Transcript TK2123-Lecture13-Assembly Language Level

TK 2123
Lecture 13: Assembly Language
Level (Level 4)
Ass. Prof. Dr Masri Ayob
Why Use Assembly Language?
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Assembly language programming is difficult.
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Writing a program in assembly language takes much
longer than writing the same program in a high-level
language.
Much longer to debug and much harder to maintain.
Why use assembly language?
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Performance
Access to the machine
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Why Use Assembly Language?
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Performance
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An expert assembly language programmer can often
produce code that is much smaller and faster than a highlevel language programmer can.
 Some application – speed and size are critical. E.g.
embedded application (code on a smart card,
handphone, etc.)
Access to the machine.
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Some procedure need complete access to the hardware.
 E.g. low-level interrupt, trap handlers in OS, device
controller, etc.
 This is usually impossible in high-level language.
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Why Use Assembly Language?
Comparison of assembly language and high-level language
programming, with and without tuning.
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Assembly Language
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Assembly language:
 Assembly language is used for most programming
because it is extremely difficult to program a
microprocessor in its native, that is hexadecimal machine
language.
Assembler:
 An assembler is a program that converts software written
in symbolic machine language (the source program) into
hexadecimal machine language (object program).
 The primary reason to use assembler is because
development and modification are always difficult in
machine language.
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Assembly Language
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Instruction is a command to the microprocessor to
perform a given task on specified data.
Each instruction has two parts:
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one is the task to be performed, called the operation code
(op-code),
the second is the data to be operated on, called the
operand.
 The operand (or data) can be specified in various ways.
 It may include an internal register or a memory
location.
 In some instructions, the operand is implicit.
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Assembly Language
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The Two-pass Assembler :
 Read programme two times.
1. Generate a table of the labels/symbols
within the source program.
2. Develop hexadecimal version of the source
program.
 Allow forward addressing (the software can
jump ahead to an instruction in a program).
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Assembly Language
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Assembly Language
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The assembler always assumes that the first instruction of the
program is stored at memory address 0000H unless otherwise
directed by the ORG command.
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Assembly Language
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Pass One:
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The assembler scans the source program during the
first pass and generates a table of the labels found
within the source program.
Each entry in the label table contains the label and
the address where the label appears in the program.
During the first pass the assembler determines the
length of each instruction by updating an internal
program counter.
This internal program counter allows the assembler
to complete the label table by equating each label
with the counter.
Once the label table is complete the second pass
begin.
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Assembly Language
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Pass Two:
 During the second pass of the source program,
the assembler forms the object program.
 This occurs by referring to the label table for any
labels that appear in the program and to an
instruction table.
 The instruction table contains all the opcodes in
both symbolic and machine language forms.
 The tables help convert the source program into
the object program.
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Assembly Language
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Assembly Language Statement:
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Format :
Label Field.
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Contains a symbolic memory address that refers to the
statement in a program. Labels are optional.
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Labels are constructed from alphanumeric characters
and must begin with any letter of the alphabet.
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Format of an Assembly Language Statement (1)
Computation of N = I + J. (a) Pentium 4.
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Format of an Assembly Language Statement (2)
Computation of N = I + J.
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(b) Motorola 680x0.
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Format of an Assembly Language Statement (3)
Computation of N = I + J. (c) SPARC.
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Assembly Language
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Opcode field:
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Operand field:
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This field must contain opcodes.
May contain register name, data or labels.
If more than one of these is present, they must be separated with
comma.
Data must be encoded as decimal, binary, octal, hexadecimal, or
ASCII.
ASCII must appear as one of more letters surrounded by apostrophe.
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Assembly Language
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Operand arithmetic operations.
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Assembly Language
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Comment field.
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Must begin with semicolon in most 8085 assemblers
and may continue to the end of the line only.
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Use asterisk * or semicolon ; if the comment should
continue into the next line.
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Example :
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Pseudoinstructions
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Assembler pseudo operations.
 Directives to the assembler program that may or
may not generate machine code.
 Examples :
 END, DB, DW, DS, ORG, EQU, IF, ENDIF, SET,
GLB, EXT, TITLE, SPC.
 All pseudo operations must appear in the opcode
field of a statement.
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Pseudoinstructions
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Define Byte (DB).
 Defines 8-bit memory data for a program.
 Multiple one byte data, comma ( , ) as a
separator.
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Assembly Language : Example
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Pseudoinstructions
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Origin (ORG).
 Changes the starting location of the
program to another address besides
0000H.
 Can be used at any place in a program to
change the location of the assembled
machine language instructions or data.
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Assembly Language : Example
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Pseudoinstructions
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Define Word (DW).
 Pseudo operation stores a 16-bit number
in the memory for use by a program.
 Defines no only numeric data but also
memory addresses and label.
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Assembly Language : Example
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Pseudoinstructions
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Define Storage (DS).
 Reserves space in a program for
variable data.
 Does not place any specific data into
the reserved area of memory.
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Assembly Language : Example
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Pseudoinstructions
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Equate (Equ).
 Equates a label to another label or
value.
 Note that the EQU statement label does
not contain a colon ( : ).
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Assembly Language
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Macro
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Assembly language programmer frequently need to
repeat sequence of instructions several times within a
program.
Solution:
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Use subroutine (CALL instruction).
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Disadvantage: Procedure CALL overhead.
Use Macro
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Easy and efficient solution
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Is a way to give name to a piece of text.
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Macro Definition, Call, Expansion
(1)
Macro
Assembly language code for interchanging P and Q twice.
(a) Without a macro.
(b) With a macro.
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Macro Definition, Call, Expansion
(2)
Comparison of macro calls with procedure calls.
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Macros with Parameters
Nearly identical sequences of statements.
(a) Without a macro. (b) With a macro.
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Thank you
Q&A
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