Computer Architecture and
Microprocessors
By
Navdeep Goyal
Purpose of Talk
• What are the advantages of teaching these
topics to physics students
• To discuss the curriculum for teaching
microprocessors to physics students
• Introduction to SAP-1
• 8085 microprocessor
Importance
. Learning about the design considerations of
microprocessors is important
. Students opting for further studies in
microelectronics are benefitted
. Knowledge of working of a processors is important
. Starting with working of simplest computer system
is always helpful
. Better to start with few basic principles of working
of computer systems
Proposed Sequence
• Which device was designed first
• Computer or microprocessor?
• Bus organised computer systems
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Data Bus
Address Bus
Control Bus
Tri-state Devices
Buffer Registers
SAP-1, SAP-2 etc
8085 microprocessor
8086 and other advanced microprocessors
BUS SYSTEM
• In all computer system, the data is transferred
from one place to other through common
DATA BUS
• The selection of device is from the address,
which is sent through common address bus
• Depending upon control signals data is written
onto or read from a particular register
TRI STATE DEVICES
• TRI-STATE Devices play important role when
we are dealing with bus organized computer
systems
• Three terminal device:
– Input, output, control
– Available in high active or low active mode
– Used at the output of registers, which are
connected with common data bus
Buffer Registers
• The loading of data and reading data from
buffer register is with the help of control
signals
• For loading data onto register a Load (L) signal
is used
• For receiving data from buffer register an
Enable (E) signal is used
• For different registers loading or enabling may
be high or low active
Introduction to Sap-1
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SAP-1 stands for simple as possible computer
SAP-1 is a computer made from discrete IC’s
Its an 8 bit computer with 4 address lines
Can handle max. of 16 address locations
Instruction set includes instructions
LDA, ADD, SUB, OUT, HLT
OPCODES
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LDA
ADD
SUB
OUT
HLT
0000
0001
0010
1100
1111
OPCODES AND OPERAND
Complete code includes opcode and operand
Like LDA 04H
0000 0100
One instruction is executed in one instruction
cycle
Machine Cycle/Instruction Cycle
• Instruction cycle may consist of many machine
cycles
• For SAP-1, Instruction cycle= machine cycle
• Instruction cycle=Fetch cycle+Execution Cycle
• Fetch cycle is generally same for all
instructions
Ring Counter
SAP-1
How to fabricate various units
• Program Counter: 4 bit counter-made from J-K flip-flops-74
LS107 with 74LS126
• Input/MAR: includes address/data switch registers includes
buffer register and 2-1 multiplexer
• 16X8 RAM
• Instruction Register: 8 bit buffer register , 2 nos 74 LS173
• Controller Sequencer: to generate control signals
• Accumulator: Buffer register to store intermediate results
• Adder Subtracter: Arithmatic unit
• B: buffer register
Control Signals
Fetch
T1
T2
T3
010 1
1 011
0011
1110
1110
0110
001 1
0011
0011
Control Words for various instructions
Control Unit of SAP-1
Microprogrammed Control Unit
• Easier to fabricate
• Slower-not useful for commercial
microprocessors
• Can be used for testing various instructions
Variable Machine Cycle
• Fixed machine cycle may give many unwanted
time states
• Execution is going to take longer time
• Better to opt for variable machine cycle
8085 Pin Diagram
Architecture of 8085
Thanks