Intel 210200-002 manual Thing

ARCHITECTURE AND INSTRUCTIONS

loaded into memory at the indicated addresses and then executed.

At the time the instructions are loaded, initial values for data items could also be loaded into memory. This means that the object code, besides containing instructions and their addresses, may also contain initial values for data items and their addresses. These initial values are specified to the assembler in the data definition statements.

The following statement will cause the assembler to produce object code that, when loaded into memory, will result in a 25 being placed in the memory address allocated to

THING;

THING DB 25 :byte initially contains 25

A question mark in place of an initial value means that we do not choose to specify an initial value for that data item; we will be satisfied with whatever initially appears in the corresponding memory location.

When the assembler sees the question mark, it still allocates memory for the data item, but does not produce object code to initialize the memory location (although it could).

In general, the initial value could be specified by an expression, since expressions are eval- uated at assembly time. So we can write statements like:

IN_PORT DB PORT_VAL

OUT_PORT DB PORT_VAL+1

what about a memory-address value? It won't fit into a byte, but the offset component fits into a word; and, both the segment and address components fit into a double word. So we can write initialization statements like those shown at the bottom of this page.

The initialization of LITTLE_CYCLE per- mits an indirect intrasegment jump or call to use the date item named LITTLE,CYCLE to transfer control to the label na~eq CYCLE. Similarly, an intersegment jump or call transfers control to CYCLE by using the data item named BIG_CYCLE.

Tables

So far we have used data-definition state- ments to define one byte, word, or double- word at a time. Often, we deal with tables of bytes, words, or double words. For example, the 8088 XLAT instruction uses a table of bytes to translate an encoded value into the s~me valJ.le under a different encoding. The 8088 interrupt mechanism uses a table of double-words, starting at memory location 0 to point to the starting addresses of the inter- rupt service routines. And, the 8088 string instructions operate on tables of bytes or words containing the string elements.

A table is defined by placing several initial values on a data-definition statement. The following statement defines a table of bytes containing powers of 2:

DB 1,2,4,8,16

Recall that expressions come in two varieties - numeric and memory address. It is mean- ingful to initialize either a byte, or a word, or a double-word with a numeric value. But,

The byte at the memory address correspond- ing to POWERS_2 will be initialized to 1 (whenthe object code is loaded into memory).

LITTLE_CYCLE	DW	CYCLE	;offset of CYCLE
BIG_CYCLE	DO	CYCLE	;offset and segment of CYCLE

CYCLE MOV BX,AX

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