Commodore 128 Programmer's Reference Manual page 330

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320 COMMODORE 128 ^
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In the program, as discussed, the instructions in $180A through $181A start the LJ
VIC and 8563 bit maps at locations $2000 and $0000 respectively. All these instructions
encompass step 1 of the algorithm. ) I
The instructions that perform step 2 of the algorithm are stored at locations LI
$182D-$1830.
Step 3 stores the VIC bit map byte at the 8563 bit map position and is performed j j
by the instruction $1831 and the subroutine stored between $1875 and $1882. This jj
subroutine writes the value stored in the accumulator into the appropriate address in
8563 RAM as specified by the contents of 8563 registers 18 ($12) and 19 ($13). , (
The instructions stored at $183A through $183D perform the fourth step of the 1
algorithm.
The stored in $1883 through $188E update the VIC bit map pointer by 8.
Steps 2, 3 and 4 are performed while the pixel row counter in $9C is less than ) [
seven. If it is less than seven, the next byte of the VIC bit map is loaded and stored into <—*
the 8563 bit map.
Step 6 is contained in the subroutine starting at ($189B). This decrements the VIC j I
bit map pointer by 311 locations. U
Step 7 of the algorithm is contained in the subroutine stored in locations $188F
through $1899. \~)
Finally, step 8 of the algorithm is performed by the instructions $186F through [j
$1872.
This program allows you to transfer one bit map to the other, but leaves the VIC ^
bit map as 40 columns. Expand on the algorithm in this section and center the bit map I
on the 80-column screen. Write a routine that proportionally spaces the 40-column bit
map and places it on the entire 80-column screen. Also add a routine that clears the
other half of the 80-column bit map. Add these routines to any business application, and
you'll have a valuable business tool which you can use completely within the 8563 chip
that display your character and bit map data.
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SCROLLING THE 8563 SCREEN \ j
The 8563 has a feature that allows the screen to scroll smoothly either vertically (up or
down) or horizontally (left or right). The 8563 scrolling feature can be thought of as a
small virtual screen window which is displayed on a portion of a full size 80-by-25 j j
character screen. The virtual screen is only visible through the window you define at the *-*
specified position of the full-sized screen. Both screens however do exist in the 8563
character RAM storage locations. ) I
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