8085 INTERNAL ARCHITECTURE
 Upon
completing this topic, you should
be able to:
• State all the register available in the 8085
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microprocessor and explain their function
Explain the function of stack
Depict the ALU subsystem
List and describe the function of control logic
signal in the processor
Elaborate the process of de-multiplexing data
bus and address bus
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6 x 8-bit registers with addresses B, C, D, E, H & L
Can be concatenated to form 3 x 16-bit registers with addresses
BC, DE & HL.
Note : 16-bit register HL used as a memory pointer with many
8085A instructions.
• e.g MOV A, M which copies the contents of the memory location whose
address is specified by the contents of the HL register pair to the CPU
register A
• Note the assembly language instruction format
MOV <destination> <source>
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16-bit register SP. It contains the address of the memory location of
the top of the stack area of memory. It is automatically controlled
by the processor but can be initialized by software.
16 – bit register PC. It contains the address of the memory location
from where the CPU is to fetch the next instruction byte.
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The program counter is a 16-bit CPU register.
The program counter always contains the address of the
memory location from where the CPU is to fetch the next
instruction byte.
The program counter is automatically incremented every
time the CPU fetches an instruction byte. Instructions are
thus executed in the sequence they are loaded into program
memory.
The contents of the program counter can be overwritten by
execution of certain instructions e.g. jmp, call, ret etc.
The program counter is initialized to 0000H after processor
reset or on power-up.
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The stack is a reserved area of memory. It operates as a lastin first-out bank of registers.
The memory locations, which constitute the stack, are used
to store binary information temporarily during program
execution.
The stack can be located anywhere in read/write memory,
but is usually defined such that it neither interferes with the
program memory space or the data memory space.
The start address of the stack is specified at the initialisation
stage of the program by loading the 16-bit CPU register,
called the stack pointer, with the desired address of the start
of the stack.
• e.g
LXI
SP, data 16
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Data from CPU register pairs are stored in the stack
area of memory when the processor executes a push rp
instruction.
The contents of the program counter is automatically
stored in the stack area of memory whenever the
processor executes a call or restart (rst n) instruction.
Data stored in the stack area of memory are returned to
processor register pairs when the processor executes a
pop rp instruction.
Data is automatically transferred from the stack area of
memory to the program counter whenever the
processor executes a return (ret) instruction.
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To execute the instruction push HL
assuming initial SP contents is 2099 H.
The stack pointer is decremented by 1
(SP=2098) and the contents of H are
written to this location.
The stack pointer is decremented by 1
(SP=2097) and the contents of L are
written to this location.
Note : When data is written to the
stack the stack pointer is first
decremented and then the data is
written
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To execute the instruction pop BC
assuming initial SP contents is 2097 H.
The contents of the memory location
at the address specified by the
contents of SP is moved to register C
and SP is incremented.
The contents of the memory location
at the address specified by the
contents of SP is moved to register B
and sp is incremented.
Note : When data is read from the
stack the data is read first and then
the stack pointer incremented.
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The ALU itself is a group of combinatorial logic
circuits that can perform 8-bit arithmetic and logical
operations. Included are :- add, sub, and, or, xor
 The accumulator ( reg A ) is an 8-bit general purpose
register that holds one of the source operands of an
ALU operation. The result of the operation is also
returned to the accumulator.
 The temporary accumulator is an 8-bit register that
holds the second operand used in a ALU operation. It
is inaccessible to the user.
 The decimal adjust unit allows the ALU to perform
BCD arithmetic. (rarely used as it is usual to use
binary, as opposed to BCD, arithmetic)
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The flag register is a collection of eight, single bit
registers. It reflects outcomes of arithmetic or logical
operations.
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The Sign Flag (S) reflects the contents of bit-7 of the accumulator
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The Zero Flag (Z) is set to ‘1’ if the accumulator contains all zero’s
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The Auxiliary Carry (AC) is a ‘1’ if the arithmetic operation caused
a carry out from the bit-3 to the bit-4 position. (The auxiliary carry
flag is used in BCD arithmetic)
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The Parity Flag (P) is a ‘1’ if the accumulator has an even number of
‘1’s
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The Carry Flag (CY) is set if the arithmetic operation caused a
carry overflow (from addition) or a borrow (from subtraction).
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Example:
ADD B
register A
register B
register A
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S=0
Z=0
AC = 1
P=1
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CY = 1
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; A contains 88 (H)
B contains 99 (H)
10001000
10011001
136
153
_____________
_____
100100001
289
decimal
decimal
decimal
Bit D7 = 0 after addition
The accumulator contains other than zero after addition
There is a carry out of bit D3 to bit D4 during addition
The accumulator contains an even number of ‘1’s
after addition
There is an overflow as a result of the addition
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The control logic subsystem of a microprocessor is a
configurable sequential state machine.
It is configured by the instruction decoding circuitry to enable
execution of a particular instruction.
It has associated with it a number of inputs and outputs, both
from other subsystems of the CPU and from external devices.
It operates with respect to a stable clock reference signal
provided by a crystal source. Typical clock frequencies for an
8085A microprocessor are in the order of 5-MHz.
The internal clock for the control logic is half the frequency of
the external crystal source (i.e internally the crystal source
clock is divided by 2)
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RD (o) A logic ‘0’ specifies the processor will read
the data on the data bus.
WR (o)
A logic ‘0’specifies that the processor has
placed data (write) onto the data bus for
an external device to read.
S0 S1 IO / M (o) A 3-bit status code that specifies the operation the
processor is performing. (e.g 110 specifies the
processor is fetching an opcode from program
memory).
reset out (o) A logic ‘1’ on the reset in input to the processor
causes the program counter to be reset to 0000H. To
indicate the processor has been reset a logic `1’,
synchronized to the processor clock, is placed on
this output.
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CLK out (o) Is a buffered version of the internal master clock of
the 8085A. It is half the crystal frequency.
HOLD (i) HLDA (o) An external device can request use of the system
ALE (o)
busses by driving the HOLD input to logic ‘1’.
The 8085A acknowledges the request by
asserting the HLDA output to logic ‘1’. It only
does after it has relinquished control of the bus
structure.
When this signal is logic ‘1’ the processor specifies
that it has valid address information on the CPU pins
AD07
RDY (i) When this signal is logic ‘0’ the processor suspends
execution of an instruction. It permits slow peripheral
devices more time to access or load data.
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The 8085A microprocessor uses only 16 connections to
interface the 8-bit data bus and 16-bit address bus.
Address bus lines A
are time multiplexed with data
07
bus lines D
07
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Peripheral devices ( memory and IO ) require stable address
data throughout a read or write operation.
The processor only provides A0 - A7 during the period ALE is
in the logic ‘1’ state after which the multiplexed lines AD0 AD7 assume the role of the data bus. ( D0 - D7 )
To provide external devices with stable address data
throughout a read or write operation it is necessary to latch
the low byte of the address using the ALE control signal.
By this mechanism it is possible for the 8085A computer
system to have a 16-bit address bus and an 8-bit data bus
whilst only using 16 processor connections ( AD0 - AD7 and
A8 - A15 )
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Note : See through latches ( eg 74LS573 ) are the preferred
type of latch.
When clk (ALE) is logic ‘1’ the latch outputs follow the inputs and on
the negative edge of clk the data is latched.
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