emu8086 does NOT use 32 bit registers (eax, ebx, etc...)

Where do we store values? How much storage? What type of binary coding?

 What are REGISTERS?
 Advantages in using?
 Are they necessary?







Examples: moving data into registers

MOV  al, 255
MOV  ax, 0f54ah
MOV  bx, -1
MOV  bh, 11010101b

Are these different from each other?
MOV ah, -1
MOV ah, 0ffh
MOV ah, 11111111b
MOV ah, 255

ugh...

MOV ax, 1110001101011101b

 What are STATUS FLAGS?




Sign: result is negative
Zero: result is zero
Carry: unsigned arithmetic out of range
Overflow: signed arithmetic out of range

MOV al, 0ffh
ADD al, 1

problem? when? how do we know???


Can't we just store everything in registers? Why not?

   why 20 bit addresses? why not just use the 16 in the register?
   
   how do you construct a 20 bit address? segment: offset
   
         1234:7890 => 1234h * 10h + 7890h = 
        
         12340 
        + 7890
         -----
         19BD0h

   emulator: 16 bit registers and memory with 20 bit addresses

   memory map

Better than registers?
======================
Where do we store values?       How much storage do we use?
What type of binary coding?     How do we define storage?

RAM

Data definition statement sets aside storage in memory.
  May optionally assign a name (label) to the data
  Syntax: [name] directive initializer [,initializer] . . .

  price  DW 0
  fname  DB "Joseph",0 




DB     8-bit unsigned & signed integer

DW     16-bit unsigned & signed integer

DD     32-bit unsigned & signed integer



  first   DB 10, 32, 41h
  second  DB 0Ah, 20h, ‘A’
  third   DB ?
---------------------------------------------
   How many bytes?      How organized?
   How represented?     Draw it!          Symbol Table?



    value1 DB 'A'   ; character constant
    value2 DB 0     ; smallest unsigned byte
    value3 DB 255   ; largest unsigned byte
    value4 DB -128 ; smallest signed byte
    value5 DB +127 ; largest signed byte
    value6 DB ?     ; uninitialized byte


    list1     DB  10, 32, 41h, 00100010b
    list2     DB  0Ah, 20h, 'A', 22h
    array1    DB  20 DUP(0)
    greeting1 DB  "Good afternoon", 0

    A string is stored as a sequence of bytes - why ZERO at end?


Labels let you use symbolic names for memory locations!
The offset of a byte in storage is the distance from the beginning of
the segment  (.data) to the first byte of the variable.

      Remember this one?

  first   db 10, 32, 41h
  second  db 0Ah, 20h, ‘A’
  third   db ?
---------------------------------------------
   How represented?     Draw it!          Symbol Table?

         Memory         Symbol Table****
         0 - 0A         first = 0
         1 - 20         second = 3
         2 - 41         third = 6
         3 - 0A
         4 - 20
         5 - 41
         6 - ??

 ; how else could you write this?
   Var1  db  'ABC'
   Var2  db  "DEFG"

 The offset of Var1 ('A') is 0
 ‘B’ is at offset Var1 + 1 = 1
 ‘D’ is at offset Var2 or at Var1+3 =  3
 ‘F’ is at offset Var2+2 or at Var1+5 = 5


  ; how else could you write this?
   Var1  db  3, 7
   Var2  db  4, 25

 The offset of Var1 (3) is 0
 7 is at offset Var1 + 1 = 1
 4 is at offset Var2 or at Var1+2 =  2
 25 is at offset Var2+1 or at Var1+3 = 3


String Examples:
 str1 DB "Enter your name",0
 str2 DB 'Error: halting program',0
 str3 DB 'A','E','I','O','U'
 greeting1 BYTE "Welcome to assembly "
           BYTE "language", 0Dh,0Ah, 0

0Dh is what?  0Ah is what?  



DUP does what?
   var1 DB 20 DUP(0)    ; 20 bytes, all equal to zero
   var2 DB 20 DUP(?)    ; 20 bytes, uninitialized
   var3 DB 10, 13, 65, 3 DUP(0), 20 ;


; ----------------- Word Values ---------------------
=======================================================

word1   DW   65535    ; largest unsigned value
word1   DW   0        ; smallest unsigned value
word2   DW   32767    ; largest signed value
word2   DW  -32768    ; smallest signed value
word3   DW   ?        ; uninitialized



What is "little Endian?
Intel’s x86 are little endian processors: the lowest order byte
(of a word or double word) is  stored at the lowest address.

   X  DW 1234h ; 2 bytes at X
        -------------------------
            offset:  value:
               0       34h   - least significant byte
               1       12h   - most

   X  DW 256 ; 2 bytes at X
        -------------------------
            offset:  value:
               0       00h   - least significant byte
               1       01h   - most
       

   X  DW 256, 1234h ; 4 bytes at X
        -------------------------
            offset:  value:
               0       00h   - least significant byte
               1       01h   - most
               2       34h   - least significant byte
               3       12h   - most


If a value fits into a byte, it will be stored in
the lowest ordered one available.

Ex:
      init DW ‘A’ ;ascii code 41h

        the value will be stored as:
        -------------------------
            offset:  value:
               0       41h   - least significant byte
               1       00h   - most

      init DW ‘AB’ ; ascii codes 41h, 42h

        -------------------------
            offset:  value:
               0       41h   - least significant byte
               1       42h   - most

 =  Symbolic Constant
   name = expression
   expression is a 32-bit integer
   MAY be redefined

      prod     = 10 * 5 ; Evaluates an expression
      maxInt   = 7FFFh  ; Maximum 16-bit signed value
      minInt   = 8000h  ; Minimum 16-bit signed value
      maxUInt  = 0FFFFh ; Maximum 16-bit unsigned value
      string   = ‘XY’      ; Up to two characters allowed
      count    = 500

     WHY? good programming??

     NO STORAGE ALLOCATED!!
     ^^
EQU 
Define a symbol as either an integer OR text expression.
May NOT be redefined

   PI       EQU <3.1416>
   pressKey EQU <"Press any key to continue...",0> ;NO STORAGE ALLOCATED!!
                                                    ^^
 prompt DB pressKey ; storage IS allocated here...
                                ^^

 $ 
Location counter
   used at assembly time
   tracks the offset from 0 (start) of
   where each byte of the program is stored as it is generated
Program counter
   used at run time
   tracks the instruction address of the next executable instruction

$ operator returns the current value of LOCATION COUNTER.
Use it to compute the string length at assembly time.

      list DB 10,20,30,40
      ListSize = ($ - list)

   so the value of List Size is ??
-----------------------------------

      list DB 10,20,30,40, 50, 60, 70
      ListSize = ($ - list)

   so the value of List Size is ??
-----------------------------------

      LongString db 'This is a piece of text that I'
                 db 'want to type on 2 separate lines'
      LongString_length = ($ - LongString)

   Offset of ‘w’(in want) = 1 + offset of ‘I’

Note: do not need to put a label on every line...
    in fact everything could be an offset from a label at ZERO - why bad?

Little Endian Order
===================

Little endian order refers to the way Intel stores integers in memory.
Multi-byte integers are stored in reverse order, with least 
significant byte stored at lowest address

For example, the doubleword 12345678h would be stored as:

    DD 12345678h



PTR can also be used to combine elements of a smaller data type 
and move them into a larger operand. 

     myBytes DB 12h,34h,56h,78h
   
     mov ax,  DW PTR [myBytes]   ;AX  = 3412h
     mov ax,  DW PTR [myBytes+1] ;AX  = 5634h
     mov ax,  DW PTR [myBytes+2] ;AX  = 7856h
     mov eax, DD PTR myBytes    ;EAX = 78563412h


       varB DB 65h, 31h, 02h, 05h
       varW DW 6543h, 1202h
       varD DD 12345678h
   

       mov ax,  WORD PTR [varB+2]   ; a.     0502h
       mov bl,  BYTE PTR varD       ; b.       78h
       mov bl,  BYTE PTR [varW+2]   ; c.       02h
       mov ax,  WORD PTR [varD+2]   ; d.     1234h
       mov eax, DWORD PTR varW      ; e. 12026543h