- Compute:
- Compute = calculate
- Computer:
- Computer = a machine that performs a series of computing instructions
I.e.: a computer can execute the kind of instructions:
- Arithmetic operations on 2 numerical values (e.g.: +, −, ×, /)
- Compare 2 numerical values (e.g., >, ≥, <, ≤, ==, != )
- Logical operations on 2 boolean values (e.g.: AND, OR, NOT)
- Function of the
Central Processing Unit is:
- Fetch (obtain) the next computer instruction
- Execute the (fetched) instruction
- In order to perform
these functions,
the CPU contains
components
(= circuits that will be discussed
in CS355) that
enable the CPU to
do its job
We will now study the logical structure (= the components and their functions) of the CPU (that enable it to perform its functions)
- The CPU contains the
following component to
help it perform its function:
The CPU contains:
- A number (about 10 or so)
general purpose registers
- A register is a memory device that can store 8, 16, or 32 bits values
- General purpose means that the register can be used in multiple ways
Use of general purpose registers:
- General purpose registers are
used in arithmetic and logical operations
performed by the CPU
Example:
- When the CPU add 2 values ....
- The first value of the addition is obtained from a general purpose register
- The second value of the addition is obtained from another general purpose register
- The sum is stored in a general purpose register
- An Arithmetic and Logic Unit (ALU):
- The ALU is a complex digital circuit can perform arithmetic (+, −, ×, /) and logical (AND, OR, NOT) operations
- A number of special purpose register
- These registers always store 32 bits values
- Special purpose means that
the value stored
in the register is used
for one (special) purpose
(I.e.: each special purpose register has a dedicated function)
There at 3 commonly used special purpose registers in the CPU:
- Program Counter (PC):
- The value in the PC register is the adddress (in the RAM) of the next computer instruction
- Instruction Register (IR):
- The value in the IR register is the instruction code of the current instruction that the CPU will execute.
- Processor Status Register (PSR):
- The value in the
PSR register is the
status code of the
result after
executing the last (executed)
instruction
Example: if the last instruction was "x − y", then PSR will contain status information that indicate whether x < y or x ≤ y or x > y etc...
- The value in the
PSR register is the
status code of the
result after
executing the last (executed)
instruction
- A number (about 10 or so)
general purpose registers
- Recall a
computer is:
- Computer = a machine that performs a series of computing instructions
I.e.: a computer can execute the kind of instructions:
- Arithmetic operations on 2 numerical values (e.g.: +, −, ×, /)
- Compare 2 numerical values (e.g., >, ≥, <, ≤, ==, != )
- Logical operations on 2 boolean values (e.g.: AND, OR, NOT)
- How the
CPU executes
an instruction:
- The current instruction
(that is being executed)
is contained inside the
Instruction Register (IR):
(We will discuss how this instruction is stored inside the IR register later).
- The IR register will
send
control signals to
the bring the
specified input values from
the registers to the
ALU:
- The IR register will
send
control signals to
the ALU to
perform the
desired operation
(e.g., +):
- The IR register will
send
control signals to
a (destination) register to
store the result:
Now the CPU has complete the instruction (add v1 and v2 and store result in another register ) and can execute the next (another) instruction
- The current instruction
(that is being executed)
is contained inside the
Instruction Register (IR):
-
How the CPU get the next instruction (to execute)