Advanced
BIOS
Functions
6-20
dis pIa y
data
buffer address:
attribute
data
buffer address:
Starting
address
of
display
data
buffer (offset, base
address; 2 words)
Starting address
of
attribute
data
buffer (offset, base
address; 2 words)
Figure 6-3 shows
how
the
DMA
transfer function works.
DISPL.A Y
REQCEST
BLOCK
--1
DISPLAY
DATA
VIDEO
MEMORY
~
Il...
I ,
I "
I
",
\
\
\
---------------
\
ARI,A
\"
;~
"
"
~
LA I
CA
NOL
-r---.
-
,'----------
DMA
TRANSHR
---
1111
I
41
I 00 I
42
I
DISPL\
Y
DATA
[I'IDIO
MIMORY
H)RMA'I)
j
I',
~-----------,~--------------~
l
--
-----.~--------~
,'L--
.......
I
--
I I
\
A
TfRIBUTE
t-
- - - - - - - - - - - - - -
DMA
TRANSII
R
\T1RIHI'TID.YIA
DATA
AREA
\r-_
i
-------------,-~--------------~
__
1
____
->----1
__
I
Figure 6-3 DMA Transfer
The display request
block
contains the addresses
of
the display
data
area
and
the
attribute
data
area.
On
the video memory, each display
character
consists
of
two
bytes
of
display
data
and
one
corresponding
byte
of
attribute
data.
The first byte
of
the
display
data
for
each character identifies whether the following
character
code
is
a
normal
character
or
an
auxiliary character. The second byte
of
the display
data
is a
character
code. The
attribute
data
is
a
color
code.
FIRST
BYTE
SECOND
BYTE
OOH=
Normal
OOH
through
FFH
Display
Data
89H = Auxiliary
Attribute
Data
OOH
=
through
FFH
N/A
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