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The Intel Microprocessor Architecture

System Overview

We learned that memory was needed so that there would be a place for data & instructions

to be stored. Data & programs which can be lost after power is removed are stored in

RAM. Data & instructions which must never be lost, even after the power is turned off, are

stored in ROM. Remember that ROM is a type of memory which cannot have its contents

changed once the ROM chip is manufactured. PROM & EPROM are used in much the

same way as ROM but can be reprogrammed after manufactured but require special

equipment to program them.

Block Diagram of a Complete Computer with Peripheral Devices (Arrows indicate Data Flow)

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Addressing • Since there are many memory locations (or,

addresses), it is necessary to have a means of referring to specific locations.

• This is done through addressing. Typically, RAM addresses are numbered 0000H (in hex) to the highest addressable memory.

• For example, using 12 binary digits, how many unique memory locations would be possible?

212 = 4096 unique addresses from 0000 0000 0000 up to 1111 1111 1111 or

0 0 0 hex F F F

μP Architecture

• Accumulator: one of the most often used parts of a μP is the accumulator – a register which often has its contents altered in some way.

• For example, we can add the contents of the accumulator to the contents of a RAM address. Usually, the result of an operation is also placed in the accumulator.

• Width of Registers: maximum size of bits is generally, 8, 16, 32, or 64

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• General-Purpose Registers: are similar to the accumulator, and are temporary storage locations. They differ from accumulator in that operations involving two pieces of data are usually not performed in them with the result going back into the register itself, as the case of the accumulator.

• Program Counter (PC) or Instruction Pointer (IP): considering the fact that there can be millions of RAM locations, it’s obvious that the μP must keep track of the location from which it will be getting its next instruction. This is the job of the PC or IP.

• Status Register: sometimes called condition code register, or flag register, is a special register which keeps track of certain facts about the outcome of arithmetic, logical, & other operations. This register makes it possible for the μP to be able to test for certain conditions, & then to perform alternate functions based on those conditions. This is done through the use of flags.

• Stack Pointer: the structure of a stack is a first-in-last-out (FILO), where unlike main memory, where you can access data item in any order. The stack is designed so that you can only access (through stack pointer) the top of the stock. We can push an item (save operation) onto the stack so that we can later pop them (retrieve operation)

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AL

BL

CL

DL

AX

BX

CX

DX

SP

BP

DI

SI

AH

BH

CH

DH

32 bits

16 bits

IP

CS

DS

ES

SS

FS

GS

Accumulator

Base index

Count

Data

Stack pointer

Base pointer

Source index

Destination index

Instruction pointer

Code segment

Data segment

Extra segment

Stack segment

32-bit names

EAX

EBX

ECX

EDX

ESP

EBP

EDI

ESI

EIP EFlags

Name

The diagram on the left is a complete programming model of the 8086/8088 μPs in addition to the RAM address available for programmers but only starting at address 0100H

0100 HH

0101 HH

0102 HH

0103 HH

0104 HH

0105 HH

0106 HH

.

:

:

:

FFFF HH

Address RAM

Intel 8086/8088 μP Programming Model

Lesson Check-Up

1. Describe how the 8088/8086 accumulator is labeled.

2. How many 8-bit general-purpose registers does 8088 have?

3. In the 8088 what has the same function as the program counter in 8-bit μPs?

4. If we had 20,00010 memory address lines, what would be the last address line needed to describe an item in hex 2 bytes wide?

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5. In simplest terms, what are general-purpose registers?

6. A register which helps μPs to work with tables of data?

7. When a flag register is . . . . . . this indicates that the condition which the flag tests has not come true.

8. When a flag register is . . . . . . this indicates that the condition which the flag tests has come true.

9. Refer to Figure shown. If we POP data item #2 (two-bytes) from the stack, will the stack pointer (SP) increment or decrement? What hex value will appear in the SP?

10. Refer to the same Figure, if we PUSH three data items onto the stack 1-byte each, will the SP increment or decrement? What hex value will appear in the SP?