Advanced Microprocessor Presentation 4

8/4/2019 Advanced Microprocessor Presentation 4

1/21

ADVANCED MICROPROCESSORS

Session IV

Prof. S. Jagannathan,

HOD Department of Electronics andCommunication Engineering,

R.V. College of Engineering, Bangalore


2/21

Contents

Control signals indicating actions in 8086

Real mode memory addressing


3/21

The control signals indicated below govern all

the actions of the pertinent blocks.

The queue status signals indicate what is the

present status of the queue.


4/21

Queue Status

Subsequent byte from the queue11

Empty queue01

First byte of the opcode from the

queue

10

No operation00

IndicationQS0QS1


5/21

The memory in 8086 is organized into odd and evenblocks of bytes of data. The memory manufacturers

never make the memory registers of size 16 bits or

32 bits etc. Normally, byte size registers are presentin memory. For example, 256 X 8 type of memories

are available indicating 256 bytes can be stored in

such a memory chip. These memory blocks can be

divided into odd group as well as even group of

memory.


6/21

BHE

In order to access even group of memory i.e. theaddresses will end in 0, 2, 4, 6, 8, etc. (the LSB is

A0=0), all that one has to do is to output the

address. On the other hand in order to access odd

group of memory i.e. the addresses will end in 1, 3,

5,7,9 (the LSB is A0

1) we have to send bus highenable ( ) signal for chip select.


7/21

Bus High Enable / Status

none11

Lower byte from or to even address01

Upper byte from or to odd address10

Whole word00

IndicationsA0

BHE


8/21

Now there are 8 status signals in 8086 as

compared to only 2 in 8085. The signals and

their functions are as given below.


9/21

Status Details

Passive111

Write memory011Read memory101

Code access001

1

1

0

0

Halt10

Write I/O port00

Read I/O port10

Interrupt Acknowledge00

Indication2S 1

S 0S


10/21

Data11

Code or none01

Stack10

Alternate data00IndicationsS3S4


11/21

5S ----- Value of Interrupt Enable flag

----- Always low (logical) indicating 8086 is

on the bus. If it is tristated another bus

master has taken control of the system bus.

6S

----- Used by 8087 numeric coprocessor to

determine whether the CPU is a 8086 or

8088

7S


12/21

Real mode memory addressing

The segment registers have contents of 16-bits.Hence, 216 = 64Kb of memory can be addressed by

segment registers. Normally, the segment base

register contains three zeroes, so that each segmentcan start from say E0000 to EFFFF. The segments

namely code segment, data segment, stack segment

and extra segment for a particular program can becontiguous, separate or in case of small programs

overlapping even.


13/21

For example, code segment is supposed to have64Kb and in case of small programs data segmentmay be within the code segment. To give you anexample of the segment base and offset, we canconsider the telephone numbers. For example,23322651 is a telephone number out of which, 2 isan universal code, 332 is the area code, and 2651 isthe offset in that area. In other words, the areatelephone numbers can occupy 23320000 to23329999.


14/21

One way four 64-Kbyte segment might bepositioned within the 1-Mbyte address space of an 8086

PHYSICAL

ADDRESS MEMORYFFFFFH HIGHEST ADDRESS

70000H

7FFFFH

EXTRA SEGMENT BASE ES=7000H

TOP OF EXTRA SEGMENT

64K

5FFFFH

STACK SEGMENT BASE SS=5000H

TOP OF STACK SEGMENT

64K

50000H

TOP OF CODE SEGMENT

CODE SEGMENT BASE CS=348AH64K

4489FH

348A0H

TOP OF DATA SEGMENT

BOTTOM OF DATA SEGMENT

64K

2FFFFH

20000H


15/21

Addition of IP to CS to produce thephysical address of the code byte

PHYSICAL

ADDRESS

MEMORY

4489FH TOP OF CODE SEGMENT

348A0H

38AB4H CODE BYTE

START OF CODE SEGMENTCS=348AH

IP=4214H


16/21

Segmentation

The 8086 microprocessor has 20 bit address pins. These

are capable of addressing 220 = 1Mega Byte memory.

To generate this 20 bit physical address from 2 sixteen bit

registers, the following procedure is adopted.

The 20 bit address is generated from two 16-bit registers.

The first 16-bit register is called the segment base register.

These are code segment registers to hold programs, data

segment register to keep data, stack segment register for

stack operations and extra segment register to keep strings

of data.


17/21

The contents of the segment registers are shiftedleft four times with zeroes (0s) filling on the right

hand side. This is similar to multiplying four hex

numbers by the base 16. This multiplication

process takes place in the adder and thus a 20 bit

number is generated. This is called the base address.

To this a 16-bit offset is added to generate the 20-

bit physical address.


18/21

Segmentation helps in the following way. The

program is stored in code segment area. The data

is stored in data segment area. In many cases theprogram is optimized and kept unaltered for the

specific application. Normally the data is variable.


19/21

So in order to test the program with a different

set of data, one need not change the program but

only have to alter the data. Same is the case withstack and extra segments also, which are only

different type of data storage facilities.


20/21

Generally, the program does not know the exact

physical address of an instruction. The

assembler, a software which converts theAssembly Language Program (MOV, ADD etc.)

into machine code (3EH, 4CH etc) takes care of

address generation and location.


21/21

Today we saw the Control signals indicating

actions in 8086, Real mode memory addressing

Advanced Microprocessor Presentation 4

Documents

Transcript of Advanced Microprocessor Presentation 4