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These notes provide a comprehensive overview of the 8085 microprocessor, covering its architecture, programming model, instruction set, and addressing modes. The document delves into the internal components of the 8085, including the control unit, alu, registers, and bus structure. It also explores the 8085's instruction set, categorized by function and word size, and explains different addressing modes used in programming. This resource is valuable for students and professionals seeking a thorough understanding of the 8085 microprocessor.
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Submitted To: Submitted By: Patna University Dr. Gargi Tiwari Guest Asst. Prof. (Physics)
Microprocessor is a Central Processing Unit (CPU) etched on a single chip. A single Integrated Circuit (IC) has all the functional components of a CPU namely Arithmetic Logic Unit (ALU), Control Unit and registers. The 8085 microprocessor is an 8-bit processor that includes on its chip most of the logic circuitry for performing computing tasks and for communicating with peripherals.
What about micro? • Micro is a new addition. – In the late 1960 ’s, processors were built using discrete elements. • These devices performed the required operation, but were too large and too slow. – In the early 1970 ’s the microchip was invented. All of the components that made up the processor were now placed on a single piece of silicon. The size became several thousand times smaller and the speed became several hundred times faster. The “Micro”Processor was born.
Programmable device: The microprocessor can perform different sets of operations on the data it receives depending on the sequence of instructions supplied in the given program. By changing the program, the microprocessor manipulates the data in different ways. – Instructions: Each microprocessor is designed to execute a specific group of operations. This group of operations is called an instruction set. This instruction set defines what the microprocessor can and cannot do. Takes in: The data that the microprocessor manipulates must come from somewhere. • It comes from what is called “input devices”. • These are devices that bring data into the system from the outside world. • These represent devices such as a keyboard, a mouse, switches, and the like. Numbers: The microprocessor has a very narrow view on life. It only understands binary numbers. A binary digit is called a bit (which comes from binary digit). The microprocessor recognizes and processes a group of bits together. This group of bits is called a “word”. The number of bits in a Microprocessor’s word, is a measure of its “abilities”.
Arithmetic and Logic Operations: Every microprocessor has arithmetic operations such as add and subtract as part of its instruction set. – Most microprocessors will have operations such as multiply and divide. – Some of the newer ones will have complex operations such as square root. • In addition, microprocessors have logic operations as well. Such as AND, OR, XOR, shift left, shift right, etc. • Again, the number and types of operations define the microprocessor’s instruction set and depends on the specific microprocessor. Stored in memory : First, what is memory? – Memory is the location where information is kept while not in current use. – Memory is a collection of storage devices. Usually, each storage device holds one bit. Also, in most kinds of memory, these storage devices are grouped into groups of 8. These 8 storage locations can only be accessed together. So, one can only read or write in terms of bytes to and form memory. – Memory is usually measured by the number of bytes it can hold. It is measured in Kilos, Megas and lately Gigas. A Kilo in computer language is 210 =1024. So, a KB (KiloByte) is 1024 bytes. Mega is 1024 Kilos and Giga is 1024 Mega. When a program is entered into a computer, it is stored in memory. Then as the microprocessor starts to execute the instructions, it brings the instructions from memory one at a time. • Memory is also used to hold the data. – The microprocessor reads (brings in) the data from memory when it needs it and writes (stores) the results into memory when it is done.
Machine Language
The number of bits that form the “word” of a microprocessor is fixed for that particular processor. – These bits define a maximum number of combinations.
For example an 8-bit microprocessor can have at most 28 = 256 different combinations.
However, in most microprocessors, not all of these combinations are used. – Certain patterns are chosen and assigned specific meanings. – Each of these patterns forms an instruction for the microprocessor. – The complete set of patterns makes up the microprocessor’s machine language.
The 8085 Machine Language
The 8085 (from Intel) is an 8-bit microprocessor. – The 8085 uses a total of 246 bit patterns to form its instruction set. – These 246 patterns represent only 74 instructions. The reason for the difference is that some (actually most) instructions have multiple different formats. Because it is very difficult to enter the bit patterns correctly, they are usually entered in hexadecimal instead of binary. For example, the combination 0011 1100 which translates into “increment the number in the register called the accumulator”, is usually entered as 3C.
00111100 translates to 3C in hexadecimal (OPCODE)
accumulator.
- Microprocessor consists of: - Control unit: control microprocessor operations. - ALU: performs data processing function. - Registers: provide storage internal to CPU. - Interrupts - Internal data bus
In addition to the arithmetic & logic circuits, the ALU includes the accumulator, which is part of every arithmetic & logic operation. • Also, the ALU includes a temporary register used for holding data temporarily during the execution of the operation. This temporary register is not accessible by the programmer.
General Purpose Registers • B, C, D, E, H & L (8 bit registers) • Can be used singly • Or can be used as 16 bit register pairs – BC, DE, HL • H & L can be used as a data pointer (holds memory address)
Special Purpose Registers • Accumulator (8 bit register) – Store 8 bit data – Store the result of an operation – Store 8 bit data during I/O transfer
Flag Register 8 bit register – shows the status of the microprocessor before/after an
operation
- S (sign flag), Z (zero flag), AC (auxillary carry flag), P (parity flag) &
CY (carry flag)
- Sign Flag - Used for indicating the sign of the data in the accumulator - The sign flag is set if negative (1 – negative) - The sign flag is reset if positive (0 – positive)
Features 8 bit microprocessor (8085 microprocessor can read or write or perform arithmetic and logical operations on 8-bit data at time) It has 8 data lines and 16 address lines hence capacity is 216 = 64 kB of memory Cock frequency is 3 MHz It requires +5V power supply. It is a single chip NMOS device implemented with 6200 transistors. It provides 74 instructions with five addressing modes. It provides 5 hardware interrupt and 8 software interrupts. Pin Configuration 40 pins classified into 6 groups:
The microprocessor 8085 has 16 bit address
lines from A15-A8 and AD7-AD0. These lines are
used to transfer 16 bit address of memory as well as
8-bit address of I/O ports.
The lower 8 lines (AD7-AD0) are often called
as multiplexed data lines.
SPECIAL SIGNAL
ALE (Address Latch Enable): The ALE signal is
used to enable or disable the external latch IC (74373/8212).
The external latch IC is used for the de-multiplexing
of AD7-AD0 lines, i.e., it is used to separate the address and data from AD7-AD0 lines.
If ALE = 1/0 then external latch IC is enabled /
disabled respectively.
