Programming Model - Microprocessors and Computer Systems - Lecture Slides, Slides of Microprocessors

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Programming Model

Outline

a

Introduces the programming models of 8088 that contains aconcise description of the internal registers and theirfunctions.

a

Describes the effective address of 8088 (segment + offset).

Microprocessor Programming Model a

The programming model (software) summarizes all the informationneeded for programming the microprocessor.

a

Example: 8088 Programming Model 4

SegmentRegisters

4 Data Registers

2 PointerRegisters2 IndexRegisters

1 FlagRegister

bit 15

8

7

0

15

0

AX

AH

AL

Accumulator

SP Stack pointer

BX

BH

BL

Base

BP Base pointer

CX

CH

CL

Count

SI

Source index

DX

DH

DL

Data

DI Destination indexIP

Instruction pointer

15

0

CS

Code segment

15

0

DS

Data segment

Flags

SS

Stack segment

ES

Extra segment

Programming Model for Intel 80386

: Only available in80386 or higher

8-bit

Names

32-bitNames

16-bitNames

EAX

AH

AX

AL

Accumulator

EBX

BH

BX

BL

Base index

ECX

CH

CX

CL

Count

EDX

DH

DX

DL

Data

ESP

SP

Stack pointer

EBP

BP

Base pointer

EDI

DI

Destination index

ESI

SI

Source index

32-bits

16-bits

EIP

IP

Instruction pointer

EFLAGS

FLAGS

Flags

CS

Code

DS

Data

ES

Extra

SS

Stack

FSGS

Intel 80X86 Registers

a

Register in the 8088/8086 and 80286 may be grouped into 4sets:

(i) General registers (AX, BX, CX, DX)(ii) Pointer and Index registers (SP, BP, SI, DI, IP)(iii) Flag Register (FLAGS)(iv) Segment Registers (CS, DS, SS, ES)

a

The 80386, 80486 and Pentium microprocessors have 32-bitregisters, and the 16-bit registers of the 8088 form a subset.

Software model of the 8088

microprocessor

00000

16

FFFFF

16

8088 CPU

IP CSDSSSES

AH

AL

AX

BH

BL

BX

CH

CL

CX

DH

DL

DX SPBPSI DI

SR

The segment registersin CPU store the initialaddress information ofthe corresponding memory segments

SR: status (flag) register

Code segment(64 K bytes)Data segment(64 K bytes)Stack segment(64 K bytes) Extra segment(64 K bytes)

memory

Q: 16-bit

→

20-bit?

The segment register is16-bit, but the physicaladdress is 20-bit

20-bit

Pointer and Index Registers

a

This set of registers usually stores offset addresses of memory. IP usuallystores the offset address of the next instruction in memory. SP, BP, DIand SI usually store the offset address of data in memory.

a

SP, BP, DI and SI may also be used for general purposes.

a

SP (Stack Pointer) is used in the PUSH and POP instructions foroperations on a LIFO (Last-In, First-Out) stack.

a

BP(Base Pointer) is often used in addressing an array of data in the stackmemory.

a

DI (Destination Index) usually stores the indirect destination address ofdata from an instruction.

a

SI (Source Index) is used when indirectly addressing source data incertain string instructions.

Destination IndexInstruction Pointer

15

0

SP

Stack pointer

BP

Base pointer

DISI

Source Index

IP

U U U

OF

ZF U AF

8086 Flag Register Format

BIT

8085 COMPATIBLE FLAGS

15141312 11

10

9

8

7

6

5 4

3 2

1

0

U

DFIF TF SF

U PF UCF

U=

CARRY FLAG - SET UP CARRY OUT OF MSB

UNDEFINED

PARITY FLAG – SET IF RESULT HAS EVEN PARITYAUXILIARY CARRY FLAG FOR BCDZERO FLAG – SET IF RESULT = 0SIGN FLAG = MSB OF RESULTSINGLE STEP TRAP FLAGINTERRUPT ENABL

E FLAG

STRING DIRECTION FLAGOVERFLOW FLAG

FLAGS

a

Flags in the FLAGS register are either condition or control flags.

a

Condition flag consist of :

`

C (carry flag) - set to 1 when the result of an addition has a carry out of themost significant byte. Other instructions can also affect C (e.g. subtraction) `

P (parity flag) - set to 1 if the low-order byte of the result contains an evennumber of ones; otherwise it is set to zero. `

A (auxiliary carry flag) - set to one if there is a carry-out of the bit-3 during anaddition or a borrow by bit-3 during subtraction. `

Z (zero flag) - set to 1 if the result is zero; Z is otherwise zero. `

S (sign flag) - equal to the most significant bit of the result (i.e. set to 1 if theresult is negative) `

O (overflow flag) - set if a result is out of range (e.g. when adding two positivenumbers and the result appears negative)

O

D

I^

T

S

Z

A

P

C

FLAGS

Control Flags

a

Three bits (D, I, T) in the flags register control the operation of themicroprocessor under the following circumstances:

`

D (direction flag) - in certain string manipulation instructions, Ddetermines whether the string is processed from the lowest address(D=0) or the highest address (D=1). `

I (interrupt flag) - determines whether a maskable interrupt isrecognized by the microprocessor. If I=1, a maskable interrupt ispossible, otherwise the interrupt is ignored. `

T (trap flag) - if T=1, a trap (e.g. for single stepping through a program)is executed after every instruction.

O

D

I^

T

S

Z

A

P

C

FLAGS

D=0: auto-incrementD=1: auto-decrement

Segment Registers & Memory Segments a

Segment Registers are 16-bit registers (in CPU) used in conjunction withthe index and pointer registers to generate the physical 20-bit address.

Memory segments (in Memory): a

Code Segment is the section of memory used to store the programinstructions and procedures. CS register stores the starting address of theprogram code. In the 8086 (and 80286) the code segment is limited to64K bytes in length (in 80386 the maximum length is 4G bytes).

a

Data Segment contains data used by the program. DS register stores thestarting address of the data segment.

Bit-

bit-

CS

CODE SEGMENT Register

DS

DATA SEGMENT Register

ES

EXTRA SEGMENT Register

SS

STACK SEGMENT Register

Segment Registers & Memory Segments a

Extra Segment is another data segment which is used by some stringinstructions.

a

Stack Segment is the section in memory called stack which is used forstorage of register contents and data.

a

A segment in the 8088 system is a continuous section of memory of up to64K bytes (64K of 1-byte information; or 64 KB) in length.

a

The 64-Kbyte block defined by the start address of a segment mayoverlap with other segments or be completely in its own separate area ofmemory.

-- Don’t try it unless you really know what you are doing.

a

Segments

allow

packages

of

information

(e.g.

a

data

table,

or

a

subroutine) to be kept separately - it is not necessary to fill all 64K of thesegment and the programmer can make the segment of arbitrary size (upto 64K bytes, in 16 bytes increments).

Segments and Offsets (Cont.)

a

Since the segment address is derived by shifting 4 bits to form the 20-bitaddress, the start address of a segment can only occur at 16 data bytesintervals. Valid start address 00000h, 00010h, 00020h, …

a

Increasing the value of segment register by 1 will increase the physicalstarting address by 16.

a

Advantages of the segment + offset method include

`

Program code can easily be reallocated in memory (useful for multi-tasking) `

Most operations can be performed by changing only the 16-bitoffset. The offset can be stored in 16-bit registers (No need for 20-bit registers to store the address), allowing for easier interface to 8-and 16-bit wide memory.

Q: How do we know which Segment Address shall be used for a given offsetaddress?

Rules of combination of Segment

registers and Offset

a

The microprocessor has a set of rules that define the segment registerand offset register combination used by certain addressing modes.(Implication: you may not need to to specify the segment register used)

a

However, the default can be overridden by using the segment overrideprefix:

MOV CL, [BP]

(move a byte from location SS:[BP] to CL)

or

MOV CL,DS:[BP]

Offset register

DefaultSegment register

Override Prefix

IP

CS

Never

SP

SS

Never

BP

SS

DS, ES or CS

SI, DI (not include strings)

DS

ES, SS or CS

DI for string instructions

ES

Never