Assembler directives
8086 Microprocessor
Assemble Directives
Instructions to the Assembler regarding the program being
executed.
Control the generation of machine codes and organization of
the program; but no machine codes are generated for
assembler directives.
Also called ‘pseudo instructions’
Used to :
› specify the start and end of a program
› attach value to variables
› allocate storage locations to input/ output data
› define start and end of segments, procedures, macros etc..
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8086 Microprocessor
Assemble Directives
DB Define Byte
DW Define a byte type (8-bit) variable
SEGMENT Reserves specific amount of memory
ENDS locations to each variable
ASSUME Range : 00H – FFH for unsigned value;
00H – 7FH for positive value and
ORG 80H – FFH for negative value
END
EVEN General form : variable DB value/ values
EQU
PROC
FAR Example:
NEAR LIST DB 7FH, 42H, 35H
ENDP
Three consecutive memory locations are reserved for
SHORT the variable LIST and each data specified in the
instruction are stored as initial value in the reserved
MACRO memory location
ENDM
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8086 Microprocessor
Assemble Directives
DB Define Word
DW Define a word type (16-bit) variable
SEGMENT Reserves two consecutive memory locations
ENDS to each variable
ASSUME Range : 0000H – FFFFH for unsigned value;
0000H – 7FFFH for positive value and
ORG 8000H – FFFFH for negative value
END
EVEN General form : variable DW value/ values
EQU
PROC
FAR Example:
NEAR ALIST DW 6512H, 0F251H, 0CDE2H
ENDP
Six consecutive memory locations are reserved for
SHORT the variable ALIST and each 16-bit data specified in
the instruction is stored in two consecutive memory
MACRO location.
ENDM
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8086 Microprocessor
Assemble Directives
DB SEGMENT : Used to indicate the beginning of
a code/ data/ stack segment
DW
ENDS : Used to indicate the end of a code/
SEGMENT data/ stack segment
ENDS
General form:
ASSUME
ORG
END Segnam SEGMENT
EVEN
…
EQU … Program code
… or
PROC … Data Defining Statements
…
FAR …
NEAR
ENDP Segnam ENDS
SHORT
MACRO User defined name of
the segment
ENDM
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8086 Microprocessor
Assemble Directives
DB Informs the assembler the name of the
program/ data segment that should be used
DW for a specific segment.
SEGMENT General form:
ENDS
ASSUME segreg : segnam, .. , segreg : segnam
ASSUME
ORG
User defined name of
END Segment Register
the segment
EVEN
EQU
PROC Example:
FAR
NEAR ASSUME CS: ACODE, DS:ADATA Tells the compiler that the
ENDP instructions of the program are
stored in the segment ACODE and
data are stored in the segment
SHORT ADATA
MACRO
ENDM
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8086 Microprocessor
Assemble Directives
ORG (Origin) is used to assign the starting address
DB
(Effective address) for a program/ data segment
DW END is used to terminate a program; statements
after END will be ignored
SEGMENT
ENDS EVEN : Informs the assembler to store program/
data segment starting from an even address
ASSUME
EQU (Equate) is used to attach a value to a variable
ORG
Examples:
END
EVEN ORG 1000H Informs the assembler that the statements
EQU following ORG 1000H should be stored in
memory starting with effective address
1000H
PROC
FAR
LOOP EQU 10FEH Value of variable LOOP is 10FEH
NEAR
ENDP
_SDATA SEGMENT In this data segment, effective address of
SHORT ORG 1200H memory location assigned to A will be 1200H
A DB 4CH and that of B will be 1202H and 1203H.
EVEN
MACRO B DW 1052H
ENDM
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8086 Microprocessor
Assemble Directives
PROC Indicates the beginning of a procedure
DB
ENDP End of procedure
DW
FAR Intersegment call
SEGMENT
ENDS NEAR Intrasegment call
General form
ASSUME
ORG
procname PROC[NEAR/ FAR]
END
EVEN …
… Program statements of the
EQU procedure
…
PROC RET Last statement of the
procedure
ENDP
FAR procname ENDP
NEAR
SHORT User defined name of
the procedure
MACRO
ENDM
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8086 Microprocessor
Assemble Directives
DB
Examples:
DW
SEGMENT ADD64 PROC NEAR The subroutine/ procedure named ADD64 is
ENDS declared as NEAR and so the assembler will
… code the CALL and RET instructions involved
… in this procedure as near call and return
ASSUME …
RET
ORG
ADD64 ENDP
END
EVEN
EQU CONVERT PROC FAR The subroutine/ procedure named CONVERT
is declared as FAR and so the assembler will
… code the CALL and RET instructions involved
PROC … in this procedure as far call and return
ENDP …
FAR
RET
NEAR CONVERT ENDP
SHORT
MACRO
ENDM
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8086 Microprocessor
Assemble Directives
DB Reserves one memory location for 8-bit
signed displacement in jump instructions
DW
Example:
SEGMENT
ENDS
ASSUME JMP SHORT The directive will reserve one
AHEAD memory location for 8-bit
ORG displacement named AHEAD
END
EVEN
EQU
PROC
ENDP
FAR
NEAR
SHORT
MACRO
ENDM
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8086 Microprocessor
Assemble Directives
DB MACRO Indicate the beginning of a macro
DW ENDM End of a macro
SEGMENT General form:
ENDS
ASSUME macroname MACRO[Arg1, Arg2 ...]
Program
… statements in
ORG … the macro
END …
EVEN
EQU macroname ENDM
PROC
ENDP
FAR User defined name of
NEAR the macro
SHORT
MACRO
ENDM
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Interfacing memory and i/o ports
8086 Microprocessor
Memory
Processor Memory
Registers inside a microcomputer
Store data and results temporarily
No speed disparity
Cost
Primary or Main Memory
Storage area which can be directly
Memory accessed by microprocessor
Store programs and data prior to
Store
execution
Programs
Should not have speed disparity with
and Data
processor Semi Conductor
memories using CMOS technology
ROM, EPROM, Static RAM, DRAM
Secondary Memory
Storage media comprising of slow
devices such as magnetic tapes and
disks
Hold large data files and programs:
Operating system, compilers,
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8086 Microprocessor
Memory organization in 8086
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8086 Microprocessor
Memory organization in 8086
Operation A0 Data Lines Used
1 Read/ Write byte at an even address 1 0 D7 – D 0
2 Read/ Write byte at an odd address 0 1 D15 – D8
3 Read/ Write word at an even address 0 0 D15 – D0
4 Read/ Write word at an odd address 0 1 D15 – D0 in first operation
byte from odd bank is
transferred
1 0 D7 – D0 in first operation
byte from odd bank is
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8086 Microprocessor
Memory organization in 8086
Available memory space = EPROM + RAM
Allot equal address space in odd and even
bank for both EPROM and RAM
Can be implemented in two IC’s (one for
even and other for odd) or in multiple IC’s
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8086 Microprocessor
Interfacing SRAM and EPROM
Memory interface Read from and write in
to a set of semiconductor memory IC chip
EPROM Read operations
RAM Read and Write
In order to perform read/ write operations,
Memory access time read / write time of
the processor
Chip Select (CS) signal has to be generated
Control signals for read / write operations
Allot address for each memory location
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8086 Microprocessor
Interfacing SRAM and EPROM
Typical Semiconductor IC Chip
No of Memory capacity Range of
Address address in
pins hexa
In Decimal In kilo In hexa
20 220= 10,48,576 1024 k = 1M 100000 00000
to
FFFFF
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8086 Microprocessor
Interfacing SRAM and EPROM
Memory map of 8086
EPROM’s are mapped at FFFFFH
Facilitate automatic execution of monitor programs
and creation of interrupt vector table
RAM are mapped at the beginning; 00000H is allotted to RAM
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8086 Microprocessor
Interfacing SRAM and EPROM
Monitor Programs
Programing 8279 for keyboard scanning and display
refreshing
Programming peripheral IC’s 8259, 8257, 8255,
8251, 8254 etc
Initialization of stack
Display a message on display (output)
Initializing interrupt vector table
Note : 8279 Programmable keyboard/ display controller
8257 DMA controller
8259 Programmable interrupt controller
8255 Programmable peripheral interface
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8086 Microprocessor
Interfacing I/O and peripheral devices
I/O devices
For communication between microprocessor and
outside world
Keyboards, CRT displays, Printers, Compact Discs
etc.
Ports / Buffer IC’s
Microprocessor I/ O devices
(interface circuitry)
Data transfer types
Memory mapped
Programmed I/ O
Data transfer is accomplished I/O mapped
through an I/O port
controlled by software
Interrupt driven I/ O
I/O device interrupts the
processor and initiate data
transfer
Direct memory access
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Data transfer is achieved by 113
bypassing the microprocessor
8086 Microprocessor
8086 and 8088 comparison
Memory mapping I/O mapping
20 bit address are provided for I/O 8-bit or 16-bit addresses are
devices provided for I/O devices
The I/O ports or peripherals can be Only IN and OUT instructions can be
treated like memory locations and used for data transfer between I/O
so all instructions related to device and processor
memory can be used for data
transmission between I/O device
and processor
Data can be moved from any Data transfer takes place only
register to ports and vice versa between accumulator and ports
When memory mapping is used for Full memory space can be used for
I/O devices, full memory address addressing memory.
space cannot be used for
addressing memory. Suitable for systems which
require large memory capacity
Useful only for small systems
where memory requirement is less
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8086 and 8088 comparison
8086 Microprocessor
8086 and 8088 comparison
8086 8088
Similar EU and Instruction set ; dissimilar BIU
16-bit Data bus lines obtained by 8-bit Data bus lines obtained by
demultiplexing AD0 – AD15 demultiplexing AD0 – AD7
20-bit address bus 8-bit address bus
Two banks of memory each of 512 Single memory bank
kb
6-bit instruction queue 4-bit instruction queue
Clock speeds: 5 / 8 / 10 MHz 5 / 8 MHz
No such signal required, since the
data width is only 1-byte
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8087 Coprocessor
8086 Microprocessor
Co-processor – Intel 8087
Multiprocessor A microprocessor system comprising of two or more
processors
system
Distributed processing: Entire task is divided in to
subtasks
Advantages Better system throughput by having more than one
processor
Each processor have a local bus to access local
memory or I/O devices so that a greater degree of
parallel processing can be achieved
System structure is more flexible.
One can easily add or remove modules to change the
system configuration without affecting the other
modules in the system
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8086 Microprocessor
Co-processor – Intel 8087
8087 Specially designed to take care of mathematical
calculations involving integer and floating point data
coprocessor
“Math coprocessor” or “Numeric Data Processor (NDP)”
Works in parallel with a 8086 in the maximum mode
Features 1) Can operate on data of the integer, decimal and real
types with lengths ranging from 2 to 10 bytes
2) Instruction set involves square root, exponential,
tangent etc. in addition to addition, subtraction,
multiplication and division.
3) High performance numeric data processor it can
multiply two 64-bit real numbers in about 27s and
calculate square root in about 36 s
4) Follows IEEE floating point standard
5) It is multi bus compatible
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8086 Microprocessor
Co-processor – Intel 8087
16 multiplexed address / data pins
and 4 multiplexed address / status
pins
Hence it can have 16-bit external
data bus and 20-bit external address
bus like 8086
Processor clock, ready and reset
signals are applied as clock, ready
and reset signals for coprocessor
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8086 Microprocessor
Co-processor – Intel 8087
BUSY
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8086 Microprocessor
Co-processor – Intel 8087
The request / grant signal from the
8087 is usually connected to the
request / grant pin of the
independent processor such as 8089
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8086 Microprocessor
Co-processor – Intel 8087
INT
The interrupt pin is connected to the
interrupt management logic.
The 8087 can interrupt the 8086
through this interrupt management
logic at the time error condition
exists
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8086 Microprocessor
Co-processor – Intel 8087
Status
1 0 0 Unused
1 0 1 Read memory
1 1 0 Write memory
1 1 1 Passive
QS0 – QS1
QS0 QS1 Status
0 0 No operation
0 1 First byte of opcode
from queue
1 0 Queue empty
1 1 Subsequent byte of
opcode from queue
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8086 Microprocessor
Co-processor – Intel 8087
8086 and 8087 reads
instruction bytes and
8087 puts them in the
instructions respective queues
are inserted
in the 8086 NOP
program
8087 instructions have
11011 as the MSB of
their first code byte
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Ref: Microprocessor, Atul P. Godse, Deepali A. Gode, Technical publications, Chap 11
8086 Microprocessor
Co-processor – Intel 8087
8086/ 8088 Coprocessor
Wake up the
coprocessor
Monitor
ESC 8086/
8088
Deactivate the
Execute the host’s TEST pin
8086 and execute the
instructions specific
operation
Activate
WAIT the TEST
pin
Wake up the
8086/ 8088
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