flag Register
Note that data copy instructions are used to store data in registers. The flag register is not a general purpose register, only 5 bits out of the 8 bits are used. Bit positions of the flag are used to test data conditions. The programmer can examine the flag by accessing the register through an instruction such as the assembly language, JNZ, JZ.
The stack pointer is a 16-bit register. It has a last in first out (LIFO) structure. It points to a memory location in read/write memory called the STACK. The stack pointer is used to call up a sub program or a sub routine which is external to the main program being executed.
The program counter (PC) is a 16-bit register that gives sequence to the execution of instructions. It is also a memory pointer. Whan a machine code is fetched the program counter increases by one to point to the next memory location. It contains the 16-bit address of the next location to be executed.
The instruction register/decoder is a form of temporary storage the holds the operation code for the current instruction to be performed by the microprocessor in the next cycle. The decoder receives from memory prior to the execution, interprets the instruction and passes it on.
The control generator generates signals within the microprocessor and enables the microprocessor carry out decoded instructions. It opens and closes connections between the different parts of the microprocessor to enable adequate operation.
The registered selector helps in carrying out selection between different registers.
The buffer serves as an electronic connection between the external and internal data buses.
An assembly language is a symbolic language that allows the use of symbols and hexadecial numbers for programming which makes programs readable, easier to write and understandable as compared to machine language.
Assemblying is known as the translation from assembly language to machine language while assembly is the process of code conversion done by an assembler that translates an assembly language source code into a machine language.
A machine language is a series of binary digits (1's and 0's) executed by a microprocessor to accomplish individual tasks.
An object code is a halfway step between source code and executable machine language which is not directly executed by the computer and must go through a linking process that resolves external and address references.
Linking is the process of resolving external and address references of an object code of which the resulting machine language is executed by the computer.
Reasons for writing with assembly language are:
1. It is used for system programming.
2. It is very fast, flexible, versatile and produces compact code.
3. It is used to write assembly language routines that augment the capabilities of the high level languages in which the program will be used.
Assembly Language Tools are:
1. Assembler
2. Linker
3. Debugger
Types of Field are:
1. Label Field: This field gives a name to an instruction which one can use as an operand in another instruction.
There are two ways one can use a Label and they are:
A. Jump Instruction: When one wants to jump to an instruction, one gives it a label and then jumps to the instruction.
B. Data Names: One can assign a name to a data storage area in the program, then and instruction can refer to the data are by name rather than by numeric address.
2. Operation Field: This field tells the computer what to do. It states the specific operation to be carried out.
3. Operand Field: This field supplies the values on which the operation stated in the operation field is to be carried out on or the memory locations where the values can be found when necessary.
There are three basic types of operand which are:
A. Register Names
B. Memory Address
C. Immediate Data
4. Comment Field: This field is used to document the programs that are ignored by the assembler. They are just comments or guide which the programmer inserts for certain reasons eg: as a guide or reminder etc.
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