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010101010101010101010101010101010101010101 0 1 0 Assembly broken down 1 0 1 0 by rustymemory of uk2duohet 1 0 .we share thus we are one. 1 010101010101010101010101010101010101010101 Session 1 - introduction

what is assembly? Assembly languages are processor-specific low-level languages. They are rarely used any more except for handling very low-level machine-specific tasks, since languages like C can generally satisfy most requirements, even low-level ones. The other main use of assembly language is to provide the "glue" to enable different languages to be used in a single program. It is however useful to know something about assembly language to get a feel for what goes on inside a processor, as well as for understanding code generation in compilers and for debugging when there is no source code available. -"BURKS version 6" __________________Assembly___________________ | | | instruction set BIOS interrupts Dos interrupts -mov -ready subroutines -add in bios -jmp -.....

Lets start by the features and draw-backs of the assembly language talking simple we can say.

[1]Features: a- Speed. b- Protection. c- i/o access. d- Size "it matters baby". [2]Draw-Backs: a- No 3d graphics. b- not good with math calculations. c- lack of documentation. Splitting "Assembly" = i) Debugger ii) Assembler

a) debugger --> for editing, No variables , No lables. types of debuggers --> gdb. soft-ice. periscope. .......... b) Assembler --> Nasm "http://www.cryogen.com/Nasm/". Tasm "http://home.comcast.net/~tasm/tasmdnl.htm". Masm "http://webster.cs.ucr.edu/AsmTools/MASM/".

LETS REALLY START

[3]Registers:- [i]General purpose registers: registers that can be divided into two equal parts of 8 bits each , each can be used individualy: AX --> accumulator register -- divided gives AL "low" ,AH "high". think of AX "int a =5;", which means that the register can hold a value. "EAX,AX,AH,AL" BX --> Base address register -- divided gives BL, BH. think of BX "char *add = 0x45", which means that BX is a pointer to memory address. "EBX,BX,BH,BL" CX --> Counter register -- divided gives CL,CH. think of CX "for(i = 0; i <= 10; i++);" which means that CX is used for loops. "ECX,CX,CH,CL" DX --> Data register -- divided gives DL, DH.It is used for I/O port access, arithmetic, some interrupt calls. "EDX,DX,DH,DL" [ii]Index Registers " cannot be divided": SI - Source index register DI - Destination index register these registeres are used to copy a block from a location to another. [iii]Segment registers "pointers that point to of 64k each":- DS - Data segment CS - Code segment --> used for instructions SS - Stack segment --> points to a stack segment ES - Extra segment

• Pointer Registers______________________________________

SP - Stack pointer --> points to the top of the stack |______ BP - Base pointer -->points to a specific location in the stack| IP - Instruction pointer - program counter. ___________________| Flags register: abv Flags Name bit nº Description OF Overflow Flag 11 indicates an overflow when set DF Direction Flag 10 used for string operations to check direction IF Interrupt Flag 9 if set, interrupt are enabled, else disabled TF Trap Flag 8 if set, CPU can work in single step mode SF Sign Flag 7 if set, resulting number of calculation is negative ZF Zero Flag 6 if set, resulting number of calculation is zero AF Auxiliary Carry 4 some sort of second carry flag PF Parity Flag 2 indicates even or odd parity CF Carry Flag 0 contains the left-most bit after calculations Lets take an example to make things more clear :) Example: 1 Mov BX,100 2 Mov AL,[bx] ----> hence [ ] == content of 3 Mov BX,0

lets analyse line 1 : Mov BX,100 --> make BX point to memory address 100 line 2 : Mov AL,[BX] --> copy the content inside memory address 100 to the AL register line 3 : Mov BX,0 --> forget about 100 ;)

Some notes: F -->15 FF -->225 FFFF -->64k FFFFF -->1024 "1mb"

• Dos can only view memory not more than 1mb.

BX is a 16-bit register, so it can only point to places up to 64k (2^16). sence a problem ?

solution is to divide memory into 64k segments(16 segments each 64k), locations in memory are determined by: [i] Segment number , [ii] offset inside the segment.

• Segment number = its from 1 > 16

HUH? "art of determining a memory location >:D" BX (offset) = 1000 DS (segment) = 3000 we add a 0 to the end of DS = 3000[0] if we add it to BX BX 1 0 0 0 + DS 3 0 0 0 0 ---------------- |3 1 0 0 0| ---------------- 3 = segment number 1000 = offset

[4]Addressing modes:-

There are several type of addressing which we will talk about, we have : [a] immediate addressing, direct addressing, [c] indirect addressing{, [i] base addressing, base relative addressing, [ii] indexed addressing, relative indexed addressing, [iii] base indexed addressing, [iv]base indexed relative addressing}, [d] stack addressing. "OUCH"

• Immediate addressing: " addressing values to registers or memory"

correct example: mov AX,5 AH AL store value 5 in the the AX register [[00000000][00000101]]

wrong example: mov DS,3000 you cannot store in a Segment register. another wrong example: mov 30,AL 30 is not a register nor a address location. lets take a wrong example and solve it :) mov [30],5 "wrong wrong wrong" solution: first of all we must determine the format of 5 either a byte or a word in other words:- mov byte ptr [30],5 ----> this means that at 30"address" there is a pointer to a byte. mov word ptr [30],5 ----> this means that at 30"address" there is a pointer to a word. 5 = 05 byte ; 5 = 0005 word "2 bytes" correct example: mov ch,0 this doesnt affect the 0 flag register because this 0 stored in ch is not result of an operation in the ALU. the zero flag is affected by the results of operation executed in the alu only.

• Direct addressing:- "addressing values from one register to another"

correct example: mov AL,BL store the value of BL in AL wrong example: mov BX,AL if you do this slap your head for sometime and think AL = 8bit , BX = 16bit ... :) solution: mov BL,AL mov BH,0 why ? --> it is not allowed to store 8 bits in 16 bits they have to have the same size wrong example: mov DS,3000 DS is not connected to the DATA bus its connected to the address bus. in other words: mov AX,3000 mov DS,AX wrong example: mov IP,AX Instruction pointer is only changed by the processor. i call this "HANDEL WITH CARE EXAMPLE": mov [1234],CX meaning: the value of CX will be stored in memory in two locations (2bytes). so it will overwrite the value of the location [1235] <--- handel with care :) wrong example: mov [1000],[1003] it is not allowed to store from a memory location to another memory location directly.

• Indirect addressing:

[i] Base addressing: "think of it: BX = base address" examples:

• mov BX,3FCF

• mov [BX],AX

No comments ofcourse "i wont spoon feed you" [ii]Base relative addressing: "base addressing with addition(mov [BX+50],AL)" example: to transfer data from address 1000 to address 1050 ???? mov BX,1000 mov AL,[BX] mov [BX+50],AL [iii]indexed Addressing: "indexed??? ---> DI , SI umm index registers" examples:

• mov [SI],DI

• mov cx,[DI]

use an index register if the segment number is different [iv] relative indexed addressing " index addressing with addition(mov [SI],[SI+30])" examples:

• mov [SI+50],DX

• mov [SI],[SI+30]

[v] Base indexed addressing:"using BX with SI or DI" example: mov DI,[BX+SI] note: take a good look at base indexed relative addressing how it differes from base indexed addressing. [vi]base indexed relative addressing:"using BX with SI or DI along with addition" example: mov DI,[BX+SI+10] hence the "10" ;) note: keep in mind you wont be thinking while coding , ill do it via relative indexed addressing or shall i use ase base indexed relative addressing no they are just some info you would start to know for yourself so dont worry :). *Stack Addressing:-"using SP and BP" example: lets say [sp] holds value 300 while DS holds 1000 and SS holds 2000 :) *mov AL,[sp] so this location is segment 2000 offset 300 :) [5]SEGMENT OVERIDE: I DONT WANT TO BE DEAULTY WRITTING TO THIS SEGMENT CAN I CHOOSE MINE ???? :) yes examples: *mov AX,ES:[DI] *mov ES:[SI],AX mov SS:[BX],CX mov DS:[BP],DX mov CS:[SI],AX looks wierd? no explanation: ":"-- Specify a segment override in a operand -- like CS:[SI],AX so CS will be used instead of ES. now its the end of our first session i hope you had fun. "we share thus we are one"-uk2duohet
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