Fundamental of Assembly Language Programming (for Microprocessor) Prima Dewi Purnamasari...

Fundamental of Assembly Language Programming (for Microprocessor)

Prima Dewi PurnamasariMicroprocessor

Electrical Engineering Department Universitas Indonesia

Computer Language

High Level language Pascal, C, C++, Java, etc

Low Level Language Assembly

Machine Codes 010010001010100101010 in binary 1234 FFAB 1234 H in hexadecimal

2 Microprocessor (c) Prima Dewi Purnamasari 2011

Why Assembly? Assembly has several features that make it a good

choice many some situations.1. It's fast – Assembly programs are generally

faster than programs created in higher level languages. Often, programmers write speed-essential functions in assembly.

2. It's powerful – You are given unlimited power over your assembly programs. Sometimes, higher level languages have restrictions that make implementing certain things difficult.

3. It's small – Assembly programs are often much smaller than programs written in other languages. This can be very useful if space is an issue.3 Microprocessor (c) Prima Dewi Purnamasari 2011

Preparation for Assembly Programming

Basically you will need: Program editor as simple as Notepad Assembler

1. MASM http://www.masm32.com/.2. TASM Made by Borland, a commercial product3. NASM http://sourceforge.net/projects/nasm/

Be careful in writing your programs, because it runs directly on your microprocessor!


http://www.masm32.com/

http://sourceforge.net/projects/nasm/

Steps to Create a Program


MASM32


TASM


Emulator

an emulator is hardware and/or software that duplicates (or emulates) the functions of a first computer system in a different second computer system, so that the behavior of the second system closely resembles the behavior of the first system.


Emu8086


Individual Assignment

Download and install emu8086 (trial)

http://www.emu8086.com/

Find corresponding tutorial on how to use it (available on the Internet!), self study!


http://www.emu8086.com/

Group assignment

Each group is responsible to bring at minimum 1 laptop (with emu8086 installed) to class every session


Creating an Assembly Language Program

An assembly language program should be written with any text editor and have the extension filename.asm.

The assembler and Linker The assembler program converts a symbolic

source module (file) into a hexadecimal object file The linker program executes as the second part of

ML, reads the object files, created by the assembler program, and links them into a single execution file (.EXE)


Assembly Program Structure


LIST File, generated automatically after program successfully assembled

Machine codes

MemoryAddre

ss


Writing Structure

NEXT: MOV AX, [BX] ; comment

1= label, followed by “:”2= opcode3= operand4= comment, preceded with”;”

1 432


Writing Structure

Each statement in an assembly language program consists of four parts or fields.

The leftmost field is called the label. used to store a symbolic name for the memory

location it represents All labels must begin with a letter or one of

the following special characters: @, $, -, or ?. a label may have any length from 1 to 35

characters The label appears in a program to identify the

name of a memory location for storing data and for other purposes.


The next field to the right is the opcode field. designed to hold the instruction, or opcode the MOV part of the move data instruction is an

example of an opcode Right of the opcode field is the operand field.

contains information used by the opcode the MOV AL,BL instruction has the opcode MOV

and operands AL and BL The comment field, the final field, contains a

comment about the instruction(s). comments always begin with a semicolon (;)


Try it in emulator! Click “View” and look the changes in every menu

list: registers Data Screen Flags etc


Computer Data Formats

Microprocessor (c) Prima Dewi Purnamasari 201121

Computer Data Formats

ASCII and Unicode Data Binary Coded Decimal (BCD) Byte-Sized Data Word-Sized Data Doubleword-Sized Data Real Numbers


ASCII Data

American Standard Code for Information Interchange (ASCII) data represent alphanumeric characters in the memory of a computer system (Table 1.7)

The standard ASCII code is a 7-bit code with the eighth and MSB used to hold parity in some systems

ASCII are most often stored in memory using a special directive to the assembler program called define byte(s) or DB


BCD Data

Binary-Coded Decimal (BCD) information is stored in either packed or unpacked forms

Packed BCD data are stored as two digits per byte

Unpacked BCD data are stored as one digit per byte

The range of a BCD digit extends from 00002 to 10012 or 0-9 decimal

Table 1.9 shows some decimal numbers converted to both packed ad unpacked BCD


Byte-Sized Data

Byte-size data are stored as unsigned and signed integers

Negative signed numbers are stored in the 2’s complement form Whenever a number is 2’s complement, its sign

changes from negative to positive or positive to negative

See example 1-22, 1-23


Define bit (DB) directive is used to store 8-bit data in memory

Microprocessor (c) Prima Dewi Purnamasari 2011 29

Word-sized Data

A word (16-bits) is formed with two bytes of data The LSB is always stored in the lowest-numbered

memory location, the MSB in the highest (i.e., little endian format)—used with Intel family of microprocessor

An alternate method (i.e., big endian format) is used with the Motorola family of micro-processors


Word-sized Data

Fig 1.11(a) & (b) shows the weight of each bit position in a word of data


Example 1.25 shows several signed and unsigned word-sized data stored in memory using the assembler program

Note that define word(s) directive or DW causes the assembler to store words in the memory


Doubleword-sized Data Doubleword-sized data requires four bytes of

memory (32-bit number) Doubleword-sized data appear as a product

after a multiplication and also as a dividend before a division

Fig. 1-12 shows the form used to store doublewords in the memory and the binary weights of each bit position


To define doubleword-sized data, use assembler directive define doubleword or DD

Microprocessor (c) Prima Dewi Purnamasari 2011 37

Real Numbers

A real number (floating-point number) contains two parts: a mantissa, significant, or fraction and an exponent

Fig. 1-13 and example 1-27 depicts both the 4-byte (single precision) and 8-byte (double precision) forms of real numbers


The exponent is stored as a biased exponent an exponent of 23 is represented as a biased

exponent of 127+3 or 130 (82H) in the single- precision form or as 1026 (402H) in the double-precision form


Assembler detail

From chapter 4

Microprocessor (c) Prima Dewi Purnamasari 20114242 Microprocessor (c) Prima Dewi Purnamasari 2011

Directives

Indicate how an operand or section of program is to be processed by the assembler

Storing Data in a Memory Segment: DB, DW, DD, SEGMENT, .DATA, ENDS, DUP, ALIGN e.g.: Example 4.12

THIS refers the data as byte or word


Memory is reserved for use in the future by using a question mark (?) as an operand for a DB, DW, or DD directive. when ? is used in place of a numeric or ASCII value,

the assembler sets aside a location and does not initialize it to any specific value

DUP: creates array with or without initial values It is important that word-sized data are placed at

word boundaries and doubleword-sized data are placed at doubleword boundaries. if not, the microprocessor spends additional

time accessing these data types


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EQU, ORG ASSUME

Equate directive (EQU) equates a numeric, ASCII, or label to another label. equates make a program clearer and simplify

debugging EX: TEN EQU 10 …. MOV AL,TEN

The ORG (origin) statement changes the starting offset address of the data or code segments.

At times, the origin of data or the code must be assigned to an absolute offset address with the ORG statement.

ASSUME tells the assembler what names have been chosen for the code, data, extra, and stack segments. Used only with full-segment definition


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PROC and ENDP Indicate start and end of a procedure (subroutine).

they force structure because the procedure is clearly defined

Both the PROC and ENDP directives require a label to indicate the name of the procedure.

RET instruction executed the end of the proc. USES directive indicates which registers are used by

the proc. The assembler automatically save and restore

them using the stack instructions. EX: PRC1 PROC USES AX BX CX Use .LISTALL directive to view all instruction

generated by assembler


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Memory Organization

The assembler uses two basic formats for developing software: one method uses models; the other uses full-

segment definitions Memory models are unique to MASM. The models are easier to use for simple

tasks. The full-segment definitions offer better

control over the assembly language task and are recommended for complex programs.


Models

There are many models available to the MASM assembler, ranging from tiny to huge.

.MODEL memsize TINY: all software and data fit into 64kb memory segment.

Useful for small programs. assembled as a command (.COM) program

SMALL: one data segment with one code segment for a total of 128kb of memory. assembled as an execute (.EXE) program

Start of segments: .CODE, .DATA, .STACK Start of instructions and load segment registers with

segment addresses: .STARTUP Exit to DOS: .EXIT End of file: END MP selection : .386, .486, .586, .686 ..


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Full Segment Definitions Full-segment definitions are also used with the

Borland and Microsoft C/C++ environments for procedures developed in assembly language

More structured form than the model method Use assume directive before the program begins. The program loader does not automatically

initialize DS and ES. These registers must be loaded in the program

STACK_SEG, DAT_SEG, CODE_SEG, END MAIN


STACK_SEG SEGMENT‘STACK’DW 100H DUP(?)

STACK_SEG ENDS

DATA_SEG SEGMENT‘DATA’LISTA DB 100 DUP(?)LISTB DB 100 DUP(?)

DATA_SEG ENDS

COSE_SEG SEGMENT‘CODE’ASSUME CS:CODE_SEG, DS:DATA_SEG, SS:STACK_SEG

MAIN PROC FARMOV AX, DATA_SEGMOV ES, AXMOV DS, AXCLDMOV SI, OFFSET LISTAMOV DI, OFFSET LISTBMOV CX, 100REP MOVSB

MAIN ENDPCODE_SEG ENDS

END MAIN


Introduction to MOV Instruction


MOV instruction provides a basis for explanation of data-addressing modes

opcode

an opcode, or operation code, tells the microprocessor which operation to perform

Data Addressing Modes


MOV Instruction

MOV instruction perform COPY of a value, either from or to memory or register MOV = COPY MOV ≠ MOVE


MOV BX, CX

► The source register’s contents do not change. the destination register’s contents do change

► The contents of the destination register or destination memory location change for all instructions except the CMP and TEST instructions.

► Note that only the rightmost 16 bits of register EBX change. The MOV BX, CX instruction does not affect the leftmost 16 bits of register EBX59 Microprocessor (c) Prima Dewi Purnamasari 2011

Some MOV Variant

MOV AX,BX MOV [BX], AX

[ ] sign represents memory location Destination = memory which has address as in BX

MOV DATA,AX DATA is a name the programmer define in DATA

SEGMENT destination=memory named DATA

MOV AX,0123H a value 0123H is copied to AX register

There are several more, but the above are the fundamental ones 60 Microprocessor (c) Prima Dewi Purnamasari 2011

Rules in addressing. DO NOT:

Mix different size of register MOV AX, BL

Perform memory to memory addressing MOV [1234H],DATA

Copy content of one segment register to another MOV DS,ES

Use CS as the destination register MOV CS,1000H


Group Assignment—Due Thursday 22/9

Make a program (altogether in one program, sequentially) Reserve place for data in data segment namely

DATA1 with type word Copy 1234 to AX Copy 0011B to AL Copy 12H to AH Copy AX to DATA1


The report

Write the program in emulator Compile it. Run the program in emulator. (single step).

Analyze the effect on the registers and memory for each line of code

Written report should be made as comprehensive as it can be (greater score for better report)

The main part of your report would be:1. Print of program (source code), provide sufficient

comment2. Print of LISTING file3. Program analysis 63 Microprocessor (c) Prima Dewi Purnamasari 2011

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