Page 4.20
THEORY OF OPERATION
By translating the physical address, it is possible to move
the mapping function along so that it operates on consecutive
"lines" of RAM. The use of this adder is analagous to that
of a magnifying glass. The magnifying glass makes a small
portion of text easier to read by enlarging the print. The video
RAM mapping module has, up to this point, made video RAM
easier to address. To reference data outside the area of the
magnifying glass, it is simply moved to the new data area.
Once the screen image has been "effectively" moved, as is
essentially done when the start address of the CRT-C is ad-
vanced, the mapping function must be moved.
When the adder is correctly initialized and maintained, the
bytes representing line 25 are always in the same logical
address. This frees the software routines from maintaining
a pointer and indexing into the screen data based on the start
address. All references to a particular line of data are fixed
with respect to the 8088. Furthermore, since the video RAM
mapping module is between the 8088 and video RAM, its
operation/maintenance do not affect screen refresh in any
way. It must only be maintained for easy references to the
CRT-C RAM from the CPU.
Pictorial 4-9 shows an 8-bit adder. This adder has been dis-
cussed as though it were a full 16-bit adder. In concept, it
is. An 8-bit adder is sufficient, however, because the value
to be added to this address will always be a multiple of 80
*
16
=
5*16*16 = 5" 256. In the implementation, adding n * (5*256
)
to the address is equivalent to adding n
*5 to the high order
byte of the address because multiplication by 256 is equivalent
to shifting left by eight bits. This does assume that the start
address will also be a multiple of 80
*16. If the start address
is initialized to 0, subsequent scrolling operations can maintain
it as a multiple of 16
*80.
The bits inside each byte of video RAM are mapped to pixels
as follows:
b it
7
6
5
4
3
2
1
0
pixel
0
1
2
3
4
5
6
7
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