; fasm example of writing simple EXE program 

format MZ 

        push    cs 
        pop     ds 
        mov     ah,9 
        mov     dx,hello 
        int     21h 

        mov     ax,4C00h 
        int     21h 

hello db 'Hello world!',24h    
    

; fasm example of writing multi-segment EXE program 

format MZ 

entry main:start                        ; program entry point 
stack 100h                              ; stack size 

segment main                            ; main program segment 

  start: 
        mov     ax,text 
        mov     ds,ax 

        mov     dx,hello 
        call    extra:write_text 

        mov     ax,4C00h 
        int     21h 

segment text 

  hello db 'Hello world!',24h 

segment extra 

  write_text: 
        mov     ah,9 
        int     21h 
        retf        
     

; fasm demonstration of writing simple ELF executable

format ELF executable 3
entry start

segment readable executable

start:

        mov     eax,4
        mov     ebx,1
        mov     ecx,msg
        mov     edx,msg_size
        int     0x80

        mov     eax,1
        xor     ebx,ebx
        int     0x80

segment readable writeable

msg db 'Hello world!',0xA
msg_size = $-msg     

I guess we can take the question one at a time.

>1. 32 bit vs 64 bit asm- Is there a major different in the different bit types?

Yes. The internal registers sizes are different. Things like overflow and carry behave differently.

>How hard would it be to convert 32 bit programming to 64 bit?

It depends upon what your code does. Usually for any non-trivial code it is quite time consuming and finicky.

>2.AMD vs Intel - I use these two CPU the most. Do I have to program in a different manner for different CPU types with assembly?

Only if you are writing very finely tuned performance applications then you could do well to consider the underlying hardware mechanics (not just the CPU but also the system board and memory).

>3. Linux and windows - I use both of these operating systems and am very familiar with both. Does the assembly programming differ for these two operating systems?

The interfacing, architecture and calling conventions are different. You have to rewrite your OS related code to make it work correctly.

>Would assembly "Hello world" written in windows work on Linux?

Not without some help from something like WINE.

>4.The different assemblers. FASM vs. NASM vs. MASM. Is the code written different in each one?

Yup.

>1.eax

An internal CPU register. 32-bits

>2.ebx

An internal CPU register. 32-bits

>3.ecx,msg

The constant "msg" (a memory pointer) and the internal CPU register ecx. 32-bits

>4.edx,msg_size

The constant "msg_size" and the internal CPU register edx. 32-bits

>5.int 0x80 (I'm guessing int is and integer, but what is 0x80?)

Nope. INT mean interrupt. In this case software interrupt number 0x80. Please read the AMD and/or Intel manuals for descriptions of each instruction.

>6.eax,1

The constant "1" and the internal CPU register eax. 32-bits

>7.xor ebx,ebx

Zero the ebx register. Xor with itself will always give a zero result.

>8.msg db 'Hello world!',0xA (What does 'db' mean and '0xA.')

Define some text at the memory address "msg". DB = define byte(s). 0xA hexidecimal for 10 (a line feed (LF) character).

>9.msg_size = $-msg

How long is the message? $ means current address.