Q:

Syntax of "segment at" in assembly language

newsegm segment at 0B800h
...
newsegm ends

Can you describe what does it mean? I have another segment with the address 0B800h?
 The reason why I ask is:
 I have a code with these strings
lea DX, message 
message DB "Hello, World!",'$'  
VIDEO DD 0B800h:072Eh 

The teacher highlighted 0B800h:072Eh and messageand said to write an expression using the vidsegm segment name. I don't know what does he want and what to do

A:

Essential part:
You can refer to the char at the position in the video segment with the handle newsegm at the position newsegm:072Eh.
Further explanation:
The address 0B800 hexadecimal is the starting address of the video segment for text mode. The directive
newsegm segment at 0B800h
...
newsegm ends

defines a shortcut for this segment address.
In this 0B800 video segment, each char on the display consists of two bytes: the color code and the char code. So the address in your second code sample, 0B800h:072Eh, points to the

072Eh/2 = 919d   ; two bytes per char
  919d/80d = 11d   ; line number
  919d%80d = 39d   ; column number

It points to the char at position line 11, column 39 (starting with 0).  

