EEL 3801
Part II
System Architecture
EEL 3801C
Components
•
•
•
•
Video Display Terminal – self explanatory
Keyboard – self-explanatory
Disk Drives – self-explanatory
System Unit – contains the motherboard or
the system board. Otherwise selfexplanatory
EEL 3801C
Components (cont.)
• Random Access Memory (RAM) – Electronic
memory where the program and the data are
kept while the program is running. It is
volatile since the contents are lost if there is
loss of power. Additionally, it is also called
dynamic since its contents must be
continuously refreshed.
EEL 3801C
Components (cont.)
• Read-Only Memory (ROM) BIOS – Contains
the information on the input output
peripherals.
• CMOS RAM – Keeps system setup
information.
• Expansion slots – Permit expansion of the
system by adding special purpose boards
such as modems, communication cards, etc.
EEL 3801C
Components (cont.)
• Power Supply – Self-explanatory
• Parallel Port – Output port that transfers a
set of bits simultaneously. Typically used for
printers. Allow for quick transfer of data but
only for short distances.
• Serial port – Output port where single bits
are produced one by one. Slower, but useful
for longer distances.
EEL 3801C
Components (cont.)
• Microprocessor – Intel microprocessors are
downwardly compatible with each othe.
– Programs written on older versions will run on
the newer ones, but programs written for the
newer versions will not run on the older ones.
• Read Section 2.1 of the textbook for more
details.
EEL 3801C
System Architecture
• The Central Processing Unit (CPU) is the
most important part of the computer. It
consists of the Arithmetic logic Unit (ALU)
and the Control Unit (CU).
• The ALU carries out arithmetic, logic and
shifting operations.
• The CU fetches data and instructions and
decodes addresses for the ALU.
EEL 3801C
System Architecture (cont.)
• Additionally, there may be a math
coprocessor, which speeds up mathematical
calculations, as well as many other support
chips. However, they are all coordinated by
the CPU.
EEL 3801C
The CPU
• The most basic tasks of the CPU are:
• Find and load the next instruction from memory.
• Execute the instruction. This is composed of several
sub-instructions that we will discuss later.
EEL 3801C
The CPU (cont.)
• The CPU, besides the ALU and CU, is
composed of several other components:
• Data bus: Wires that move data within the CPU itself.
• Registers: High-speed memory elements within the
CPU itself on which can significantly speed up the
performance of the computer.
• Clock: A timing device whose ticks coordinate all
individual operations that take place in the computer.
These ticks are called machine cycles.
EEL 3801C
Registers
• Registers are special work areas inside the
CPU that can store data and/or instructions.
• These memory elements are very fast.
• There are several registers on the Intel 8088
family of microprocessors:
–
–
–
–
–
Data registers
Segment registers
Index registers
Special registers
Flag register
EEL 3801C
Data Registers
• Also called general purpose registers.
• Are used for arithmetic and data
manipulation operations.
• Can be addressed as either 8 or 16 bit
values, or as both.
• The 80386 and newer CPU’s use 32-bit
registers addressable as 16-bit ones.
EEL 3801C
Data Registers (cont.)
• There are several of these.
– The AX Register: the accumulator register is
used by the CPU for arithmetic operations.
– It is a 16-bit register, but can be addressed as
two independent 8-bit registers called AH (for
high) and AL (for low).
EEL 3801C
Data Registers (cont.)
– The BX Register: the base register is also general
purpose (like the AX).
– Has the ability to hold addresses for other
variables (pointers).
– Also 16-bit that can be independently addressed
as two 8-bit bytes (BH and BL).
EEL 3801C
Data Registers (cont.)
– The CX register: the counter register best serves
as the counter for repeating looping instructions.
• These instructions automatically repeat and
decrement the CX register, and quit when it equals 0.
– Also 16-bit that can be independently addressed
as two 8-bit bytes (CH and CL).
EEL 3801C
Data Registers (cont.)
– The DX Register: the data register is also general
purpose but has a special role when doing
multiplication or division.
EEL 3801C
Segment Registers
• These registers are used to store memory
locations of either instructions or data in
main memory.
• These registers contain the base segment of
the memory location – where the memory
segment begins.
EEL 3801C
Segment Registers (cont.)
• There are several of these:
– The CS Register: the code segment register
contains the base location of the executable
instructions that make up the program.
– Note that the base location can only address the
initial location where these instructions can be
found, not the entire segment..
EEL 3801C
Segment Registers (cont.)
– The DS Register: the data segment register is
the default base location in memory for variables
– The SS Register: the stack segment register
contains the base location of the run-time stack.
– The ES Register: the extra register is an
additional memory location where additional
base locations can be stored.
EEL 3801C
Index Registers
• Contain the offset (the distance from the
base segment) where a specific variable or
instruction may be found. The base
segment and the offset can uniquely identify
any addressable location of any length in
memory. Base segment + offset = memory
location.
EEL 3801C
Index Registers (cont.)
• There are several of these:
– The SI Register: the source index takes
name from the instruction used to move
strings.
• SI usually contains an offset from the DS
register, but can address any variable.
EEL 3801C
Index Registers (cont.)
– The DI Register: generally acts as a destination
for string movement instructions. Typically
contains an offset for the ES register, but not
necessarily so.
– The BP Register: the base pointer register
contains an offset from the stack register (SS).
• Used to locate variables in the stack.
EEL 3801C
Special Registers
• Do not fit into any other categories.
– The IP Register: the instruction pointer register
contains the offset of the next instruction to be
executed.
• Combines with CS to form the complete address of
the next executable instruction.
EEL 3801C
Special Registers (cont.)
– The SP Register: the stack pointer register
contains the offset from the beginning of the
stack segment to the top of the stack.
– SS and SP combine to form the complete
address for the top of the stack.
EEL 3801C
Flags Register
• One single 16-bit register whose individual
bit positions serve as flags to indicate the
status of the CPU or the result of some
arithmetic operation.
• The individual positions are predefined,
although not all 16 are defined.
EEL 3801C
Flags Register (cont.)
• Bit positions and flags:
– 0  Carry flag: Set when result of unsigned
arithmetic operation is too large to fit into
destination. Values are 1=carry; 0=no carry.
– 1  undefined
– 2  Parity flag: reflects the number of bits that
are set in the result of an operation. Can be
even or odd.
EEL 3801C
Flags Register (cont.)
– 3  undefined
– 4  Auxiliary carry: set when operation causes a
carry from bit 3 to bit 4. Rarely ever used.
– 5  undefined.
– 6  Zero flag: Set when result of an operation
results in zero. Used in jumping to other
instructions based on comparison of two values. Has
a value of 1when 0; 0 when ~0.
EEL 3801C
Flags Register (cont.)
– 7  Sign flag: Set when result of an operation
results in negative number. Value is 1 when
negative; 0 when positive.
– 8  Trap flag: Determines whether or not the
CPU will be halted after each instruction is
executed. Allows Trace or stepping through a
program’s execution. Allows the programmer to
control the CPU in this way through the INT 3
instruction.
EEL 3801C
Flags Register (cont.)
– 9  Interrupt flag: Makes it possible for external
interrupts to occur. Interrupts can be disabled
by setting this flag to 0. Controlled by the
programmer through the CLI and STI
instructions.
– A  Direction flag: controls the assumed
direction used by the string processing
instruction. Values are 1=up; 0=down.
Programmer can control this flag through the
STD and CLD instructions.
EEL 3801C
Flags Register (cont.)
– B  Overflow flag: Like the Carry flag, but for
signed arithmetic operations. Value is
1=overflow; 0=no overflow.
– C, D, E and F  undefined
EEL 3801C
The Run-Time Stack
• The run-time stack is an important element
in the execution of a stored program.
• It is a temporary holding area for addresses
and data.
• It resides in the stack segment identified in
the SS and SP registers.
• Each “cell” in the stack is 16 bits.
EEL 3801C
Run-Time Stack (cont.)
• The stack pointer holds the last element to
be added or pushed into the stack.
• This is also the first element to be taken off
the stack, or popped.
• This is referred to as Last-In-First-Out
(LIFO).
EEL 3801C
The Run-Time Stack (cont.)
• There are three typical uses for the run-time
stack:
– If we want to save the contents of a register, the
stack makes a great place to store their values
temporarily.
EEL 3801C
The Run-Time Stack (cont.)
– When a subroutine is called from another part of
the program, it is important that the processor
return to the place where the function was called
after it exits. The address of the instruction that
called the subroutine is saved on the stack so as
to be able to return to it later.
EEL 3801C
The Run-Time Stack (cont.)
– Local variables can be created when a subroutine
is active and then popped off the stack when the
subroutine returns to the calling instruction. This
is done in an area inside the run-time stack
called the stack frame.
EEL 3801C
The Run-Time Stack (cont.)
• Operations:
– The push operation: Used to put values of data
or instructions onto the stack. There is only on
place in the stack into which things can be
inputted – the top of the stack.
mov ax,00A5
; move 00A5 into AX
push ax
; pushes content of ax into stack
push bx
; assume BX has a value of 0001
push cx
; assume cx has a value of 0002
EEL 3801C
The Run-Time Stack (cont.)
• The push instruction
does not change the
value of the source
register (typically the
ax register, but could
be others). Rather it
simply copies its value
to the top of the stack.
High memory
0006
00A5
0001
0002
Low memory
EEL 3801C
SP
The Run-Time Stack (cont.)
– The pop operation: Used to remove the value in
the stack pointed to by the stack pointer and
places it in a register or memory location
(variable). Immediately upon removing the
element popped, the SP moves to the
immediately previous element in the stack.
pop ax
; pops stack and puts value into AX
EEL 3801C
The Run-Time Stack (cont.)
• Note that the value
remains in the stack,
but not being
pointed by the stack
pointer, it is subject
to be overwritten by
the next push
operation.
High memory
0006
00A5
0001
0002
Low memory
EEL 3801C
SP
Microinstructions
• Machine level instructions are not the lowest
level instructions in the computer.
Microinstructions are. These are very lowlevel operations that carry out the machinelevel instructions.
EEL 3801C
Microinstructions (cont.)
• There are three basic ones:
– fetch: the control unit fetches the instruction,
copies it into the CPU (register).
– decode: this operation decodes the instruction
as well as any operands specified by the
instruction. If any operands, the control unit
fetches the operand from main memory.
EEL 3801C
Microinstructions (cont.)
– execute: the ALU executes the operation and
passes the result operands to the CU, where
they are returned to the registers and/or to main
memory.
– Get next instruction
– Go back to step 1
• Microcode is the interface between the
binary code level and the electronic level.
EEL 3801C
Memory organization of DOS
• The Intel 8086 processor can access 1 Mb of
memory (actually, 1,048,576 bytes, which is
FFFFF in a 20-bit address). This is called the
Real Mode.
• The main memory is divided into RAM and
ROM.
– RAM occupies low memory, and starts at 00000h
and continues up to BFFFFh.
EEL 3801C
Memory organization of DOS
(cont’d)
– ROM occupies high memory and begins at C0000
and continues to FFFFF.
– This is mostly used for the ROM BIOS (the hard
disk controller).
– The BIOS contains diagnostic and configuration
software, as well as input-output subroutines.
EEL 3801C
Memory organization of DOS
(cont’d)
– Addresses begin with a hex address of
00000 and continue incrementally until
FFFFF.
– DOS allows only the first 640kB of RAM to
be used for programs.
– This is misleading because DOS (74kB) itself
has to occupy this area as well.
– Remaining RAM used by video display and
hard disk controller.
EEL 3801C
System Memory (cont.)
• The 80286 and more notably, the 80386 and
80486 processors can run in Protected Mode.
– This means that they can radically increase the
amount of memory they can address (16MB).
EEL 3801C
System Memory (cont.)
– The Pentium can address significantly more than
that.
– Unfortunately, DOS can only run in real mode.
– However, Windows runs in protected mode and
liberates the programmer from the 1MB memory
limit.
EEL 3801C
System Memory (cont.)
• The 80386 and beyond processor also has
the virtual 8086 mode, which allows
concurrent real mode processes to be
executed in by a single CPU.
• The total memory being used can total more
than the available RAM. The processor uses
external memory (hard disk drive or floppy)
to page currently unused portions of the
program to these devices.
EEL 3801C
Address Calculations
• An address is a number that refers to an 8bit (byte) memory location.
• The addresses are numbered consecutively,
starting at 00000h and going up to the
highest location in memory, depending on
the amount of memory available.
EEL 3801C
Address Calculations (cont.)
• Addresses can be expressed in one of two
ways:
– A 32-bit (16 + 16) segment-offset address. This
combines a base location (the base segment)
with the offset to represent the actual address.
For example, 08F1:0100, where 08F1 is the base
location (segment) from which to start counting,
and 0100 is the offset, or how much to count.
The address points to the first byte in the
address.
EEL 3801C
Address Calculations (cont.)
– A 20 bit absolute address, which refers to an
exact memory location. For example, F405Bh.
• Using 20 bits, the processor can only address
1 Mb (actually, 1,048,576 bytes) of memory.
EEL 3801C
Address Calculations (cont.)
• But address registers are only 16 bits wide,
limiting the addressable memory to 65,535.
• Thus, the segment-offset technique is used
to expand the range of accessible memory
beyond the 65,535 limit.
• Thus, when addressing memory locations,
the registers combine the values of two
registers, the base segment and the offset.
EEL 3801C
Address Calculations (cont.)
• The CPU uses the segment and offset value
to generate an absolute address. It adds the
segment and the offset to create the
absolute address. The segment value is
always known to have an implied half-byte at
the right (0000).
• Example: Given an address such as
08F1:0100. The absolute address (20 bit)
would be calculated as follows:
EEL 3801C
Address Calculations (cont.)
Segment value plus implied byte: 0 8 F 1 0 h
Add
the offset value:
0 1 0 0h
__________________________________________
Absolute Address:
0 9 0 1 0h
• The advantages to the segment offset method
is that it allows the program to be loaded into
any segment address in memory without
having to recalculate the addresses of all
variables.
EEL 3801C
Address Calculations (cont’d)
• Furthermore, large data structures that
occupy a large block of memory can be
easily accessed by knowing their base
segment and offset.
EEL 3801C