Control sequence of ADD and SUB instruction Hardwire approach

1. Birla Vishwakarma
Mahavidhyalaya
• Subject : Control Sequences and
Hardwire Approach
• Batch : B3 Group : G2
• Made by : 15CP032:-Siddhapura Romil
15CP033:-Kavaiya Hiren
15CP018:-Ahalpara Keval
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2. INTERNALBUS
MAR
Z
Interrupt
control
MDR
Serial I/O
Flag
MUX
Y
A
Instruction
decoder
IR
B (8) C(8)
D(8) E(8)
H(8) L(8)
PC(16)
SP(16)
ALU
Control
signal
MEM
ORY
Select 1
SINGLE BUS
ARCHITECTURE
OF 8085 MICRO-
PROCESSOR

3. Add Register to Accumulator
Opcode : ADD Reg.
Machine Cycle : 1
T – states : 4
Cycles : Fetch
Instruction : ADD B
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4. Control Sequence of ADD B
Instruction
 1. PC out, MAR in, READ, WMFC, PC++
 2. MDR out, IR in
 3. B out, Y in, Select Y, ADD ,Z in
 4. Z out ,A in, End.
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5. Control Sequence of ADC B
Instruction
 1. PC out, MAR in, READ, WMFC, PC++
 2. MDR out, IR in
 3. B out, Y in, Select Y, CY out, ADD ,Z in
 4. Z out ,A in, End.
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6. Subtract Register from
Accumulator
Opcode : SUB Reg.
Machine Cycle : 1
T – states : 4
Cycles : Fetch
Instruction : SUB B
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7. Control Sequence of SUB B
Instruction
 1. PC out, MAR in, READ, WMFC, PC++
 2. MDR out, IR in
 3. B out, Y in, Select Y, SUB ,Z in
 4. Z out ,A in, End.
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8. Control Sequence of SBB B
Instruction
 1. PC out, MAR in, READ, WMFC, PC++
 2. MDR out, IR in
 3. B out, Y in, Select Y, CY out, SUB ,Z in
 4. Z out ,A in, End.
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9. Hardwire Approach
• To execute instructions, the processor must have
some means of generating the control signal in
proper sequence.
• The approach fall into one of two categories:
1. Hardwire Control
2. Microprogrammed Control
• Let us discuss a hardwired control unit organization
as shown in this figure,
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10. CLK
Control step
counter
Step decoder
Combinational
Logic
Circuit
Instruction
decoder
IR
External
inputs
Condition
codes
CLK
INS1
INS2
T1 T2
Run
Reset
End
Control signal

11. Logical Representation of Z in
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T3 ADD T3 SUB
. . . . . . . .
Z in
Z in = T3.ADD + T3.SUB + ……….

12. Logical representation of IR in
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. . . .
IRIN
OTHER INST.
T2 ADD
IR in = T2.ADD + …….

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