Transcript ICS312 Lecture1

ICS312 Set 3
Pentium Registers
Intel 8086 Family of
Microprocessors
All of the Intel chips from the 8086
to the latest pentium, have similar
architectures and use the
same core assembly language.
REGISTERS AVAILABLE TO
ASSEMBLER PROGRAMMER
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General registers
Pointer and Index Registers
Flags register
Segment registers
The registers are 8-bit, 16-bit, or 32-bit, highspeed storage locations inside the CPU which
can be accessed through the CPU's internal
data bus much more quickly than data stored
in external memory.
General Purpose or Data
Registers
The general purpose registers are used for
arithmetic operations and moving data
Upper Half
Register
Name
Lower Half
Name and Special Functions (besides arithmetic):
AX
EAX
no name
AH
EBX
Accumulator
AL
Base Register
BX
no name
BH
Used for indirectly addressing items in the data segment
BL
CX
ECX
CH
Count Register
CL
DX
EDX
DH
Used for I/O
Used for counting
Data Register
DL
Pointer and Index Registers
o These are used to hold the offset addresses of data and instructions.
o Offset addresses refers to the distance of a variable, label, or
instruction from the beginning of its segment
O Can also be used for arithmetic and moving data
Register Name
Destination Index
Abbreviation
EDI (32 bit) or DI
(16 bit)
Function
,
Index into data segment
but for the string
instructions it’s the index into segment ES
Source Index
ESI or SI
Index into data segment
Base Pointer
EBP or BP
2nd Index into stack
Stack Pointer
ESP or SP
Indexes top of stack
Flags Register (EFLAGS or
FLAGS)
o Individual bits in the FLAGS register give
information about the status of the processor.
o The use of flags will be discussed further later.
o Status flags - reflect the results of
instructions (add, subtract, multiply, divide)
executed by the processor.
o Frequently used flags
Zero flag (ZF)
Sign flag (SF)
Carry flag (CF)
Overflow Flag (OF)
Segment Registers
16-bit registers that hold the base locations(base
addresses) for program instructions, data, and the stack
The function of the segment whose starting address is
encoded in the segment register involved
Register Name
Abbreviation
Code Segment
CS
Holds program code
Data Segment
DS
Holds program data
Extra Segments
ES, FS, GS
Stack Segment
SS
Also used for data
Holds program stack
Memory Segments
o Memory segments contain 216 (64K) consecutive
memory addresses
o Each segment has a 16 bit segment number
(0-FFFFh) which is placed in a associated
segment register
o Within a segment a memory address is specified
by a 16-bit offset (0-FFFFh) which indicates
the number of bytes from the beginning of the
segment
Memory Address
Logical memory addresses are given in SEGMENT:OFFSET form.
Examples:
0FF0Eh:0010h
CS:IP
DS:SI
ES:DI
SS:SP
Physical (absolute) memory addresses are calculated using
the formula: Physical Address = 10h * SEGMENT + OFFSET
Example: Logical Address (using hex nos.): 0FF0E:0010
Physical Address: 0FF0E0 + 0010 = 0FFF00
Note. Logical Addresses are not unique.
e.g. 0FF0E:0010 =
0FF0D:0020
Instruction Pointer (EIP or IP)
o Gives the offset address (in the code
segment) for the next instruction to
be executed.
o The IP cannot be used as the operand
of any instruction.
Textbook Reading (Jones):
Read chapter 2, which provides an Assembler overview