C Assembly Programming - Binary Operations and Concept of Registers ~ My C Files

Assembly Programming series by MyCFiles.com
This post will explain different binary operations and the concept of registers.
Read Part 1 First - Introduction to C Assembly Programming

THEORY
When it comes to assembly or lower level programming there is no way to ignore the concept of bits and bytes and therefore the operations on them. Believe it or not if you neglect binary operations you miss 60 % and therefore whole language..

WHY BINARY ONLY???

Actually the whole essence of bits and bytes lies in the way electronic instruments work. Since it is 100 times easy to work with only 2 nos (binary sys) then 10 (dec sys). Hence binary was adopted in electronics. You better grab a book on Boolean algebra to dig it out how computer internals work (call it ALU whatever).

THE BINARY OPERATIONS
BITWISE AND

first of all consider 2 bin nos

1 0 1 0 1 0 0 0 => 168

1 1 0 0 1 1 0 0 => 204

as is the tradition we represent AND by .(dot) in B-algebra following the rules

1 . 1 = 1

0 . 1 = 0

1 0 1 0 1 0 0 0 => 168

AND 1 1 0 0 1 1 0 0 => 204

-------------

1 0 0 0 1 0 0 0 => 200

BITWISE OR

it is simply the opposite of AND represented by + sign in B algebra following the rules

1 + 1 = 1

0 + 1 = 1

0 + 0 = 0

1 0 1 0 1 0 0 0 => 168

OR 1 1 0 0 1 1 0 0 => 204

-------------

1 1 1 0 1 1 0 0 => 236

understandable enough...

LOGICAL NEGATION - NOT

it is the operation is easy all you have to do it change 0 to one or vice-versa..

Its represented by 'over no or bar over the no.

The rules are

1' = 0

0' = 1

1 0 1 0 1 0 0 0 -> NOT -> 0 1 0 1 0 1 1 1

BITWISE XOR

it has no representation in B-algebra since it is combination of AND OR and NOT... but i will use ^ for it. You may have heard of it as MAGIC OPERATOR whe reason it that it speeds up things incredibaly faster and help generate more efficient code.

The rules are

give 1 ans only and only if 1 s in ques are odd else 0

1 0 1 0 1 0 0 0 => 168

XOR 1 1 0 0 1 1 0 0 => 204

0 1 1 0 0 1 0 0 => 100

XOR when decoded translates to

(a + b) . (a . b)'

better in C

(a || b) && !(a && b)

good programmers use it as efficient way to turn some var to 0 notice

a ^ a = 0

even

a ^= b

b ^= a

a ^= b

will swap a and b(much faster then using any other var).

CPU REGISTERS

Registers are the core of CPU and therefore heart of assembly. infarct learning assembly is to apply instructions to these regs rest is all same as c/c++.

For DOS Registers are 16 bit variables in your intel chip which guides cpu in the sense what to do next.

They are of type.

1. General regs

2. Segment regs

3. Pointer(offset) regs

GENERAL REGISTERS

there are 4 under this category.

AX - accumulator -> AH,AL

BX - base -> BH,BL

CX - counter -> CH,CL

DX - data -> DH,DL

xH,xL stands for there msb(15 to 8) and lsb(8 to 0) respectively.

SEGMENT REGISTER

they are 4

CS - code segment

DS - data segment

ES - extra segment

SS - stack segment

no access to lower or high bit.

POINTER(OFFSET)REGISTERS

they are of 5 type

IP - instruction pointer

SI - source index

DI - destination index

SP - stack pointer

BP - base pointer

no high low access to them too.

THE CONCEPT OF SEGMENT AND OFFSET

If you consider your whole computer memory as 2D-array then segment represents column and offset a row. Any physical entity(data or code) is accessible only if it is in its respective segment(CS or DS) means code within CS is accessible data within DS is accessible..

UTILITY

AX - normally as a var

BX - as a var, segment addressing.

CX - as a counter in loops

DX - same as BX

CS - segment having current code

DS - segment having global vars

ES - normally for interrupt calls

SS - segment containing stack of current program

IP - pointer to instruction of current code

SI - in array addressing

DI - same as SI