Thursday, December 15, 2011

Thursday, December 15, 2011 - No comments

Draw and explain the internal architecture of 8085.

Figure 3: Internal Architecture of 8085(click the above pic to view larger image)
8085 System Bus
Typical system uses a number of busses, collection of wires, which
transmit binary numbers, one bit per wire. A typical microprocessor
communicates with memory and other devices (input and output)
using three busses: Address Bus, Data Bus and Control Bus.

Address Bus
One wire for each bit, therefore 16 bits = 16 wires. Binary number
carried alerts memory to ‘open’ the designated box. Data (binary) can
then be put in or taken out. The Address Bus consists of 16 wires,
therefore 16 bits. Its “width” is 16 bits. A 16 bit binary number allows
216 different numbers, or 32000 different numbers, ie
0000000000000000 up to 1111111111111111. Because memory
consists of boxes, each with a unique address, the size of the address
bus determines the size of memory, which can be used. To
communicate with memory the microprocessor sends an address on
the address bus, eg 0000000000000011 (3 in decimal), to the
memory. The memory selects box number 3 for reading or writing
data. Address bus is unidirectional, i.e numbers only sent from
microprocessor to memory, not other way.
Data Bus
Data Bus: carries ‘data’, in binary form, between microprocessor and
other external units, such as memory. Typical size is 8 or 16 bits. The
Data Bus typically consists of 8 wires. Therefore, 28 combinations of
binary digits. Data bus used to transmit “data”, ie information, results
of arithmetic, etc, between memory and the microprocessor. Bus is bidirectional.
Size of the data bus determines what arithmetic can be
done. If only 8 bits wide then largest number is 11111111 (255 in
decimal). Therefore, larger number have to be broken down into
chunks of 255. This slows microprocessor. Data Bus also carries
instructions from memory to the microprocessor. Size of the bus
therefore limits the number of possible instructions to 256, each
specified by a separate number.
Control Bus
Control Bus are various lines which have specific functions for
coordinating and controlling microprocessor operations. Eg:
Read/NotWrite line, single binary digit. controls whether memory is
being ‘written to’ (data stored in mem) or ‘read from’ (data taken out
of mem) 1 = Read, 0 = Write. May also include clock line(s) for
timing/synchronising, ‘interrupts’, ‘reset’ etc. Typically microprocessor
has 10 control lines. Cannot function correctly without these vital
control signals.
CPU Architecture
Control Unit
Generates signals within microprocessor to carry out the instruction,
which has been decoded. In reality causes certain connections
between blocks of the microprocessor 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.
Registers
The 8085/8080A-programming model includes six registers, one
accumulator, and one flag register, as shown in Fig. 2.1 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). This 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.
Temporary Register: It is used to hold the data during the arithmetic
and logical operations.
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. 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 is 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 Fig. 2.1 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 register 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
Stack Pointer (SP)
The stack pointer is also a 16-bit register used as a memory pointer. It
points to a 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.
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 microprocessor to carry out the instruction
which has been decoded. In reality causes certain connections
between blocks of the microprocessor 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
Microprocessor 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.
Timing and Control unit:
· It has three control signals ALE, RD (Active low) and WR (Active
low) and three status signals IO/M(Active low), S0 and S1.
· ALE is used for provide control signal to synchronize the
components of microprocessor and timing for instruction to
perform the operation.
· RD (Active low) and WR (Active low) are used to indicate whether
the operation is reading the data from memory or writing the
data into memory respectively.
· IO/M(Active low) is used to indicate whether the operation is
belongs to the memory or peripherals.
· If,
Interrupt Control Unit:
It receives hardware interrupt signals and sends an acknowledgement
for receiving the interrupt signal.

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