CIS-261 Home http://www.c-jump.com/bcc/

Language Components of MASM and General-Purpose Registers

1. General-Purpose Registers
2. Typical Uses of General-Purpose Registers
3. ESP Stack Pointer Register
4. EIP Instruction Pointer Register
5. EFLAGS Register
6. EFLAGS Bit Labels
7. EFLAGS Individual Bit Flags
8. Write Fast Code
9. Language Components of MASM
10. Data-Related Operators and Directives
11. Identifiers
12. Integer Constants
13. EQU Directive and Symbolic Integer Constants
14. TYPE, LENGTHOF and SIZEOF Operators
15. masm_operators.ASM
16. Operand Addressing Mode Types
17. Register Operands
18. Immediate Operands
19. The OFFSET Operator and LEA Instruction
20. More about LEA Instruction
21. Ambiguous moves: PTR directive
22. OFFSET and PTR Example
23. ADDR and OFFSET
24. Direct Memory Operands
25. Plus, Minus, and Index
26. Directives BYTE PTR, WORD PTR, DWORD PTR
27. Pointers
28. Pointer Types
29. NEAR Pointers
30. The TYPEDEF Operator

1. General-Purpose Registers

• Similarly, AL, DL, CL, and BL represent the low-order 8 bits of the registers.

   

2. Typical Uses of General-Purpose Registers

Register	Size	Special Uses
EAX	32-bit	Accumulator for operands and results
EBX	32-bit	Base pointer to data in the data segment
ECX	32-bit	Counter for loop operations
EDX	32-bit	I/O pointer
EBP	32-bit	Frame Pointer - useful for stack frames
ESP	32-bit	Stack Pointer - hardcoded into PUSH and POP operations
ESI	32-bit	Source Index - required for some array operations
EDI	32-bit	Destination Index - required for some array operations
EIP	32-bit	Instruction Pointer
EFLAGS	32-bit	Result Flags - hardcoded into conditional operations

3. ESP Stack Pointer Register

• The ESP register points to the current location within the stack segment.

• Pushing a value onto the stack decreases the value of ESP.

• Popping from the stack increases the value of ESP.

   

4. EIP Instruction Pointer Register

• The EIP register always contains the address of the next instruction to be executed.

• You cannot directly access or change the instruction pointer.

• However, instructions that control program flow, such as calls, jumps, loops, and interrupts, automatically change the instruction pointer.

   

5. EFLAGS Register

• See also: wikipedia article about EFLAGS Register

   

6. EFLAGS Bit Labels

•     0       CF      Carry flag
      2       PF      Parity flag
      4       AF      Auxiliary carry flag
      6       ZF      Zero flag
      7       SF      Sign flag
      8       TF      Trap flag
      9       IF      Interrupt enable flag
      10      DF      Direction flag
      11      OF      Overflow flag
      12-13   IOPL    I/O Priviledge level
      14      NT      Nested task flag
      16      RF      Resume flag
      17      VM      Virtual 8086 mode flag
      18      AC      Alignment check flag (486+)
      19      VIF     Virutal interrupt flag
      20      VIP     Virtual interrupt pending flag
      21      ID      ID flag
  

   

7. EFLAGS Individual Bit Flags

• Although all flags serve a purpose, most programs require only the carry, zero, sign, and direction flags.

   

8. Write Fast Code

• Whenever possible, use registers rather than constant values, and constant values rather than memory.

• Minimize changes in program flow.

• Smaller is often better. For example, the instructions

     dec  bx
     sub  bx, 1
  

  accomplish the same thing and have the same timings on 80386/486 processors, but DEC BX is 3 bytes smaller than the second, and so may reach the processor faster.

   

9. Language Components of MASM

• Reserved words in MASM include:

  • Instructions, which correspond to operations the processor can execute.

  • Directives, which give commands to the assembler.

  • Attributes, which provide a value for a field, such as segment alignment.

  • Operators, which are used in expressions.

  • Predefined symbols, which return information to your program.

• MASM reserved words are not case sensitive except for predefined symbols.

• The assembler generates an error if you use a reserved word as a variable, code label, or other identifier.

   

10. Data-Related Operators and Directives

• Operators and directives are not part of the Intel instruction set.

• They are only understood by the Assembler (in our case, Microsoft MASM.)

• Various assemblers have differing syntaxes for operators and directives...

  • ...There is no single-defined assembler standard.

• Useful MASM operators and directives:

  • The OFFSET operator returns the distance of a variable from the beginning of its enclosing segment.

  • The PTR operator lets you override a variable's default size.

  • The TYPE operator returns the size (in bytes) of each element in an array.

  • The LENGTHOF operator returns the number of elements in an array.

  • The SIZEOF operator returns the number of bytes used by an array initializer.

   

11. Identifiers

• An identifier is a name that you invent and attach to a definition.

• Identifiers can be symbols representing variables, constants, procedure names, code labels, segment names, and user-defined data types such as structures, unions, records, and types defined with TYPEDEF.

• Identifiers longer than 247 characters generate an error.

• Certain restrictions limit the names you can use for identifiers. Follow these rules to define a name for an identifier:

  • The first character of the identifier can be an alphabetic character (A–Z) or any of these four characters: @ _ $ ?

  • The other characters in the identifier can be any of the characters listed above or a decimal digit (0–9).

   

12. Integer Constants

•         mov     ax, 25
          mov     bx, 0B3h
  

• Numerals are followed by an optional radix specifier.

• Radix is the number base suffix:

  • y for binary (or b if the default radix is not hexadecimal)

  • o or q for octal

  • t for decimal (or d if the default radix is not hexadecimal)

  • h for hexadecimal.

• Default radix is decimal, but you can change the default with the .RADIX directive.

• Hexadecimal numbers must always start with a decimal digit (0–9). If necessary, add a leading zero.

   

13. EQU Directive and Symbolic Integer Constants

• Two MASM directives,

      symbol EQU expression
      symbol = expression
  

  generate integer constants. For example,

         .CONST
      column  EQU    80                ; Constant    80 
      row     EQU    25                ; Constant    25
      screen  EQU    column * row      ; Constant  2000
      line    EQU    row               ; Constant    25
  
         .DATA
  
         .CODE
         mov     cx, column
         mov     bx, line
  

• Using symbolic constants instead of undescriptive numbers makes your code more readable and easier to maintain.

• The assembler does not allocate data storage when you use either EQU or = .

• The assembler simply replaces each occurrence of the symbol with the value of the expression.

• Integers defined with the = directive can be redefined with another value in your source code, but those defined with EQU cannot.

   

14. TYPE, LENGTHOF and SIZEOF Operators

• The TYPE operator returns the size (in bytes) of each element in an array.

• The LENGTHOF operator returns the number of elements in an array.

• The SIZEOF operator returns the number of bytes used by an array initializer.

• See listing file masm_operators.lst of the masm_operators.asm program for operator examples.

   

TITLE MASM Operators
; CIS-261
; masm_operators.asm
; Demonstration of TYPE, LENGTHOF, SIZEOF operators

.386                ; Tells MASM to use Intel 80386 instruction set.
.MODEL FLAT         ; Flat memory model
option casemap:none ; Treat labels as case-sensitive

.CONST          ; Constant data segment
    byte1    BYTE  10,20,30
    array1   WORD  30 DUP(?),0,0
    array2   WORD  5 DUP(3 DUP(?))
    array3   DWORD 1,2,3,4
    digitStr BYTE  '12345678',0
    myArray  BYTE  10,20,30,40,50,
                   60,70,80,90,100

    ;---------------------------------------------
    ; You can examine the following constant values
    ; by looking in the listing file masm_operators.lst
    ;---------------------------------------------
    X = LENGTHOF byte1     ; 3
    X = LENGTHOF array1    ; 30 + 2
    X = LENGTHOF array2    ; 5 * 3
    X = LENGTHOF array3    ; 4
    X = LENGTHOF digitStr  ; 9
    X = LENGTHOF myArray   ; 10

    X = SIZEOF byte1       ; 1 * 3
    X = SIZEOF array1      ; 2 * (30 + 2)
    X = SIZEOF array2      ; 2 * (5 * 3)
    X = SIZEOF array3      ; 4 * 4
    X = SIZEOF digitStr    ; 1 * 9

    X = TYPE byte1     ; 1
    X = TYPE array1    ; 2
    X = TYPE array2    ; 2
    X = TYPE array3    ; 4
    X = TYPE digitStr  ; 1

.DATA           ; Begin initialized data segment
    
.CODE           ; Begin code segment
_main PROC      ; Beginning of code

    ret
    
_main ENDP
END _main       ; Marks the end of the module and sets the program entry point label

15. Operand Addressing Mode Types

• Instructions work on sources of data called operands.

• Here are the Operand Types and corresponding Addressing Modes:

  • Register - an 8-bit or 16-bit register on the 8086–80486, 32-bit on the 80386/486/Pentium.

  • Immediate - a constant value contained in the instruction itself.

  • Direct memory - a fixed location in memory.

  • Indirect memory - a memory location determined at run time by using the address stored in one or two registers.

• Instructions that take two or more operands always work right to left:

  • mov destination, source

• The right operand is the source operand.

  The source specifies data that will be read, but not changed by the intruction.

• The left operand is the destination operand.

  It specifies the data that will be acted on and possibly changed by the instruction.

   

16. Register Operands

17. Immediate Operands

• An immediate operand is a constant value or the result of a constant expression.

• The assembler encodes immediate values into the instruction at assembly time.

• Here are some typical examples showing immediate operands:

          mov     cx, 20          ; Load constant to register
          add     var, 1Fh        ; Add hex constant to variable
          sub     bx, 25 * 80     ; Subtract constant expression
  

• Immediate data is never permitted in the destination operand.

• If the source operand is immediate, the destination operand must be either a register or direct memory to provide a place to store the result of the operation.

• Immediate expressions often involve the useful OFFSET operator.

   

18. The OFFSET Operator and LEA Instruction

• An address constant is a special type of immediate operand that consists of an offset or segment value.

• The OFFSET operator returns the offset of a memory location relative to the beginning of the segment to which the location belongs:

          mov     bx, OFFSET var  ; Load offset address
  

• Since data in different modules may belong to a single segment, the assembler cannot know for each module the true offsets within a segment.

• Thus, the offset for var, although an immediate value, is not determined until link time.

• Instruction

          lea eax, [LocalVar]     ;  Load effective address of LocalVar into eax
          lea eax, LocalVar       ;  This does exactly the same thing
  

  generally equates to

          mov eax, OFFSET LocalVar
  

  but is 1 CPU cycle slower, so mov with OFFSET is preferred in cases other than local variables.

• Also, if you need the program to be really small, and LEA already used 100 times, you can make the program 100 bytes smaller simply by using MOV x, OFFSET y instead.

• Effective address is the physical address of the data in memory.

The OFFSET operator returns the offset of a data label. The offset represents the distance, in bytes, of the label from the beginning of the data segment. In Protected mode, an offset is always 32 bits long. In Real-address mode, offsets are only 16 bits.

19. More about LEA Instruction

• LEA does loading of memory offset value into a register.

• Suppose we want to load EBX with the offset value of table1.

• We could write

          mov    ebx, OFFSET table1
  

• OFFSET operator resolves offset at the assembly time.

• Another way of loading memory offset is by using the LEA instruction:

• LEA resolves offset at run time:

          lea    ebx, [table1]     ;  load effective address of table1 into ebx
  

• The format of LEA instruction is

  • LEA register, source

• We must to use LEA when the needed offset is available at run time only.

• Consider array index, passed as a parameter to a procedure in register ESI, would require

• To load EBX with the address of the element of table1, where element index is in ESI,

          lea    ebx, [ table1 + esi ]
          lea    ebx, [ table1 ] + esi   ;  does the same thing
          lea    ebx, table1[ esi ]      ;  does the same thing
  

  NOTE: We cannot use the MOV instruction to do this !

   

20. Ambiguous moves: PTR directive

• Consider fragment of data definitions and code:

          .DATA
          table1 WORD 20 DUP (0)
          status BYTE 7 DUP (1)
          ..
          mov    EBX, OFFSET table1
          mov    ESI, OFFSET status
          mov    [EBX], 100
          mov    [ESI], 100
  

  The last two MOV instructions are ambiguous, since it is not clear whether the assembler should use byte or word equivalent of 100.

• WORD PTR and BYTE PTR type specifiers must be used for clarification:

          mov    WORD PTR [EBX], 100
          mov    BYTE PTR [ESI], 100
  

   

21. OFFSET and PTR Example

 
 ; offset_ptr.asm
 ; OFFSET and PTR demo program
 .586P
 ; Flat memory model
 .MODEL FLAT, STDCALL
 ;---------------------------------------
 ; Data segment
 _DATA SEGMENT
         num  DWORD   0
 _DATA ENDS
 ; Code segment
 _TEXT SEGMENT
 START:
         lea     ESI, num           ; Load effective address
         mov     ESI, OFFSET num
         mov     bx, WORD PTR num   ; WORD PTR needed because num declared DWORD
         mov     [ESI], bx          ; Copy a word-size value (BX is 16-bit)
         mov     BYTE PTR [ESI], 5  ; Store 8-bit value
         mov     WORD PTR [ESI], 5  ; Store 16-bit value
         mov     DWORD PTR [ESI], 5 ; Store 32-bit value
         ret                        ; Exit
 _TEXT ENDS
 END START
 
 

22. ADDR and OFFSET

• The OFFSET operator returns the address of a variable. It is used to specify the location rather than the content of the variable:

          .DATA
              MyVar  DB   77h       ;  byte-sized variable called MyVar initialized to 77h
          .CODE
              .
              mov eax, MyVar        ;  copy 77h into eax
              mov ebx, offset MyVar ;  copy memory address where value 77h stored into ebx
  

• OFFSET can also pass the address of a variable to a procedure in an INVOKE statement.

• However, OFFSET will only work for global variables declared in the .DATA or .DATA? segments.

• OFFSET will fail with local variables, which are declared upon entry into procedure using the LOCAL statement.

• Local variables inside a procedure do not have offset, because they are created on the stack at runtime.

• The ADDR operator solves this problem. It is used exclusively with INVOKE to pass the address of a variable to a procedure.

• For global variables ADDR operator translates to a simple PUSH instruction, just as if OFFSET had been used:

          push GlobalVar
  

• However, for local variables ADDR translates to: lea eax, LocalVar ; load effective address of LocalVar into eax push eax

• Effective address is the physical address of the data in memory.

• It is important to remember that when using ADDR with local variables, the EAX register is modified rather than leaving it available for other usages within the calling procedure.

   

23. Direct Memory Operands

• A direct memory operand specifies the data at a given address.

• The instruction acts on the contents of the address, not the address itself.

• Except when size is implied by another operand, you must specify the size of a direct memory operand so the instruction accesses the correct amount of memory.

• The following example shows how to explicitly specify data size with the BYTE directive:

              .DATA?                  ; Segment for uninitialized data
      var     BYTE   ?                ; Reserve one byte, labeled "var"
              .CODE
              .
              .
              .
              mov    var, al          ; Copy AL to byte at var
  

• Any location in memory can be a direct memory operand as long as a size is specified (or implied) and the location is fixed.

• The data at the address can change, but the address cannot.

• By default, instructions that use direct memory addressing use the DS register.

   

24. Plus, Minus, and Index

• The plus and index operators perform in exactly the same way when applied to direct memory operands.

• For example, both the following statements move the second word value from a memory array into the AX register:

          mov     ax, array[ 2 ]
          mov     ax, array + 2
  

• The index operator can contain any direct memory operand. The following statements are equivalent:

          mov     ax, var
          mov     ax, [var]
  

• Many programmers prefer to enclose the operand in brackets to show that the contents, not the address, are used.

  Memory operands in brackets make it easier to port code over to other assemblers.

• The minus operator behaves as you would expect. Both the following instructions retrieve the value located at the word preceding array:

          mov     ax, array[-2]
          mov     ax, array-2
  

   

25. Directives BYTE PTR, WORD PTR, DWORD PTR

• There are times when we need to assist assembler in translating references to data in memory.

• For example, instruction

          mov     [ESI], al  ; Store a byte-size value in memory location pointed by ESI
  

  suggests that an 8-bit quantity should be moved because AL is an 8-bit register.

• When instruction has no reference to operand size,

          mov     [ESI], 5   ; Error: operand must have the size specified
  

• To get around this instance, we must use a pointer directive, such as

          mov     BYTE PTR [ESI], 5  ; Store 8-bit value
          mov     WORD PTR [ESI], 5  ; Store 16-bit value
          mov     DWORD PTR [ESI], 5 ; Store 32-bit value
  

• These instructions require operands to be the same size.

• In general, PTR operator forces expression to be treated as a pointer of specified type:

          .DATA
          num  DWORD   0
  
          .CODE
          mov     ax, WORD PTR [num] ; Load a word-size value from a DWORD
  

   

26. Pointers

• A variable that contains the address of another variable is called a pointer variable, or, simply, a pointer.

• Pointers are essential when manipulating arrays and other data structures in memory.

• High-level language s such C/C++ and Java purposely hide the implementations of pointers,
  because such details are not portable across different machine architectures.

• However, assembly language programmers deal with pointers at the physical level.

   

27. Pointer Types

• Intel-based programs use two basic types of pointers, NEAR and FAR.

• Pointer sizes are affected by the processor's current mode:

  • l6-bit Real Mode

  • or 32-bit Protected Mode.

    (Our discussion about x86 memory modes comes later in this course.)

     

   

28. NEAR Pointers

• The Protected-mode programs use NEAR pointers.

• NEAR pointers are stored as double word variables. For example,

      .DATA               ; Begin data segment
          b_array     BYTE    0, 1, 2, 4, 8
          w_array     WORD    1000h, 2000h, 3000h
          ptr_b_arr   DWORD   OFFSET b_array
          ptr_w_arr   DWORD   OFFSET w_array
  
      .CODE               ; Begin code segment
  
          mov esi, ptr_b_arr
          inc esi
          inc esi
          mov al, [esi]   ; AL <- 2
          
          mov esi, ptr_w_arr
          add esi, TYPE w_array
          mov ax, [esi]   ; AX <- 2000h
  

   

29. The TYPEDEF Operator

• The TYPEDEF operator creates a user-defined type.

• A user-defined type has the status of a built-in type when defining variables.

• TYPEDEF is ideal for creating pointer variables.

• For example, PBYTE and PWORD are pointers to bytes and words, respectively:

      PBYTE TYPEDEF PTR BYTE
      PWORD TYPEDEF PTR WORD
  
      .DATA               ; Begin data segment
          b_array     BYTE    0, 1, 2, 4, 8
          w_array     WORD    1000h, 2000h, 3000h
          ptr_b_arr   PBYTE   OFFSET b_array
          ptr_w_arr   PWORD   OFFSET w_array
  
      .CODE               ; Begin code segment
  
          mov esi, ptr_b_arr
          inc esi
          inc esi
          mov al, [esi]   ; AL <- 2
          
          mov esi, ptr_w_arr
          add esi, TYPE w_array
          mov ax, [esi]   ; AX <- 2000h