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Computer Organization
Instructors :
Dr. Abdul Raouf Khan
Mr.Marwan El-Haj
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Program Control
Program Counter keeps the address of the next
instructions to be executed.
A program Control type of instruction, when
executed, may change the address value in the
PC and cause the flow of control to be altered.
Program control instructions specify conditions
for altering the contents of the program counter
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Program Control
Some typical program control instructions are
1. Branch
2. Jump
3. Skip
4. Call
5. Return
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Status bit Conditions
It is sometimes convenient to provide the
ALU circuit in the CPU with a status bit
register, where status bit conditions can be
stored for further analysis. Status bits are
also called condition-code bits or flag bits
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Status bit register
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Program Interrupt
Program interrupts refer to the transfer of
program control from a currently running
program to another service program, as a result
of external or internal generated request.
Control returns to the original program after the
service program is executed
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Program Interrupt & subroutine call
Three variations
1. The interrupt is usually initiated by an internal or
external signal rather than from the execution of an
instruction
2. the address of the interrupt service program is
determined by the hardware rather than from the
address field of an instruction
3. An interrupt procedure usually stores all the information
necessary to define the state of CPU rather than storing
only the program counter
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Types of Interrupts
1. External interrupts
2. Internal Interrupts
3. Software interrupts
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External interrupts
External interrupts come from input-output
(I/O) devices, from a timing device, from a
circuit monitoring the power supply, or from
any other external source.
example I/O devices requesting transfer of
data, I/O finish transfer of data, elapse time
of an event, or power failure
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Internal Interrupts
Internal interrupts arise from illegal or
erroneous use of an instruction or data.
Internal interrupts are also called Traps.
Examples: caused by internal error
conditions are register overflow, attempt to
divide by zero, an invalid operation code,
stack overflow, and protection violation.
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Software interrupts
A software interrupt is initiated by executing
an instruction. Software interrupt is a special
CALL instruction that behaves like an
interrupt rather than a sub routine call. It can
be used by the programmer to initiate an
interrupt procedure at any desired point in
the program.
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CISC Computer
A computer with a large number of
instructions is called a complex instruction
set computer.
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Characteristics of CISC
1. A large number of instructions(100 to 250)
2. Some instruction that perform specialized
tasks and are used in frequently
3. A large variety of addressing modes (5 – 20)
4. Variable length instruction formats
5. Instructions that manipulate operands in
memory
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RISC Computers
Computers which is fewer instructions with
simple constructs so they can be excuted
much faster within the CPU without having to
use memory as often
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Characteristics of RISC
1. Relatively few instructions
2. Relatively few addressing modes
3. Memory access limited to load & store
4. All operations done within the registers of the CPU
5. Fixed-length, easily decoded instruction format
6. Single-Cycle instruction execution
7. Hardwired control
8. A relatively large number of registers in the CPU
9. Use of overlapped register windows to speed procedure call &
return
10. Efficient instruction pipeline
11. Compiler support for efficient translation of HLL program to MLP
programs
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