8085 notes g scheme 2016

1. Microprocessors and programming Chapter1 Mrs. RadhikaKamath, VPMP,Thane 1
Evolution of Microprocessors
Microprocessor:
Microprocessor is a multipurpose, programmable, clock-driven, register based electronic device
that reads binary instructions from a storage device called memory, accepts binary data as input
and processes data according to those instructions, and provides as output.
A common way of categorizing microprocessors is by the number of bits that their ALU can work
time. In other words, a microprocessor with a 4-bit will be referred to as a 4-bit microprocessor,
regardless of the number of address lines or the number of data bus lines that it has.
 Intel 4004 :
The first commercially available microprocessor was the Intel 4004, produced in 1971. It
contained 2300 PMOS transistors. The 4004was a 4 bit device intended to be used with some other
devices in making a calculator. Some logic designers however saw that this device could be used
to replace PC- boards full of combinational and sequential logic devices. Also, the ability to
change the function of a system by just changing the programming, rather than redesigning the
hardware is very appealing.
 Intel 8008 :
In 1972 Intel came out with the 8008, which was capable of working with 8-bit words. The
8008however required 20 or more additional devices to form a functional CPU.
 Intel 8080 :
In 1974 Intel announced the 8080, which had a much larger instruction set than the 8008 and
required only two additional devices to form a functional CPU. Also, the 8080 used NMOS
transistors. So it operated much faster than the 8008. The 8080 is referred to as a second-
generation microprocessor. It requires +12 V power supply.
 Motorola MC6800
Soon after Intel produced the 8080, Motorola came out with the MC6800, another 8-bit
general-purpose CPU. The 6800 had the advantage that it required only a +5-V supply rather
than the -5-V, +5-V and + 12V supplies required by the 8080.
For several years the 8080 and the 6800 were the top-selling 8-bit microprocessors. Some of
their competitors were the MOS Technology 6502, used as the CPU in the Apple II microcom-
puter andthe Zilog Z80, used as the CPU in the Radio Shack TRS microcomputer.
Three major directions of microprocessor Evolutions are
(i) Dedicated or Embedded Controllers
(ii) Bit Slice Processors
(iii) General purpose CPUs
(i) Dedicated or Embedded Controllers:
One direction has been dedicated or embedded controllers. These devices are used to control
"smart" machinessuch as microwave ovens, clothes washers, sewing machines, auto ignition
systems and metal lathes.
 Texas Instruments has produced millions of their TMS-1000 family of 4-bit microprocessors
for this type of application.
 In 1976 Intel introduced the 8048, which contains an 8-bit CPU. RAM. ROM. and some I/O
ports all in one 40-pin package. Other manufacturers have followed with similar products.
These devices are often referred to as microcontrollers.
 Some currently available devices in this category-the Intel 8051 and the Motorola MC6801.
 A more recently introduced single chip microcontroller, the Intel 8096 contains a 16-bit CPU,
ROM, RAM, a UARTports, timers and a 10-bit analog-to-digital converter.
(ii) Bit Slice processors:
A second direction of microprocessor evolution has been bit-slice processors.For some
applications, general purpose CPUs such as the 8080 and 6800 are not fast enough or do not
have suitable instruction sets. For these applications, several manufacturers produce devices

2. Microprocessors and programming Chapter1 Mrs. RadhikaKamath, VPMP,Thane 2
which can be used to build a custom CPU.
 An example is the Advanced Micro Devices 2900 family of devices. This family includes 4-
bit ALUs, Multiplexers, sequencers and other parts needed for custom-building a CPU. The
term slice comes from the fact that these parts can be connected in parallel to work with 8-bit
words, 16-bit words or 32-bit words. In other words, a designer can add as many slices as
needed for a particular application.
 The designer not only custom-designs the hardware of the CPU, but also custom-makes the
instruction set for it using “microcode”.
(iii) General Purpose CPUs
The third major direction of microprocessor evolution has been toward general-purpose CPUs
which give a microcomputer most or all of the computing power of earlier minicomputers.
 Intel 8085 :
After Motorola came out with the MC6800, Intel produced the 8085 an upgrade of the 8080
that required only a +5-V supply.
 Motorola MC 6809:
Motorola then produced the MC6809. which has a few 16-bit instructions. but is still basically
an 8-bit processor.
 Intel 8086 :
In 1978 Intel came out with the 8086, which is a full 16bit processor. Some 16-bit
microprocessors, such as the National PACE and the Texas Instruments 9900 family of devices
had been available previously, but the market apparently was not ready.
 Motorola MC68000
Soon after Intel came out with the 8086, Motorola came out with the 16-bit MC68000.
 The 8086 and the 68000 work directly with 16-bit words instead of with 8-bit words. they can
address a million or more bytes of memory instead of the 64 Kbytes addressable by the 8-bit
processors and they execute instructions much faster than the 8-bit processors. Also. these 16bit
processors have single instructions for functions such asmultiply and divide,which required a
lengthy sequence of instructions on the 8-bit processors.
 The evolution along this last path has continued on to 32-bit processors that work with gigabytes
(109 bytes) or terabytes (1012 bytes) of memory. Examples of these devices are the Intel 80386,
the Motorola MC68020, and the National 32032.
The Table below shows the Microprocessor evolution with comparison:
Name Date Transistors Microns Clock
Speed
Data
Width
MIPS
8080 1974 6000 6 2 MHZ 8 bits 0.64
8088 1979 29,000 3 5 MHZ 16 bits, 8
bit bus
0.33
80286 1982 1,34000 1.5 6 MHZ 16 bits 1
80386 1985 275,000 1.5 16 MHZ 32 bits 5
80486 1989 1,200,000 1 25 MHZ 32 bits 20
Pentium 1993 3,100,000 0.8 60 MHZ 32 bits, 64
bit bus
100
Pentium
II
1997 7,500,000 0.35 233 MHZ 32 bits,6 4
bit bus
~300
Penium
III
1999 9,500,000 0.25 450 MHZ 32 bits ,
64 bit bus
~510
Pentium
IV
2000 42,000,000 0.18 1.5 MHZ 32 bitx, 64
bit bus
~1,700

3. Microprocessors and programming Chapter1 Mrs. RadhikaKamath, VPMP,Thane 3
8085 Microprocessor
Features of 8085
• 8-bit microprocessor,
• 16-bit address bus, so max. 64KB of memory
• 8-bit data bus
• Generates 8-bit I/O device address, so can access 256 I/O devices
• Requires +5V supply
• On-chip clock generator
• Has 5 h/w interrupts
• Has serial I/O lines
• Has accumulator, flag register, 6 GPR, 2 SPR, 3 TR
• Has 74 instructions, 4 addressing modes
Architecture of 8085
The 8085 is an 8-bit general-purpose microprocessor capable of addressing 64K of memory.
It includes the ALU (arithmetic /Logic Unit), Timing and Control Unit, Instruction Register and
Decoder, Register Array, Interrupt Control, and Serial I/O Control.
ALU
The arithmetic and logic unit performs the computing functions; it includes the accumulator, the
temporary register, the arithmetic and logic circuits, and five flags. The temporary register is used
to hold data during an arithmetic and logic operation. The result is stored in the accumulator, and
the flags are set or reset according to the result of the operation. All operations are performed on 8
bit data.
The flags are affected by the arithmetic and logic operations in the ALU.
The flags are:

4. Microprocessors and programming Chapter1 Mrs. RadhikaKamath, VPMP,Thane 4
D7 D6 D5 D4 D3 D2 D1 D0
S Z X AC X P X CY
S- Sign Flag . If D7 =1 , then sign flag is set, otherwise rest.
Z-Zero flag. If ALU operation results in zero, then this flag is set, otherwise it is reset.
AC-Auxilliary flag. In an arithmetic operation ,when a carry is generated by digit D3 and passed
on to digit D4, the AC flag is set. Otherwise it is reset.
P-Parity Flag. If the result of an arithmetic or logic operation has an even number of 1’s then this
flag is set. Otherwise it is reset.
CY-Carry Flag. If an arithmetic operation results in a carry, the carry flag is set. Otherwise it is
reset.
Among the five flags, the AC flag is used internally for BCD arithmetic; the instruction set does
not include any conditional jump instructions based on the AC flag. Of the remaining four flags,
the Z and CY flags are those most commonly used.
Timing and Control Unit
This unit synchronizes all the microprocessor operations with the clock and generates the control
signals necessary for communication between the microprocessor and peripherals.
The 𝑅𝐷̅̅̅̅ and𝑊𝑅̅̅̅̅̅signals indicate the availability of data on the data bus.
Instruction Register and Decoder.
The instruction register and the decoder are part of the ALU. When an instruction is fetched from
memory, it is loaded in the instruction register. The decoder decodes the instruction and establishes
the sequence of events to follow. The instruction register is not programmable and cannot be
accessed through any instruction.
Registerorganization:
It consists of 3 types of registers
1. Temporary registers
2. General purpose register
3. Special purpose register
1. Temporary registers: 8085 microprocessor provides two 8 bit temporary registers W & Z.
These registers are not available to the user. They are used by the control sections to hold the
address of the program memory, data memory and intermediate data temporarily.
2. General Purpose registers: 8085 microprocessor provides only 6 eight bit general purpose
register, B, C, D, E, H and L. These registers are available for the user. These are used to hold 8 bit
operands in 8 bit operations. A pair of two 8 bit registers such as BC, DE, and HL can be used with
16 bit operations.
3. Special purpose registers: 8085 provides two 16 bit special purpose registers, Program
Counter & Stack Pointer
a. Program Counter (PC): It is a 16 bit register used to hold the address of the next
instruction to be executed by the processor. It is automatically incremented by the control section
during the instruction fetch operations.
b. Stack pointer (SP): It is a 16 bit register. It is used to access Stack memory in
LIFO mode. The 8085 microprocessor always stores bytes on the stack from the highest address
(top) to lowest address i.e., the stack is grown from top.

5. Microprocessors and programming Chapter1 Mrs. RadhikaKamath, VPMP,Thane 5
Signal lines (Pins) of 8085 Microprocessor
All the signals can be classified into six groups: (1) address bus, (2) data bus, (3) control
and status signals, (4) power supply and frequency signals, (5) externally initiated signals, and (6)
serial I/O ports.
Address Bus
The 8085 has 16 signal lines (pins), which are used as the address bus. These lines are split
into two segments A15 – A8 and AD7 – AD0. The eight signal lines A15 – A8 are unidirectional and
used for most significant bits, called the high order address of a 16 bit address.
Multiplexed Address/Data bus
The signal lines AD7-ADo are bidirectional: they serve a dual purpose. They are used as
the low-order address bus as well as the data bus. In executing an instruction, during the earlier
part of the cycle, these lines are used as the low-order address bus. During the later part of the
cycle, these lines are used as the data bus. (This is also known as multiplexing the bus.) However,
the low-order address bus can be separated from these signals by using a latch.
Diagram showing demultiplexing address bus:
The figure shows a latch and ALE signal to demultiplex the bus. The bus AD7 – AD0 is connected
at the input latch such as 74373. The ALE signal is connected to the enable pin of the latch.
When ALE = 1 during first cycle, the latch is transparent, i.e., the output is available at the
low order address bus as address lines (A7 – A0). When ALE=0 during other cycles, this address is
latched. And during this, the lines AD7 – AD0 are available at Data bus (D7 – D0 ) which are
bidirectional.
Control and status signals
ALE (Address Latch Enable): This is a positive going pulse generated every time the 8085
begins an operation (machine cycle); it indicates that the bits on AD7 – AD0 are address bits. This
signal is used primarily to latch the low-order address from the multiplexed bus and generate a
separate set of eight address lines, A7-A0.
𝑹𝑫̅̅̅̅̅ (Read): This is a Read control signal (active low). This signal indicates that the selected I/O or
memory device is to be read and data are available on the data bus.
𝑾𝑹̅̅̅̅̅ (Write): This is a Write control signal (active low). This signal indicates that the data on the
data bus are to be written into a selected memory or I/O location.
IO/𝑴̅ : This is a status signal used to differentiate between I/O and memory operations. When it is
high, it indicates an I/O operation. when it is low, it indicates a memory operation. This signal is
combined with 𝑅𝐷̅̅̅̅ (Read) and 𝑊𝑅̅̅̅̅̅ (Write) to generate I/O and memory control signals.

6. Microprocessors and programming Chapter1 Mrs. RadhikaKamath, VPMP,Thane 6
S1 and S0: These status signals, similar to IO/𝑀̅, can identify various operations such as opcode
fetch, read, write, Interrupt Acknowledge based on IO/𝑀̅.
Power supply and Clock Frequency :
VCC: +5 V power supply.VSS: Ground Reference.
Xl, X2: A crystal (or RC, LC network) is connected at these two pins. The frequency is internally
divided by two; therefore, to operate a system at 3 MHz, the crystal should have a frequency of 6
MHz.
CLK (OUT)-Clock Output: This signal can be used as the system clock for other devices.
Externally Initiated Signals including Interrupts:
INTR (input) : Interrupt Request. It is used as a general purpose interrupt.
𝑰𝑵𝑻𝑨̅̅̅̅̅̅̅̅ (Output) : Interrupt Acknowledge It is used to acknowledge the interrupt.
RST 7.5 (Inputs) : Restart Interrupts. These are vectored maskableinterrupts that transfer the
program control to specific memory locations. They have higher priorities than the INTR interrupt.
Among these three, the priority order is RST7.5, RST6.5,RST5.5.
TRAP (Input) : It is non-maskable interrupt and has the highest priority.
HOLD(Input) : It indicates that a peripheral such as a DMA (Direct memory Access) controller is
requesting the use of the address and data buses.
HLDA (Output) : Hold Acknowledge .It acknowledges the HOLD request.
READY(Input) : It is used to delay the microprocessor Read or write cycles until a slow
responding peripheral is ready to send or accept data.
𝑹𝑬𝑺𝑬𝑻 𝑰𝑵̅̅̅̅̅̅̅̅̅̅̅̅̅̅ : When the signal on this pin goes low, the program counter is set to zero, the buses
are tristated, and the MPU is reset.
RESET OUT: It indicates that the MPU is being reset.It can be used to reset other devices.
Serial I/O pins:
8085 has two signals for serial transmission. It consists of serial input and serial output ports. It
transmits or receives data. The pins SID and SOD are used for serial input and output operations
respectively.
Features of 8085
 8-bit microprocessor performing all 8bit arithmetic and logical operations
 16-bit address bus, so max. 64KB of memory
 8-bit data bus
 Generates 8-bit I/O device address, so can access 256 I/O devices
 Requires +5V supply
 40 pin IC with On-chip clock generator
 Has 5 h/w interrupts
 Has serial I/O lines
 Has accumulator, flag register, 6 GPR (General purpose register), 2 (Special Purpose
Register), 3 Temporary Registers.
 Has 74 instructions, 4 addressing modes