Microprocessors used in Personal Computers

Manufacturer Processor Data Bus Address Bus Memory size

Intel 8086 16 20 1M8088 8 20 1M80386 32 32 4G80486 32 32 4G + 8K CachePentium 64 32 4G + 16K CachePentium Pro 64 36 64G + 16K L1 Cache

+ 256 L2 CacheMotorola 6800 8 16 64K

68000 16 24 16M68040 32 32 4G + 8K CachePowerPC 64 32 4G + 32K Cache

The Memory Map of a Personal Computers

• Transient Program Area (TPA): Holds the operating system (interrupt vectors, DOS BIOS, devices drivers, Command.com) and application programs.

• System Area: Holds the ROM BIOS, the video buffer area, and the BASIC language ROM.

• Extended Memory: Memory above the 1M conventional memory found in AT compatible computers . Its size depends on the RAM installed on the motherboard, and is limited by the size of the processor’s address bus.

TPA640K

System Area384K

ExtendedMemory

15M . . 64G

The 80x86 Programming Model

AH ALAX AccumulatorEAX

BH BLBX BaseEBX

CH CLCX CountECX

DH DLDX DataEDX

SP Stack PointerESP

BP Base PointerEBP

DI Destination IndexEDI

SI Source IndexESI

IP Instruction PointerEIP

Flags Flag RegisterEFLAGS

CS Code Segment0

DS 0

0

ES 0

SS

FS 0

GS 0

Data Segment

Stack Segment

Extra Segment

078151631

80386 to Pentium only

The Flag Register

• Z (zero): This flag is set to one if the result of the last arithmetic or logic instruction is zero.

• S (sign): This flag is set to one if the MSBit of the result of the last arithmetic or logic instruction is one, indicating a negative number .

• C (carry): This flag is set to one if the last arithmetic instruction gave a carry out or a borrow in.

• O (overflow): This flag is set to one if the result of the last arithmetic operation on signed numbers exceeded the capacity of the destination register.

NT

14

IOP1

13

IOP0

12

O

11

D

10

I

9

T

8

S

7

Z

6

A

4

P

2

C

0

VIF

19

AC

18

VM

17

RF

1631

ID

21

VIP

20

8088/8086

80286

80386

80486

Pentium

80x86 Modes of operation

• Protected mode– Programs are given separate memory areas (segments)– Programs are not allowed to access memory outside of their

segments

• Real-address mode– Environment of 8086 processor– Direct access to system memory and hardware devices– The operating system could crash

• System management mode– Provides an operating system for implementing power

management and system security functions (computer manufacturers)

Segmentation

• Segmentation is used to allow relocation of programs, i.e. programs can be loaded on different memory areas and still run correctly.

• Segmentation is used in the 8088/8086 microprocessors to allow the generation of 20-bit addresses using 16-bit registers.

• In the Real Mode Operation a 20-bit address (effective address) is obtained by shifting the segment address 4 bits to the left (X10H) and then adding the offset address.

• The offset address is specified in the program. The segment address is specified by the operating system whenever the program is loaded.

• The code segment holds the machine codes of the program.The Instruction Pointer specifies the offset address in the code segment.

• The data segment holds the data used by the program. Most data references are specified in the data segment.

• The stack segment holds the stack of the program. The offset address in the stack segment is specified with the registers SP and BP.

• The extra segment is used as a data segment by some data movement instructions.

Segmentation (Example)

If DS =1600H find the maximum area occupied by the data segment. Find also the effective address, if the offset address is 1F00H.

Starting address = DS X 10H = 1600H X 10H

= 16000H

Ending address = Starting address + FFFF

= 16000H + FFFFH = 25FFFH

Effective address = Segment:Offset = 1600:1F00

= Segment address X 10H +

Offset

= 1600H X 10H + 1F00H

= 16000H + 1F00H = 17F00H

64Kbytesegment

FFFFF

25FFF

17F00

16000

00000

1600

SegmentRegister

Offset 1F00

Assembler Directives

Directives are instructions given by the programmer to the assembler on how to assemble the program.

Directives are not part of the instruction set of the microprocessor.

The use of directives might vary from assembler to assembler.

Some of the MASM 6.12 assembler directives are:– ORG (Origin): Tells the assembler where to store the machine code of

the next instruction.

– EQU (Equate): Tells the assembler to assign a value to an identifier.

– SEGMENT: Tells the assembler to begin a new segment.

– ASSUME: Tell the assembler to associate a segment with a segment register.

– PROC (Procedure): Tells the assembler to begin a new procedure.

– MACRO: Assigns the sequence of instructions to an identifier.

– END: Ends a program, segment (ENDS), a procedure (ENDP), or a macro (ENDM).

Data Definition Directives

• Data Definition directives tell the assembler to store the specified data in the next memory locations. Data that occupies more than one location is stored with the LSByte in the lower address.

– (DB) Define Byte (1 byte or 8 bits - Same as BYTE)

– (DW) Define Word (2 bytes or 16 bits - Same as WORD)

– (DD) Define Double Word (4 bytes or 32 bits - Same as DWORD)

– (DQ) Define Quad Word (8 bytes or 64 bits - Same as QWORD)

– (DT) Define Ten Bytes (10 bytes or 80 bits - Same as TBYTE)

Examples:• ABC DB 26H ;ABC=26H• XYZ DB 35H,87H,0A4H ;XYZ= {35H,87H,A4H}• YOU DB ‘JOHN’ ;YOU = ‘JOHN’• VAL DW 1254H ;VAL = 1254H• X1 DB ? ;X1 = UNSPECIFIED• X4 DB 3 DUP(20H) ;X4 = {20H,20H,20H}• N1 DB 123H ;INVALID

Data Definitions (Example)

Show the content of the memory based on the following data definitions:

ORG 100H

VAL1 EQU 21H

AB10 DB 37

NEW DB 23H,56H,’$’

LOT DW 1245H

XY11 DB ‘NEXT’

A123 DD 123H

B561 DB 4 DUP(40H)

VAL2 DB VAL1

AB10

Label

NEW+0

NEW+1

NEW+2

LOT

XY11+0

XY11+1

XY11+2

A123

XY11+3

B561+1

B561+0

B561+2

B561+3

VAL2

DS:0100

Address

DS:0101

DS:0102

DS:0103

DS:0105

DS:0104

DS:0106

DS:0107

DS:0108

DS:010A

DS:0109

DS:010B

DS:010C

DS:010D

DS:010F

DS:010E

DS:0110

DS:0111

DS:0112

Content

12H

45H

Format of DOS programs

• All programs must have a code and a stack.

– Code is the part of the program that contains the instructions of the program.

– Stack is an area in the RAM used by the system to store return addresses, and by the programmer to store temporarily data. It is a Last In First Out (LIFO) buffer.

• Programs can also have a data area, where all data (variables) is stored.

• There are two basic types of programs:

– Commands (.COM). The data and the stack of the program are part of the Code segment. The stack is always located at the end of the segment. The first 256 bytes of the segment are reserved.

– Executable (.EXE). The code and stack and data of the program are located in different segments.

Format of the .COM programs

CSEG SEGMENT PARA 'CODE’ ;Start a Code segment

ASSUME CS:CSEG, DS:CSEG, SS:CSEG

ORG 100H ;Reserve first 256 locations

START: JMP MAIN ;Skip data area

{Place the data of the program here}

MAIN PROC NEAR ;Beginning of main procedure

{Place the code of the program here}

RET ;Get return DOS address

MAIN ENDP ;End of main procedure

CSEG ENDS ;End of the segment

END START ;End of the program

Addressing Modes

Addressing mode refers to the way the data

needed by an instruction is specified.

Immediate Addressing Mode

• The data needed is specified as a number in the machine code of a program. The data is specified by the programmer:

– as a numeric operand in the instruction,

e.g. MOV AL,87H ;AL 87H

MOV CX,34A6H ;CX 34A6H

MOV BL,8C2H ;Invalid (Data Mismatch)

– or as a label. The actual value is determined by the assembler.

e.g. MOV BX,OFFSET VAL3 ;BX Address of VAL3

MOV AH,CON1 ;AH CON1

Immediate Addressing Mode (Example)

Instructions

AX

MOV CX,CON2

BX SI

Label Address Data

0105

0106

0107

0108

0109

010A

010B

010C

010D

MOV BX,OFFSET DAT1

MOV SI,2310H

MOV AL,CON1

MAIN PROC NEAR

MOV AX,283CH

MOV AX,OFFSET DAT3

CX

ORG 100H

CON1 EQU 6CH

CON2 EQU 245AH

STRT: JMP MAIN

AH AL BH BL CH CL

DAT1 DB 2FH,48H

DAT3 DW 37AH

DAT2 DB 4 DUP (0)

0100

0101

0102

0103

0104

Register Addressing Mode

• Both of the operands are the contents of registers.

e.g. MOV AL,BH ;AL BH

MOV BX,CX ;BX CX

MOV AX,DL ;Invalid (Data Mismatch)

• Example:Instructions AX

MOV CX,AX

BX SI

MOV BX,CX

MOV AL,DH

MOV AH,AL

MOV SI,DX

CX DX

AH AL BH BL CH CL DH DL

36 20 EE A5 90 89 73 F6 2006

Direct Addressing Mode

• One of the operands is the contents of the memory location that is specified directly in the instruction.

e.g. MOV AL,[1008H] ;AL [1008H]

MOV BX,VALUE ;BX [VALUE]

Instructions

AX

MOV DX,VAL1

BX Label Address Data

0105

0106

MOV BX,[0108H]

MOV AL,[0104H]

MOV CH,FRST

MOV AH,ARR1

MOV CL,ARR1+3

CX DX

AH AL BH BL CH CL DH DL

0107

0108

0109

0103

0104

FRST

VAL1

ARR1

C1

56

87

9F

A6

75

8C

Register Indirect Addressing Mode

• One of the operands is the contents of the memory location that is specified by a register, or a combination of registers and an offset, in the instruction.

Index: Use of SI or DI to specify a memory location.

e.g. MOV AL,[SI] ;AL [SI]

- Base: Use of BX or BP to specify a memory location.e.g. MOV AH,[BP] ;AL [BP]

- Base Relative: Use of BX or BP in combination with an offset to specify a memory location.

e.g. MOV AL,[BX+ 2] ;AL [BX + 2]

- Base Relative plus Index: Use of BX or BP in combination with an index register (SI or DI) and an offset to specify a memory location.

e.g. MOV AL,[BX+SI+8] ;AL [BX+SI+8]

MOV BX,ARR[BX+DI] ;BX ARR[BX+DI]

Register Indirect Addressing Mode (Example)

Instructions

AX

MOV AX,DAT1[DI]

BX SI

Label Address Data

0105

0106

0107

0108

0109

010A

010B

010C

010D

MOV AL,[BX]

MOV AH,DAT2[SI]

MAIN PROC NEAR

MOV AL,[BX+SI]

MOV AH,[BX+DI+3]

DI

ORG 100H

STRT: JMP MAIN

AH AL BH BL

DAT1 DB 2FH,48H

DAT3 DW 7A5H

DAT2 DB 12H,10H,18H

0100

0101

0102

0103

0104

DAT4 DW 37H

DAT5 DB 10H

77 45 01 04 0001 0002

Addressing Modes (Example)

Instructions AX

MOV AX,2F8AH

BX SI Address Label Address Data

0112

0111

0110

010F

0105

0106

0107

0108

0109

010A

010B

010C

010D

010E

4F

8C

5A

AC

93

59

4F

A3

7E

F4

09

8A

5C

6A

MOV BX,OFFSET LAB4

MOV SI,3

MOV AL,LAB1

MOV AH,BL

MOV AL,LAB2+3

MOV AX,LAB3

MOV AL,LAB2[SI]

MOV AL,[BX+1]

MOV AL,[SI+102]

MOV AL,[BX+SI-1]

MOV AL,LAB4[SI+2]

MOV AL,LAB2[SI+5]

MOV AL,LAB3+1

Addr. Mode

Lab1

Lab2+0

Lab2+1

Lab2+2

Lab2+3

Lab3+0

Lab3+1

Lab4+0

Lab4+1

Lab4+2

Lab4+3

Lab4+4

ExamplesIndicate whether or not each of the following MOV instructions is valid or invalid• MOV AX,BX• MOV DX,BL• MOV ECX,EDX• MOV SI,DI• MOV DS,AX• MOV AL,DH• MOV AX,DH• MOV IP,AX• MOV SI,CL• MOV EDX,AX• MOV AX,ES

• MOV AX,16

• MOV DX,7F65H

•MOV ECX,6F23458H

•MOV SI, -1

•MOV DS,1000H

•MOV AL,100H

•MOV 123,DH

•MOV 0FABH,AX

•MOV SI, CL

•MOV EDX,ESI

•MOV EDX,-2