- Topics discussed in this webnote:
- Structure and operation of the CPU
- Function of each component in the CPU
- Operation of the CPU (instruction execution cycle)
- Program flow
- The pointer concept
- Program counter is a pointer
- Default program flow is sequential
- Branch instruction can change the default program flow
- Types of instructions that a computer can execute
- Arithmetic and logic operations
- Data transfer operations between CPU and RAM memory
- Branch instructions that change program flow (branch instructions can be conditional)
- Structure and operation of the CPU
- Structure of a CPU:
Components in a CPU:
- General purpose registers:
- Register is a synonym for memory in computer science
- A general purpose register is a memory cell
- Each general purpose register has a unique name
- It is used to store (and recall) intermediate result of complex computations
- Arithmetic and Logic Unit (ALU):
- Is a complex electrical circuit that can perform Mathematical (+, -, ×, /) and logical operations (<, ≤ >, ≥, and, or)
- The output (result) of the computation (obtained by the ALU) is often stored in a general purpose register
- Instruction register (IR):
- Contains the current instruction being executed by the CPU
- The CPU will perform
the operation indicated by the
instruction code
contained in the
instruction register
(E.g.: code 0 = "add", code 1 = "subtract", and so on)
- Program counter (PC):
- The program counter is a register (memory cell) !
- This register contains the address (location in memory) of the next instruction after the CPU finishes executing the current instruction in the instruction register
- The value in the program counter will be increased after the CPU finishes executing one instruction
- Processor status register (PSR):
- This register contains the various information about the CPU
- Among the information contained in the PSR is:
- The result of a comparison operation
- When the CPU compares
2 numbers a and b, the
outcome of the comparison
is stored in the PSR
The outcome of a compare operation will allow the CPU to determine the following fact between a and b:
- equal
- not equal
- less than
- less than or equal
- greater than
- greater than or equal
- General purpose registers:
- The CPU
performs the following
sequence of
operations
repeatedly:
- After executing an instruction, the
CPU
obtains ("fetches")
the (next)
instruction
at the address (location)
given in the
program counter
- The fetched instruction is
stored in the
instruction register in the CPU
and the
program counter
is
increased to
point to the next instruction
in the memory
- The fetched instruction (stored in the instruction register) is the executed (the CPU will do the operation indicated by the instruction code in the instruction register)
- After executing an instruction, the
CPU
obtains ("fetches")
the (next)
instruction
at the address (location)
given in the
program counter
- Example:
- Supposed the CPU has just
finished
executing the
instruction
(instruction code 0) in the
instruction register:
(Instruction code 0 in the instruction register means "add" and it has just been executed)
- The following figures
illustrats the
CPU's execution cycle
(that goes on indefinitely):
- The CPU first sends
a request to retrieve (recall)
the data stored at memory location given by the
program counter (PC) (in the figure, the value of the PC = 1):
The CPU will cause the memory to retrieve (recall) the value stored at memory address (location) 1
- In response, the
RAM memory sends the
value stored in memory location
1 (= the number 4)
to the CPU which stores it in the
instruction register (IR):
This number (4) will be interpretated as a code for a computer instruction.
The old instruction code (0) will be over-written by the new instruction code
- The program counter is then
increased:
Notice that the CPU is now ready to fetch the next instruction when the current instruction (stored in the instruction register) is processed.
- The CPU will now
execute the
instruction in the
Instruction Register
(instruction code = 4)
When the CPU is finished, it will repeat these steps again (and again, until you turn the cmputer off)...
- The CPU first sends
a request to retrieve (recall)
the data stored at memory location given by the
program counter (PC) (in the figure, the value of the PC = 1):
- Supposed the CPU has just
finished
executing the
instruction
(instruction code 0) in the
instruction register:
- Pointer:
- A pointer is
"something" that points to somewhere
Examples:
3 pointers to different directions 
- In Computer Science, a
pointer always
points to
a location in memory
Examples:
Pointer to memory location 1 Pointer to memory location 4 
- A pointer is
"something" that points to somewhere
- The computer does not have
"pointing finger" that point
to some location in memory
The pointer is represented in the computer by the value of the address (location) of the memory where the pointer is pointing at
Examples:
- Storing the
value 1
in some register/memory cell will
remember the memory location 1
- The value 1 in the register/memory cell is effectively pointing to the memory location 1
- Storing the
value 4
in some register/memory cell will
remember the memory location 4
- The value 4 in the register/memory cell is effectively pointing to the memory location 4
Pointer to memory location 1 Pointer to memory location 4 
- Storing the
value 1
in some register/memory cell will
remember the memory location 1
- The program counter (PC) is
in fact a pointer
Example:
- When the program counter (PC)
contains the value 1:
the program counter (PC) is in fact pointing to the memory location 1:
- When the program counter (PC)
contains the value 1:
- Definition: program flow
- Program flow = the sequence of instructions from the program executed by the CPU
- Default program flow:
- After executing a non-branching (or non-jumping) instruction at memory location n, then next instruction that is executed is the instruction at memory location n+1
Example:
- All of the instructions in the figure are
non-branching
- Therefore, the sequence of instructions
executed by the CPU starting at
memory location 0
is:
- 0 ("add")
- 4 ("compare")
- 1 ("subtract")
- And so on (in sequence)
- Consider the mechanical piano:
- The program
used by the
mechanical piano is:
- stored as spikes on the rotating cylinder (in the center of the mechanical piano)
The cylinder can only rotate forward.
Therefore:
- There is only one kind
of program flow
possible in the
mechanical piano:
- The default program flow
- The program flow in a
mechanical piano is
restricted by the
physics
(The spikes on the cylinder are always accessed in the "rotational order")
- This physical restriction
does not exist
for the computer
- The computer can
change (update)
the program pointer
to an arbitrary value
Changing the program counter (it contains the location of the next instruction) will change the program flow
- Therefore:
- The computer can change the default program flow !!!
- This physical restriction
does not exist
for the computer
- Branch instruction:
- A branch instruction can
alter (change) the
default program flow
- When the CPU executes a branch x instruction, the next instruction that will be executed by the CPU is the instruction at memory location x
Example:
- Notice there is a
branch to location 4
instruction in the example.
- Therefore, the sequence of instructions
executed by the CPU starting at
memory location 0
is:
- 0 ("add")
- 64 ("branch to location 4")
- 56 ("negate" --- this is the instruction at memory location 4)
- 0 ("add")
- And so on (in sequence again)
- A branch instruction can
alter (change) the
default program flow
- The computer is a
computing (reckoning) machine
- All existing computers (actually the CPU) execute the
following 3 types of
instructions:
- Arithmetic and logic operations
- +
- −
- ×
- /
- AND
- OR
- NOT
The result of an arithmetic and logic operation is often stored in a general purpose register
- Memory transfer operations
- Transfer the
content
from some specific memory location
to a specific register (memory cell)
in the CPU.
- and vice versa (transfer the content from some specific register (memory cell) in the CPU to some specific memory location).
- Transfer the
content
from some specific memory location
to a specific register (memory cell)
in the CPU.
- Branch operations
- A branch instruction
will cause the CPU to
branch (jump)
to the specified location
in memory
- After the jump has occured,
the CPU will
continue to execute the instructions
in sequence,
until
another branch/jump instruction
is encountered
- There are 2 kinds
of branch operations:
- A unconditional branch
instruction
will always cause the
CPU to
jump to the
target location
- A conditional branch instruction will only cause the CPU to jump to the target location when the specified condition is met
- A unconditional branch
instruction
will always cause the
CPU to
jump to the
target location
- A branch instruction
will cause the CPU to
branch (jump)
to the specified location
in memory
- Arithmetic and logic operations