Intel 210200-002 manual Dontcare DB ?,?,?,?,?,?,?,?, Allzero DB 6 DUP

ARCHITECTURE AND INSTRUCTIONS

The next four bytes will be initialized to 2,4,8, and 16, respectively. A table of bytes, all initialized to zero, can be defined by

ALL2ERO DB 0,0,0,0,0,0

or by the shorthand notation

ALLZERO DB 6 DUP (0)

And, finally, an un-initialized table can be defined by either of the following equivalent statements:

DONT_CARE DB ?,?,?,?,?,?,?,?

DONT_CARE DB 8 DUP (?)

TYPES OF MEMORY LOCATIONS

ASM-86 associates a type with every memory location referred to in the program so it can generate the correct code for instructions that accesses memory.· For example, the data- definition statement

SUM DB ?

informs the assembler that the memory loca- tion SUM is of type BYTE. Later, when the assembler encounters an instruction state- ment such as

INC SUM

the assembler will know to generate a byte- increment instruction, rather than a word- increment instruction.

Amemory location can be one of the follow- ing types:

1) BYTE of data, as in:

SUM DB ? ;defining a byte

2)WORD of data (two consecutive bytes), as

m:

BIGGER.J)UM DW ? ;defining a word

3)DWORD of data (four consecutive bytes), as in:

BIGGEST_SUM DD ? ;defining a doubleword

4) NEAR instruction location, as in:

CYCLE: CMP SUM,100

5) FAR instruction location:

(means of defining such locations will be discussed shortly)

An instruction location can appear in a jump or call instruction statement. The assembler will generate an intrasegment jump or call if the location type is NEAR, and an interseg- ment jump or call if it is FAR. For example, the labeled instruction statement

CYCLE: CMP SUM,100

informs the assembler that the memory loca- tion CYCLE is of type NEAR. (We will see shortly how the synthetic operators PTR and THIS are used to define a memory location of type FAR). Later, when the assembler encounters an instruction such as

JMP CYCLE

the assembler will know to generate an intra- segment jump. instruction, rather than an intersegment jump instruction.

A memory address built by adding or sub- tracting a numeric value to or from some other memory address has the same type as the original memory address. For example, SUM+2 is a BYTE, BIGGER_SUM-3 is a WORD, and CYCLE+I is a NEAR instruc- tion location.

ANALYTIC AND SYNTHETIC OPERATORS

We now know enough about memory addres- ses to complete the discussion of operators.

The analytic operators decompose memory- address operands into their components. These operators are:

1)SEG

2)OFFSET

3)TYPE

4)SIZE

5)LENGTH

The SEG operator returns the segment com- ponent of the memory-address operand. The OFFSET operator returns the offset compo- nent. Both of these components are generally numeric values.

The TYPE operator returns a numeric value, which is the type component of the memory- address operand. The value of the type

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