Transcript Digital Image Processing

CoE3DJ4
Digital Systems Design
Chapter 7: Assembly Language Programming
Assembly language
• Assembly language is a computer language lying between
extremes of machine language and high level languages (e.g.,
C or Pascal)
• An assembly language program is a program written using
labels, mnemonics and so on in which each statement
corresponds to a machine instruction
• Assembly language programs, often called source code or
symbolic code, cannot be executed by a computer
• A machine language program (object code), binary bytes
representing instructions, is executable by a computer
• An assembler translates an assembly language program into a
machine language program
Assembly language
• Many 8051 assemblers are available
• Intel’s original MCS-51 family assembler (ASM51) is no
longer available but has set the standard for other assemblers
• We focus on the assembly language programming using the
common features of ASM51 which are supported by most
assemblers.
Assembly language programs
• Assembly language programs contain the following:
– Machine instructions
• Mnemonics of executable instructions
– Assembler directives
• Instructions to assembler defining program structure, symbols, data
constants and …
– Assembler controls
• Set assembler modes and direct assembly flow
– Comments
• Enhance the readability of programs
• General format of each line:
[label]
mnemonic
[operand][,operand][…][;comment]
Assembly language programs
• Label field: represents the address of the instruction that
follows
– Used in branching to this instruction
• Mnemonic field: Instruction mnemonics or assembler
directives
– Example: ORG, ADD, MOV
• Operand field: Contains the address or data used by the
instruction
– A label may be used to represent the address or a symbol may be used
to represent a data constant
• Comment field: Remarks to clarify the program
Assembly language programs
• Special Assembler Symbols: are used for register-specific
addressing modes
– Example: A, R0, DPTR, PC, C and AR0 to AR7
• AR0 to AR7 represent addresses of R0 through R7 in the
current register bank.
• $ can be sued to refer to the address of current instruction
Example: JNB TI, $ is the same as
HERE:
JNB
TI, HERE
Generic Jumps and Calls
• Assembler allows programmers to use a generic JMP instead
of SJMP, AJMP, or LJMP and CALL can be used instead of
ACALL or LCALL.
• Assembler converts JMP to SJMP if no forward references are
used and the jump destination is within -128 locations
• JMP and CALL are converted to AJMP and ACALL if no
forward references are used and the instruction following the
JMP or CALL instruction is in the same 2K block as the
destination instruction
• If short or absolute forms cannot be used, the conversion is to
the long form
Assemble-time expression evaluation
• Values and constants in the operand field may be expressed
with an expression (e.g., 2+3)
• Use of expressions makes assembly language programs more
readable and more flexible
• When an expression is used, assembler calculates a value and
inserts it into instruction
• Expression calculations are performed using 16-bit arithmetic,
however, either 8 or 16 bits are inserted into the instruction as
needed
Assemble-time expression evaluation
• Number bases: base for numerical constants is indicated by B
for binary, O or Q for octal, D or nothing for decimal and H
for hexadecimal.
MOV A, #15
MOV A,#1111B
MOV A, #17Q
• Character String: strings of one or two characters may be used
as operands. ASCII codes are converted to binary by
assembler. Character constants are enclosed in single quotes
(‘)
Example:
MOV DPRT, #’AB’
MOV DPRT, #4142H
Assemble-time expression evaluation
• Arithmetic operators: addition, subtraction, multiplication,
division and modulo
– Example: MOV A, #25 MOD 7 is the same as MOV A, #4
• Logical operators: OR, AND, XOR and NOT
– Example: MOV A, #’9’ AND 0FH is the same as MOV A, #9
• Special operators: shift right (SHR), shift left (SHL), highbyte (HIGH) , low byte (LOW)
– Example: MOV A, #HIGH 1234H and MOV A, #12H are the same
Assemble-time expression evaluation
• Relational operators: When a relational operator is used
between two operands, result is always false (0000H) or true
(FFFFH). The operators are:
EQ
NE
LT
LE
GT
GE
=
<>
<
<=
>
>=
– Example: MOV A, #5 NE 4
equals
not equals
less than
less than or equal
greater than
greater than or equal
is the same as MOV A, FFH
Assemble-time expression evaluation
• Operator precedence: from highest to lowest is
()
HIGH, LOW
*, /, MOD, SHL, SHR
+, EQ, NE< LT, LE, GT, GE,
NOT
AND
OR, XOR
Example: ‘A’ OR ‘A’ SHL 8
will result in 4141H
Assembler directives
• Assembler directives are instructions to assembler
• They are placed in mnemonic field of program
• They have no effect on the content of memory (except DB
and DW)
• Assembler directives:
–
–
–
–
–
Assembler state control
Symbol definition
Storage initialization/reservation
Segment selection
Program linkage
Assembler state control
• ORG: set origin
• Format:
ORG expression
• Example: ORG 100H
• END: last statement in source file
• No label is permitted
Assembler state control
• Some 8051 instructions (i.e., PUSH and POP) allow only register
addresses to be used.
• AR0 to AR7 are used to represent addresses of R0 through R7 in the
current register bank
• USING: informs the assembler of the currently active register bank
• By combining USING with symbols AR0 to AR7 we can specify the
address of any register in register banks.
• Format is: USING expression
• Example:
USING
PUSH
USING
PUSH
3
AR7
1
AR7
– First push will be assembled to PUSH 1FH whereas the second one will be
assembled to PUSH 0FH
• Note: USING does not actually switch register banks, it only informs the
assembler of the active bank
Symbol definition
• EQU: equate
• EQU assigns a numeric value to a specified symbol name
• Example:
N27
CR
EQU
EQU
27
0DH
Storage initialization/reservation
• A segment is a block of code or data memory created by
assembler
• There are two types of segments: generic and absolute
• Location counter: assembler maintains a location counter for
each segment
• Location counter is a pointer to the address space of active
segment and represents an offset for generic segment or the
actual address for absolute segment
• Memory initialization and reservation directives (i.e., DS, DB,
DW or DBIT) change the value of location counter as they
allocated memory
• ORG sets a new value for location counter
Storage initialization/reservation
•
•
•
•
Generic segments are created using the SEGMENT directive
Format:
symbol
SEGMENT segment_type
“symbol” is name of the segment
“segment_type” could be:
–
–
–
–
CODE: code segment
XDATA: external data space
DATA: internal data space accessible by direct addressing (00H-7FH)
BIT: bit space, overlapping byte locations 20H-2FH
• Example: EPROM
SEGMET
CODE
• In order to select a segment we use RSEG directive
– Example: RSEG
EPROM
• When RSEG is used to select a segment, that segment
becomes active and will be used by assembler until segment is
changed with RSEG or with an absolute segment directive
Storage initialization/reservation
• Absolute segments: reside in a fixed memory location
• They are created using BSEG, CSEG, DSEG and XSEG
directives
• Format:
–
–
–
–
BSEG
CSEG
DSEG
XSEG
[AT
[AT
[AT
[AT
• Example: BSEG
address]
address]
address]
address]
AT
30H
Storage initialization/reservation
• If an address is provided, assembler terminates the last
absolute segment of the specified type (if any) and creates a
new segment starting at that address
• If an address is not specified, the last address of the specified
type is continued
• If no absolute segment of this type was previously selected
and the address is omitted, a new segment is created starting
at location 0
• By default, assembler selects CODE segment as the active
segment and initializes counter to 000H.
Storage initialization/reservation
• Storage initialization and reservation directives initialize and
reserve space in word, byte or bit units.
• Space is reserved starts at the location indicated by the current
value of location counter in the currently active segment
• DS (define storage)
• Format:
[label:]
DS expression
• DS reserves space in byte units
• It can be used in any segment type except BIT
• Label represents the address of the first location of memory
Storage initialization/reservation
• Example: Create a 40 byte data buffer starting at address 30H
and fill it with zero.
LENGTH:
BUFFER:
LOOP:
DSEG AT
30H
EQU
40
DS
LENGTH
MOV
MOV
MOV
INC
DJNZ
R7,#LENGTH
R0,#BUFFER
@R0,#0
R0
R7,LOOP
Storage initialization/reservation
•
•
•
•
•
DBIT (define bit)
Format:
[label:]
DBIT expression
DBIT reserves space in bit units
It can be used only in BIT segment
Label (if used) represents the address of the first location of
memory
• Example:
KBFLAG:
BSEG
DBIT
1
Storage initialization/reservation
• DB (define byte)
• Format:
[label:]
DB expression
• DB initializes code memory with byte values (places data
constants in code memory)
• A CODE segment must be active for DB
• DB permits character strings
• Label (if used) represents the address of the first location of
memory
Storage initialization/reservation
• Example:
SQUARES:
MESSAGE:
CSEG
DB
DB
AT
0100H
0,1,4,9,16,25
‘Login’
• Resulting memory:
address
0100
0101
0102
0103
0104
0105
0106
0107
0108
0109
010A
value
00
01
04
09
10
19
4C
6F
67
69
6E
Storage initialization/reservation
• DW (define word)
• Format:
[label:]
DW expression
• DW is the same as DB except two memory locations are assigned to each
data
• Example:
CSEG
DW
address
0200
0201
0202
0203
0204
0205
0206
0207
AT
200H
$,‘A’,1234H,2
value
02
00
00
41
12
34
00
02