COMPUTER ORGANIZATION
BASIC PROCESSING UNIT
SOME FUNDAMENTAL CONCEPTS
• To execute an instruction, processor has to perform following 3 steps:
1) Fetch contents of memory-location pointed to by PC. Content of this location is an instruction
to be executed. The instructions are loaded into IR, Symbolically, this operation is written as:
IR [[PC]]
2) Increment PC by 4.
PC [PC] +4
3) Carry out the actions specified by instruction (in the IR).
• The first 2 steps are referred to as Fetch Phase.
Step 3 is referred to as Execution Phase.
• The operation specified by an instruction can be carried out by performing one or more of the
following actions:
1) Read the contents of a given memory-location and load them into a register.
2) Read data from one or more registers.
3) Perform an arithmetic or logic operation and place the result into a register.
4) Store data from a register into a given memory-location.
• The hardware-components needed to perform these actions are shown in Figure 5.1.
5-1
, COMPUTER ORGANIZATION
SINGLE BUS ORGANIZATION
• ALU and all the registers are interconnected via a Single Common Bus (Figure 7.1).
• Data & address lines of the external memory-bus is connected to the internal processor-bus via MDR
& MAR respectively. (MDR Memory Data Register, MAR Memory Address Register).
• MDR has 2 inputs and 2 outputs. Data may be loaded
→ into MDR either from memory-bus (external) or
→ from processor-bus (internal).
• MAR‟s input is connected to internal-bus;
MAR‟s output is connected to external-bus.
• Instruction Decoder & Control Unit is responsible for
→ issuing the control-signals to all the units inside the processor.
→ implementing the actions specified by the instruction (loaded in the IR).
• Register R0 through R(n-1) are the Processor Registers.
The programmer can access these registers for general-purpose use.
• Only processor can access 3 registers Y, Z & Temp for temporary storage during program-execution.
The programmer cannot access these 3 registers.
• In ALU, 1) „A‟ input gets the operand from the output of the multiplexer (MUX).
2) „B‟ input gets the operand directly from the processor-bus.
• There are 2 options provided for „A‟ input of the ALU.
• MUX is used to select one of the 2 inputs.
• MUX selects either
→ output of Y or
→ constant-value 4( which is used to increment PC content).
• An instruction is executed by performing one or more of the following operations:
1) Transfer a word of data from one register to another or to the ALU.
2) Perform arithmetic or a logic operation and store the result in a register.
3) Fetch the contents of a given memory-location and load them into a register.
4) Store a word of data from a register into a given memory-location.
• Disadvantage: Only one data-word can be transferred over the bus in a clock cycle.
Solution: Provide multiple internal-paths. Multiple paths allow several data-transfers to take place in
parallel.
5-2
, COMPUTER ORGANIZATION
REGISTER TRANSFERS
• Instruction execution involves a sequence of steps in which data are transferred from one register to
another.
• For each register, two control-signals are used: Riin & Riout. These are called Gating Signals.
• Riin=1 data on bus is loaded into Ri.
Riout=1 content of Ri is placed on bus.
Riout=0, bus can be used for transferring data from other registers.
• For example, Move R1, R2; This transfers the contents of register R1 to register R2. This can be
accomplished as follows:
1) Enable the output of registers R1 by setting R1 out to 1 (Figure 7.2).
This places the contents of R1 on processor-bus.
2) Enable the input of register R2 by setting R2out to 1.
This loads data from processor-bus into register R4.
• All operations and data transfers within the processor take place within time-periods defined by the
processor-clock.
• The control-signals that govern a particular transfer are asserted at the start of the clock cycle.
Input & Output Gating for one Register Bit
• A 2-input multiplexer is used to select the data applied to the input of an edge-triggered D flip-flop.
• Riin=1 mux selects data on bus. This data will be loaded into flip-flop at rising-edge of clock.
Riin=0 mux feeds back the value currently stored in flip-flop (Figure 7.3).
• Q output of flip-flop is connected to bus via a tri-state gate.
Riout=0 gate's output is in the high-impedance state.
Riout=1 the gate drives the bus to 0 or 1, depending on the value of Q.
5-3
BASIC PROCESSING UNIT
SOME FUNDAMENTAL CONCEPTS
• To execute an instruction, processor has to perform following 3 steps:
1) Fetch contents of memory-location pointed to by PC. Content of this location is an instruction
to be executed. The instructions are loaded into IR, Symbolically, this operation is written as:
IR [[PC]]
2) Increment PC by 4.
PC [PC] +4
3) Carry out the actions specified by instruction (in the IR).
• The first 2 steps are referred to as Fetch Phase.
Step 3 is referred to as Execution Phase.
• The operation specified by an instruction can be carried out by performing one or more of the
following actions:
1) Read the contents of a given memory-location and load them into a register.
2) Read data from one or more registers.
3) Perform an arithmetic or logic operation and place the result into a register.
4) Store data from a register into a given memory-location.
• The hardware-components needed to perform these actions are shown in Figure 5.1.
5-1
, COMPUTER ORGANIZATION
SINGLE BUS ORGANIZATION
• ALU and all the registers are interconnected via a Single Common Bus (Figure 7.1).
• Data & address lines of the external memory-bus is connected to the internal processor-bus via MDR
& MAR respectively. (MDR Memory Data Register, MAR Memory Address Register).
• MDR has 2 inputs and 2 outputs. Data may be loaded
→ into MDR either from memory-bus (external) or
→ from processor-bus (internal).
• MAR‟s input is connected to internal-bus;
MAR‟s output is connected to external-bus.
• Instruction Decoder & Control Unit is responsible for
→ issuing the control-signals to all the units inside the processor.
→ implementing the actions specified by the instruction (loaded in the IR).
• Register R0 through R(n-1) are the Processor Registers.
The programmer can access these registers for general-purpose use.
• Only processor can access 3 registers Y, Z & Temp for temporary storage during program-execution.
The programmer cannot access these 3 registers.
• In ALU, 1) „A‟ input gets the operand from the output of the multiplexer (MUX).
2) „B‟ input gets the operand directly from the processor-bus.
• There are 2 options provided for „A‟ input of the ALU.
• MUX is used to select one of the 2 inputs.
• MUX selects either
→ output of Y or
→ constant-value 4( which is used to increment PC content).
• An instruction is executed by performing one or more of the following operations:
1) Transfer a word of data from one register to another or to the ALU.
2) Perform arithmetic or a logic operation and store the result in a register.
3) Fetch the contents of a given memory-location and load them into a register.
4) Store a word of data from a register into a given memory-location.
• Disadvantage: Only one data-word can be transferred over the bus in a clock cycle.
Solution: Provide multiple internal-paths. Multiple paths allow several data-transfers to take place in
parallel.
5-2
, COMPUTER ORGANIZATION
REGISTER TRANSFERS
• Instruction execution involves a sequence of steps in which data are transferred from one register to
another.
• For each register, two control-signals are used: Riin & Riout. These are called Gating Signals.
• Riin=1 data on bus is loaded into Ri.
Riout=1 content of Ri is placed on bus.
Riout=0, bus can be used for transferring data from other registers.
• For example, Move R1, R2; This transfers the contents of register R1 to register R2. This can be
accomplished as follows:
1) Enable the output of registers R1 by setting R1 out to 1 (Figure 7.2).
This places the contents of R1 on processor-bus.
2) Enable the input of register R2 by setting R2out to 1.
This loads data from processor-bus into register R4.
• All operations and data transfers within the processor take place within time-periods defined by the
processor-clock.
• The control-signals that govern a particular transfer are asserted at the start of the clock cycle.
Input & Output Gating for one Register Bit
• A 2-input multiplexer is used to select the data applied to the input of an edge-triggered D flip-flop.
• Riin=1 mux selects data on bus. This data will be loaded into flip-flop at rising-edge of clock.
Riin=0 mux feeds back the value currently stored in flip-flop (Figure 7.3).
• Q output of flip-flop is connected to bus via a tri-state gate.
Riout=0 gate's output is in the high-impedance state.
Riout=1 the gate drives the bus to 0 or 1, depending on the value of Q.
5-3
