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.
Registers
The 8085/8080A-programming model includes six registers, one accumulator, and
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.
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.
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 microprocessor.
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. 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. 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.
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 a
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.
2. 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 the selects box number 3 for reading or writing data. Address bus is
unidirectional, ie numbers only sent from microprocessor to memory, not other way.
Question?: If you have a memory chip of size 256 kilobytes (256 x 1024 x 8 bits),
how many wires does the address bus need, in order to be able to specify an address in
this memory? Note: the memory is organized in groups of 8 bits per location,
therefore, how many locations must you be able to specify?
Data Bus
Data Bus: carries ‘data’, in binary form, between μP and other external units, such as
memory. Typical size is 8 or 16 bits. Size determined by size of boxes in memory and
μP size helps determine performance of μP. 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 bi-directional. 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 uP operations. Eg: Read/NotWrite line, single binary digit. Control
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 μP has 10 control lines.
Cannot function correctly without these vital control signals.
The Control Bus carries control signals partly unidirectional, partly bi-directional.
Control signals are things like "read or write". This tells memory that we are either
reading from a location, specified on the address bus, or writing to a location
specified. Various other signals to control and coordinate the operation of the system.
Modern day microprocessors, like 80386, 80486 have much larger busses. Typically
16 or 32 bit busses, which allow larger number of instructions, more memory
location, and faster arithmetic. Microcontrollers organized along same lines, except:
because microcontrollers have memory etc inside the chip, the busses may all be
internal. In the microprocessor the three busses are external to the chip (except for the
internal data bus). In case of external busses, the chip connects to the busses via
buffers, which are simply an electronic connection between external bus and the
internal data bus.
3. 8085 Pin description.
Properties
Single + 5V Supply
4 Vectored Interrupts (One is Non Maskable)
Serial In/Serial Out Port
Decimal, Binary, and Double Precision Arithmetic
Direct Addressing Capability to 64K bytes of memory
The Intel 8085A is a new generation, complete 8 bit parallel central processing unit
(CPU). The 8085A uses a multiplexed data bus. The address is split between the 8bit
address bus and the 8bit data bus. Figures are at the end of the document.
Pin Description
The following describes the function of each pin:
A6 - A1s (Output 3 State)
Address Bus; The most significant 8 bits of the memory address or the 8 bits of the I/0
address,3 stated during Hold and Halt modes.
AD0 - 7 (Input/Output 3state)
Multiplexed Address/Data Bus; Lower 8 bits of the memory address (or I/0 address)
appear on the bus during the first clock cycle of a machine state. It then becomes the
data bus during the second and third clock cycles. 3 stated during Hold and Halt
modes.
ALE (Output)
Address Latch Enable: It occurs during the first clock cycle of a machine state and
enables the address to get latched into the on chip latch of peripherals. The falling
edge of ALE is set to guarantee setup and hold times for the address information.
ALE can also be used to strobe the status information. ALE is never 3stated.
SO, S1 (Output)
Data Bus Status. Encoded status of the bus cycle:
S1 S0
O O HALT
0 1 WRITE
1 0 READ
1 1 FETCH
S1 can be used as an advanced R/W status.
RD (Output 3state)
READ; indicates the selected memory or 1/0 device is to be read and that the Data
Bus is available for the data transfer.
WR (Output 3state)
WRITE; indicates the data on the Data Bus is to be written into the selected memory
or 1/0 location. Data is set up at the trailing edge of WR. 3stated during Hold and Halt
modes.
READY (Input)
If Ready is high during a read or write cycle, it indicates that the memory or
peripheral is ready to send or receive data. If Ready is low, the CPU will wait for
Ready to go high before completing the read or write cycle.
HOLD (Input)
HOLD; indicates that another Master is requesting the use of the Address and Data
Buses. The CPU, upon receiving the Hold request. will relinquish the use of buses as
soon as the completion of the current machine cycle. Internal processing can continue.
The processor can regain the buses only after the Hold is removed. When the Hold is
acknowledged, the Address, Data, RD, WR, and IO/M lines are 3stated.
HLDA (Output)
HOLD ACKNOWLEDGE; indicates that the CPU has received the Hold request and
that it will relinquish the buses in the next clock cycle. HLDA goes low after the Hold
request is removed. The CPU takes the buses one half clock cycle after HLDA goes
low.
INTR (Input)
INTERRUPT REQUEST; is used as a general purpose interrupt. It is sampled only
during the next to the last clock cycle of the instruction. If it is active, the Program
Counter (PC) will be inhibited from incrementing and an INTA will be issued. During
this cycle a RESTART or CALL instruction can be inserted to jump to the interrupt
service routine. The INTR is enabled and disabled by software. It is disabled by Reset
and immediately after an interrupt is accepted.
INTA (Output)
INTERRUPT ACKNOWLEDGE; is used instead of (and has the same timing as) RD
during the Instruction cycle after an INTR is accepted. It can be used to activate the
8259 Interrupt chip or some other interrupt port.
RST 5.5
RST 6.5 - (Inputs)
RST 7.5
RESTART INTERRUPTS; These three inputs have the same timing as I NTR except
they cause an internal RESTART to be automatically inserted.
RST 7.5 ~~ Highest Priority
RST 6.5
RST 5.5 o Lowest Priority
The priority of these interrupts is ordered as shown above. These interrupts have a
higher priority than the INTR.
TRAP (Input)
Trap interrupt is a nonmaskable restart interrupt. It is recognized at the same time as
INTR. It is unaffected by any mask or Interrupt Enable. It has the highest priority of
any interrupt.
RESET IN (Input)
Reset sets the Program Counter to zero and resets the Interrupt Enable and HLDA
flipflops. None of the other flags or registers (except the instruction register) are
affected The CPU is held in the reset condition as long as Reset is applied.
RESET OUT (Output)
Indicates CPlJ is being reset. Can be used as a system RESET. The signal is
synchronized to the processor clock.
X1, X2 (Input)
Crystal or R/C network connections to set the internal clock generator X1 can also be
an external clock input instead of a crystal. The input frequency is divided by 2 to
give the internal operating frequency.
CLK (Output)
Clock Output for use as a system clock when a crystal or R/ C network is used as an
input to the CPU. The period of CLK is twice the X1, X2 input period.
IO/M (Output)
IO/M indicates whether the Read/Write is to memory or l/O Tristated during Hold and
Halt modes.
SID (Input)
Serial input data line The data on this line is loaded into accumulator bit 7 whenever a
RIM instruction is executed.
SOD (output)
Serial output data line. The output SOD is set or reset as specified by the SIM
instruction.
Vcc
+5 volt supply.
Vss
Ground Reference.
internet
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.
Registers
The 8085/8080A-programming model includes six registers, one accumulator, and
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.
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.
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 microprocessor.
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. 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. 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.
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 a
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.
2. 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 the selects box number 3 for reading or writing data. Address bus is
unidirectional, ie numbers only sent from microprocessor to memory, not other way.
Question?: If you have a memory chip of size 256 kilobytes (256 x 1024 x 8 bits),
how many wires does the address bus need, in order to be able to specify an address in
this memory? Note: the memory is organized in groups of 8 bits per location,
therefore, how many locations must you be able to specify?
Data Bus
Data Bus: carries ‘data’, in binary form, between μP and other external units, such as
memory. Typical size is 8 or 16 bits. Size determined by size of boxes in memory and
μP size helps determine performance of μP. 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 bi-directional. 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 uP operations. Eg: Read/NotWrite line, single binary digit. Control
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 μP has 10 control lines.
Cannot function correctly without these vital control signals.
The Control Bus carries control signals partly unidirectional, partly bi-directional.
Control signals are things like "read or write". This tells memory that we are either
reading from a location, specified on the address bus, or writing to a location
specified. Various other signals to control and coordinate the operation of the system.
Modern day microprocessors, like 80386, 80486 have much larger busses. Typically
16 or 32 bit busses, which allow larger number of instructions, more memory
location, and faster arithmetic. Microcontrollers organized along same lines, except:
because microcontrollers have memory etc inside the chip, the busses may all be
internal. In the microprocessor the three busses are external to the chip (except for the
internal data bus). In case of external busses, the chip connects to the busses via
buffers, which are simply an electronic connection between external bus and the
internal data bus.
3. 8085 Pin description.
Properties
Single + 5V Supply
4 Vectored Interrupts (One is Non Maskable)
Serial In/Serial Out Port
Decimal, Binary, and Double Precision Arithmetic
Direct Addressing Capability to 64K bytes of memory
The Intel 8085A is a new generation, complete 8 bit parallel central processing unit
(CPU). The 8085A uses a multiplexed data bus. The address is split between the 8bit
address bus and the 8bit data bus. Figures are at the end of the document.
Pin Description
The following describes the function of each pin:
A6 - A1s (Output 3 State)
Address Bus; The most significant 8 bits of the memory address or the 8 bits of the I/0
address,3 stated during Hold and Halt modes.
AD0 - 7 (Input/Output 3state)
Multiplexed Address/Data Bus; Lower 8 bits of the memory address (or I/0 address)
appear on the bus during the first clock cycle of a machine state. It then becomes the
data bus during the second and third clock cycles. 3 stated during Hold and Halt
modes.
ALE (Output)
Address Latch Enable: It occurs during the first clock cycle of a machine state and
enables the address to get latched into the on chip latch of peripherals. The falling
edge of ALE is set to guarantee setup and hold times for the address information.
ALE can also be used to strobe the status information. ALE is never 3stated.
SO, S1 (Output)
Data Bus Status. Encoded status of the bus cycle:
S1 S0
O O HALT
0 1 WRITE
1 0 READ
1 1 FETCH
S1 can be used as an advanced R/W status.
RD (Output 3state)
READ; indicates the selected memory or 1/0 device is to be read and that the Data
Bus is available for the data transfer.
WR (Output 3state)
WRITE; indicates the data on the Data Bus is to be written into the selected memory
or 1/0 location. Data is set up at the trailing edge of WR. 3stated during Hold and Halt
modes.
READY (Input)
If Ready is high during a read or write cycle, it indicates that the memory or
peripheral is ready to send or receive data. If Ready is low, the CPU will wait for
Ready to go high before completing the read or write cycle.
HOLD (Input)
HOLD; indicates that another Master is requesting the use of the Address and Data
Buses. The CPU, upon receiving the Hold request. will relinquish the use of buses as
soon as the completion of the current machine cycle. Internal processing can continue.
The processor can regain the buses only after the Hold is removed. When the Hold is
acknowledged, the Address, Data, RD, WR, and IO/M lines are 3stated.
HLDA (Output)
HOLD ACKNOWLEDGE; indicates that the CPU has received the Hold request and
that it will relinquish the buses in the next clock cycle. HLDA goes low after the Hold
request is removed. The CPU takes the buses one half clock cycle after HLDA goes
low.
INTR (Input)
INTERRUPT REQUEST; is used as a general purpose interrupt. It is sampled only
during the next to the last clock cycle of the instruction. If it is active, the Program
Counter (PC) will be inhibited from incrementing and an INTA will be issued. During
this cycle a RESTART or CALL instruction can be inserted to jump to the interrupt
service routine. The INTR is enabled and disabled by software. It is disabled by Reset
and immediately after an interrupt is accepted.
INTA (Output)
INTERRUPT ACKNOWLEDGE; is used instead of (and has the same timing as) RD
during the Instruction cycle after an INTR is accepted. It can be used to activate the
8259 Interrupt chip or some other interrupt port.
RST 5.5
RST 6.5 - (Inputs)
RST 7.5
RESTART INTERRUPTS; These three inputs have the same timing as I NTR except
they cause an internal RESTART to be automatically inserted.
RST 7.5 ~~ Highest Priority
RST 6.5
RST 5.5 o Lowest Priority
The priority of these interrupts is ordered as shown above. These interrupts have a
higher priority than the INTR.
TRAP (Input)
Trap interrupt is a nonmaskable restart interrupt. It is recognized at the same time as
INTR. It is unaffected by any mask or Interrupt Enable. It has the highest priority of
any interrupt.
RESET IN (Input)
Reset sets the Program Counter to zero and resets the Interrupt Enable and HLDA
flipflops. None of the other flags or registers (except the instruction register) are
affected The CPU is held in the reset condition as long as Reset is applied.
RESET OUT (Output)
Indicates CPlJ is being reset. Can be used as a system RESET. The signal is
synchronized to the processor clock.
X1, X2 (Input)
Crystal or R/C network connections to set the internal clock generator X1 can also be
an external clock input instead of a crystal. The input frequency is divided by 2 to
give the internal operating frequency.
CLK (Output)
Clock Output for use as a system clock when a crystal or R/ C network is used as an
input to the CPU. The period of CLK is twice the X1, X2 input period.
IO/M (Output)
IO/M indicates whether the Read/Write is to memory or l/O Tristated during Hold and
Halt modes.
SID (Input)
Serial input data line The data on this line is loaded into accumulator bit 7 whenever a
RIM instruction is executed.
SOD (output)
Serial output data line. The output SOD is set or reset as specified by the SIM
instruction.
Vcc
+5 volt supply.
Vss
Ground Reference.
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