UMBC CMSC 211

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Defining Segments

You have already defined two segments in your programs, because the .DATA (and .CODE) directive is the equivalent to: 
    @data   SEGMENT  PUBLIC
            .... data declarations ....
    @data   ENDS
Segments not declared to be PUBLIC are assumed to be PRIVATE. That means that if separately compiled procedures are link with the same segment name, they will not be combined.

There is an AT option: 

    Video   SEGMENT  AT 0B800h
            .... data declarations ....
    Video   ENDS
This assumes that you are using a Color Graphics Adaptor (CGA) that starts at 0b800:0h. Note that AT segments do not generate any code or data of their own, they just give you a symbolic menas to reference things that already exist at predefined locations.

An extension of this is where physical address 410h is word that uses bits 4 and 5 to specify whether the display is color (10B or monochrome(11B): 

    BIOSData   SEGMENT  AT 40h
    ORG		10h
    EquipList  DW		?
    BIOSData   ENDS
Then we can have: 
        mov	AX, BIOSData
	mov	es, AX
        ASSUME	es : BIOSData
        test	EquipList, 10000B
        jz       Color
(The BIOS INT 11 call which returns the equipment list is a much safer and more machine-independent method of getting the equipment list.)
SEGMENT AT 40h and ORG 10h put EquipList at address 40h:10h or 40h * 16 + 10h which is 410h. This could have been represented as 0h:410h, 41h:0 or 25h:1C0h. You are responsible to insure that the proper paragraph is put into the segment register.

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