Control Unit :
Generates signals within uP to carry out the instruction, which has been decoded. In
reality causes certain connections between blocks of the uP to be opened or closed, so
that data goes where it is required, and so that ALU operations occur.
Arithmetic Logic Unit
The ALU performs the actual numerical and logic operation such as ‘add’, ‘subtract’,
‘AND’, ‘OR’, etc. Uses data from memory and from Accumulator to perform
arithmetic. Always stores result of operation in Accumulator.
one flag register, as shown in Figure. In addition, it has two 16-bit registers: the stack
pointer and the program counter. They are described briefly as follows.
The 8085/8080A has six general-purpose registers to store 8-bit data; these are
identified as B,C,D,E,H, and L as shown in the figure. They can be combined as
register pairs - BC, DE, and HL - to perform some 16-bit operations. The
programmer can use these registers to store or copy data into the registers by using
data copy instructions.
register is used to store 8-bit data and to perform arithmetic and logical operations.
The result of an operation is stored in the accumulator. The accumulator is also
identified as register A.
The ALU includes five flip-flops, which are set or reset after an operation according
to data conditions of the result in the accumulator and other registers. They are called
Zero(Z), Carry (CY), Sign (S), Parity (P), and Auxiliary Carry (AC) flags; they are
listed in the Table and their bit positions in the flag register are shown in the Figure
below. The most commonly used flags are Zero, Carry, and Sign. The microprocessor
uses these flags to test data conditions.
For example, after an addition of two numbers, if the sum in the accumulator id larger
than eight bits, the flip-flop uses to indicate a carry -- called the Carry flag (CY) -- is
set to one. When an arithmetic operation results in zero, the flip-flop called the
Zero(Z) flag is set to one. The first Figure shows an 8-bit register, called the flag
register, adjacent to the accumulator. However, it is not used as a register; five bit
positions out of eight are used to store the outputs of the five flip-flops. The flags are
stored in the 8-bit register so that the programmer can examine these flags (data
conditions) by accessing the register through an instruction. These flags have critical importance in the decision-making process of the micro-
processor. The conditions (set or reset) of the flags are tested through the software
instructions. For example, the instruction JC (Jump on Carry) is implemented to
change the sequence of a program when CY flag is set. The thorough understanding
of flag is essential in writing assembly language programs.
is a memory pointer. Memory locations have 16-bit addresses, and that is why this is a
16-bit register.
The microprocessor uses this register to sequence the execution of the instructions.
The function of the program counter is to point to the memory address from which the
next byte is to be fetched. When a byte (machine code) is being fetched, the program
counter is incremented by one to point to the next memory location
memory location in R/W memory, called the stack. The beginning of the stack is
defined by loading 16-bit address in the stack pointer. The stack concept is explained
in the chapter "Stack and Subroutines."
Instruction Register/Decoder
Temporary store for the current instruction of a program. Latest instruction sent here
from memory prior to execution. Decoder then takes instruction and ‘decodes’ or
interprets the instruction. Decoded instruction then passed to next stage.
Memory Address Register
Holds address, received from PC, of next program instruction. Feeds the address bus
with addresses of location of the program under execution.
reality causes certain connections between blocks of the uP to be opened or closed, so
that data goes where it is required, and so that ALU operations occur.
Register Selector
This block controls the use of the register stack in the example. Just a logic circuit
which switches between different registers in the set will receive instructions from
Control Unit. General Purpose Registers
uP requires extra registers for versatility. Can be used to store additional data during a
program. More complex processors may have a variety of differently named registers.
binary number? The microprogram in a uP/uC is written by the chip designer and tells
the uP/uC the meaning of each instruction uP/uC can then carry out operation.
------------------------- Next>> 8085 SYSTEM BUS DESCRIPTION<<----------------------------
Generates signals within uP to carry out the instruction, which has been decoded. In
reality causes certain connections between blocks of the uP to be opened or closed, so
that data goes where it is required, and so that ALU operations occur.
Arithmetic Logic Unit
The ALU performs the actual numerical and logic operation such as ‘add’, ‘subtract’,
‘AND’, ‘OR’, etc. Uses data from memory and from Accumulator to perform
arithmetic. Always stores result of operation in Accumulator.
 |
| Internal Architecture of 8085 |
Registers :
The 8085/8080A-programming model includes six registers, one accumulator, andone flag register, as shown in Figure. In addition, it has two 16-bit registers: the stack
pointer and the program counter. They are described briefly as follows.
The 8085/8080A has six general-purpose registers to store 8-bit data; these are
identified as B,C,D,E,H, and L as shown in the figure. They can be combined as
register pairs - BC, DE, and HL - to perform some 16-bit operations. The
programmer can use these registers to store or copy data into the registers by using
data copy instructions.
Accumulator :
The accumulator is an 8-bit register that is a part of arithmetic/logic unit (ALU). Thisregister is used to store 8-bit data and to perform arithmetic and logical operations.
The result of an operation is stored in the accumulator. The accumulator is also
identified as register A.
Flags :
The ALU includes five flip-flops, which are set or reset after an operation according
to data conditions of the result in the accumulator and other registers. They are called
Zero(Z), Carry (CY), Sign (S), Parity (P), and Auxiliary Carry (AC) flags; they are
listed in the Table and their bit positions in the flag register are shown in the Figure
below. The most commonly used flags are Zero, Carry, and Sign. The microprocessor
uses these flags to test data conditions.
For example, after an addition of two numbers, if the sum in the accumulator id larger
than eight bits, the flip-flop uses to indicate a carry -- called the Carry flag (CY) -- is
set to one. When an arithmetic operation results in zero, the flip-flop called the
Zero(Z) flag is set to one. The first Figure shows an 8-bit register, called the flag
register, adjacent to the accumulator. However, it is not used as a register; five bit
positions out of eight are used to store the outputs of the five flip-flops. The flags are
stored in the 8-bit register so that the programmer can examine these flags (data
conditions) by accessing the register through an instruction. These flags have critical importance in the decision-making process of the micro-
processor. The conditions (set or reset) of the flags are tested through the software
instructions. For example, the instruction JC (Jump on Carry) is implemented to
change the sequence of a program when CY flag is set. The thorough understanding
of flag is essential in writing assembly language programs.
Program Counter (PC)
This 16-bit register deals with sequencing the execution of instructions. This registeris a memory pointer. Memory locations have 16-bit addresses, and that is why this is a
16-bit register.
The microprocessor uses this register to sequence the execution of the instructions.
The function of the program counter is to point to the memory address from which the
next byte is to be fetched. When a byte (machine code) is being fetched, the program
counter is incremented by one to point to the next memory location
 |
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Stack Pointer (SP)
The stack pointer is also a 16-bit register used as a memory pointer. It points to amemory location in R/W memory, called the stack. The beginning of the stack is
defined by loading 16-bit address in the stack pointer. The stack concept is explained
in the chapter "Stack and Subroutines."
Instruction Register/Decoder
Temporary store for the current instruction of a program. Latest instruction sent here
from memory prior to execution. Decoder then takes instruction and ‘decodes’ or
interprets the instruction. Decoded instruction then passed to next stage.
Memory Address Register
Holds address, received from PC, of next program instruction. Feeds the address bus
with addresses of location of the program under execution.
Control Generator :
Generates signals within uP to carry out the instruction which has been decoded. Inreality causes certain connections between blocks of the uP to be opened or closed, so
that data goes where it is required, and so that ALU operations occur.
Register Selector
This block controls the use of the register stack in the example. Just a logic circuit
which switches between different registers in the set will receive instructions from
Control Unit. General Purpose Registers
uP requires extra registers for versatility. Can be used to store additional data during a
program. More complex processors may have a variety of differently named registers.
Microprogramming :
How does the µP knows what an instruction means, especially when it is only abinary number? The microprogram in a uP/uC is written by the chip designer and tells
the uP/uC the meaning of each instruction uP/uC can then carry out operation.
------------------------- Next>> 8085 SYSTEM BUS DESCRIPTION<<----------------------------
