Explain the following directives CODE, ASSUME, ALINE, EQU, EVEN. Various Data & Model directives.

1) .CODE-

• This assembler directive indicates the beginning of the code segment. Its format is as follows: .CODE [name]
• The name in this format is optional.
• For tiny, small and compact models the segment name is –TEXT always.
• The medium and large memory models use more than one code segments which can be distinguished by name.

2) ASSUME-

• The directive is used for telling the assembler the name of the logical segment which should be used.

• The format of the assume directive is as follows:

  ASSUME segment register: segment-name:

• The segment register can be CS, DS, SS and ES.

• The example of assume directive is as follows:

  ASSUME CS: Code, DS: Data, SS: Stack:

• ASSUME statement can assign up to 4 segment registers in any sequences.
• In this example, DS: Data means that the assembler is to associate the name of data segment with DS register.
• Similarly CS: Code tells the assembler to associate the name of code segment with CS register and so on.

3) ALIGN-

• This directive will tell the assembler to align the next instruction on an address which corresponds to the given value.

• Such an alignment will allow the processor to access words and double words.

  ALIGN number

  This number should be 2, 4, 8, 16…. i.e. it should be a power of 2.

• The example of align directive are ALIGN 2 and ALIGN 4.

• ALIGN 2 is used for starting the data segment on a word boundary whereas

  ALIGN 4 will start the data segment on a double boundary word.

4) EQU-

• It is used to give a name to some value or symbol in the program.
• Each time when the assembler finds that name in the program, it replaces that name with the value assigned to that variable.

• Its format is [name] EQU initial value

  E.g. FACTORIAL EQU 05H

• This statement is written during the beginning of the program and whenever now FACTORIAL appears in an instruction or another directive, the assembler substitutes the value 5.
• The advantage of using EQU in this manner is that if FACTOPRIAL is used several times in a program and the value has to be changed, all that has to change the EQU statement and reassemble the program.
• The assembler will automatically put the new value each time it finds the name FACTORIAL.

5) EVEN-

• It tells the assembler to increment its location counter if required, so that the next defined data item is aligned on an even storage boundary.
• The 8086 can read a word from memory in one bus cycle if the word is at an even address.
• If the word starts at an odd address, the microprocessor must do two read cycles to get 2 bytes of the word. In the first cycle it will read the LSB and in the second it will read MSB.

• Therefore, a series of even words can be read more quickly if they are at an even address.

  $\text{E.g. EVEN TABLE DB 10 DUP (0)} \hspace{1cm} \text{; It declares an array named TABLE} \\ \hspace{7.3cm} \text{; of 10 bytes which are starting from} \\ \hspace{7.3cm} \text{; an even address}$

6) Various data directives-

• .DATA- This directive indicates the beginning of the data segment.
• Define Byte [DB]- This directive defines the byte type variable.
• Define Word [DW]- The DW directive defines items that are one word (two bytes) in length.
• Define Double word [DD]- It defines the data items that are a double word (four bytes) in length.
• Define Quad word [DQ]- This directive is used to tell the assembler to declare variable 4 words in length or to reserve 4 words of storage in memory.
• Define Ten bytes [DT]- It is used to define the data items that are 10 bytes long.

7) Model directives-

• .MODEL
  • This directive is used for selecting a standard memory model for the assembly language program.
  • Each memory model has various limitations depending on the maximum space available for code and data.
  • The general format for defining the model directive is as follows: .MODEL [memory model]:
  • The size of a memory model can be anything from small to huge.
  • The tiny model is meant for the .COM programs because they have their code, data and stack in only one 64 kB segment of memory.
  • On the other hand the flat model is the biggest which define one area up to 4GB for code and data.
  • The small model is useful for the student level programs because for this model the assembler assumes that the addresses are within a span of 64 kB and hence generates 16 kB offset addresses.
  • In the compact model, the assembler can use 32 bit addresses. So the execution time for this model is longer.
  • The huge model contains variables such as arrays which need a larger space than 64kB.
  • The memory model is chosen based on the user’s requirement by referring to the following table