Intel 815 Manuel D’Utilisation

  

125 

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Underline Location Register 

I/O (and Memory Offset) Address:  3B5h/3D5h (index=14h) 
Default: 

0UUU UUUUb (U=Undefined) 

Attributes: Read/Write 

 

7 

6 5 

4      

0 

Reserved 

(0) 

DWord 

Mode 

Count By 

4 

Underline Location 

 

7 

Reserved. Read as 0s. 

6 

DWord Mode.  

0 = Frame buffer addresses are interpreted by the frame buffer address decoder as being either byte 

addresses or word addresses, depending on the setting of bit 6 of the CRT Mode Control Register 
(CR17).  

1 = Frame buffer addresses are interpreted by the frame buffer address decoder as being DWord 

addresses, regardless of the setting of bit 6 of the CRT Mode Control Register (CR17). 

Note that this bit is used in conjunction with bits 6 and 5 of the CRT Mode Control Register (CR17) to 
select how frame buffer addresses from the processor are interpreted by the frame buffer address 
decoder as shown below: 

0 0 Word 

Mode 

0 1 Byte 

Mode 

1 0 DWord 

Mode 

1 1 DWord 

Mode 

5 

Count By 4.  

0 =   The memory address counter is incremented either every character clock or every other character 

clock, depending upon the setting of bit 3 of the CRT Mode Control Register. 

1 =   The memory address counter is incremented either every 4 character clocks or every 2 character 

clocks, depending upon the setting of bit 3 of the CRT Mode Control Register. 

Note that this bit is used in conjunction with bit 3 of the CRT Mode Control Register (CR17) to select the 
number of character clocks are required to cause the memory address counter to be incremented as 
shown, below: 

0 0 every 

character 

clock 

0 

1 

every 2 character clocks 

1 

0 

every 4 character clocks 

1 

1 

every 2 character clocks 

4:0 

Underline Location. This field specifies which horizontal line of pixels in a character box is to be used 
to display a character underline in text mode. The horizontal lines of pixels in a character box are 
numbered from top to bottom, with the top-most line being number 0. The value specified by these 5 bits 
should be the number of the horizontal line on which the character underline mark is to be shown.