ENGR 4862 Microprocessors Lecture 7

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ENGR 4862 MicroprocessorsLecture 7
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Register Review

• What is a register
• A single memory element located inside the CPU
  (microprocessor chip)
• In 8086/8088, each register has 16 bits of info
• From a programming point of view, a register is a
  permanently declared variable
• Assembly code C code
• INC AX ax

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8086/8088 Programming Model
General Purpose Registers
Segment Registers
Code Segment
Accumulator
AX
AH
AL
CS
Data Segment
Base
BX
BH
BL
DS
Extra Segment
Count
CX
CH
CL
ES
Stack Segment
Data
DX
DH
DL
SS
Pointer Registers
Flag Register
Status and Control Flags
Stack Pointer
SP
FlagsH
FlagsL
Base Pointer
BP
Instruction Register
Index Registers
Instruction Pointer
Source Index
SI
IP
Destination Index
DI
Except in Data group, all registers are 16 bits
(2 bytes) long.
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Intel 80x86 Family Microprocessors

• Overview of Intel 80x86 Family
• Numbering, Coding and Converting
• 8086/8088 Architecture
• Segment Programming and Assembly Language
• Offset Address, Logical Address and Physical
  Address
• Addressing Mode

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Addresses

• Three types of addresses frequently used
• Physical address
• The 20-bit address that is actually put on the
  address pins of the uP and decoded by the memory
  interfacing circuitry
• Range from 00000H to FFFFFH for 8086/88 and 286,
  386, 486 for real mode
• Actual physical location in RAM or ROM within the
  1MB memory range
• Offset address
• A location within a 64K-byte segment range,
  0000HFFFFH
• Logical address
• Consist of a segment value and an offset address

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Program Segments

• A segment is an area of memory that includes up
  to 64KB and begins on an address evenly dividable
  by 16 (i.e., an address ends in 0H)
• Why 64K for segment?
• Its from 8085 uP (16 address pins) to ensure
  compatibility. 8085 has 64KB for all code, data,
  and stack. In 8086, therere up to 64KB to each
  segment.
• A typical assembly language program consists of
  at least three segments a code segment (CS), a
  data segment (DS), and stack segment (SS). Extra
  segment (ES) is not required for every program

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Code Segment

• To execute a program, the 8086/88 fetch
  instruction (including opcodes and operands) from
  the code segment
• The logical address of an instruction always
  consists of a CS and IP (CSIP)
• The physical address of the instruction is
  generated by shifting the CS left one hex digit
  (4-bit) and then adding it to the IP. The
  resulting 20-bit address is the physical address
  and it is put on the external address bus to be
  decoded by memory circuitry

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Example

• CS 2200H, IP 957BH
• Offset address 957BH
• Logical address 2200H957BH
• Physical address 2B57BH
• It can have many different logical addresses for
  one single physical address

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Data Segment

• Store data information that needs to be processed
  by the instructions in the code segment
• Data segment uses register DS and an offset value
• E.g., ADD AL, 200H AL ? AL DS0200H
• The offset address is enclosed in brackets, which
  indicates that the operand represents the address
  of the data and not the data itself
• Without the , it will be an immediate number
• The 8086/88 allows only the use of registers BX,
  SI, and DI as offset registers for the data
  segment

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Data and Extra Segment

• The physical address of data is calculated using
  the same rules as for the code segment
• Extra Segment (ES)
• Many normal programs do not use
• Essential in String operations

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Little Endian, Big Endian

• When deal with 16-bit data fetch or store
• E.g., MOV AX, 35F3H
• MOV 1500H, AX
• Result DS1500H ??, DS1501H ??
• Little/Big Endian from Gullivers Travel Story
• Little Endian Convention low byte goes to the
  low memory location and the high byte goes to the
  high memory address
• All Intel uP, DEC VAX use little endian
  convention, while Motorola, Macintosh uP, and
  some mainframes use big endian convention