Commodore VIC-20 User Manual page 223

Chapter 6: Graphics
209
programming technique. Using custom characters, you set up the character
memory and change the display by altering screen memory. In high-
resolution displays, screen memory usually stays constant, but character
memory changes.
To understand high-resolution graphics, let's review our discussion of
character memory, but with a different perspective. From a text display
point of view, we think of screen memory as containing characters to be
displayed, with character memory holding the dot patterns that represent
the characters. In high-resolution terms, character memory becomes a
dot-by-dot representation of the display, and screen memory is simply a set
of pointers to remind the VIC chip of where it is on the screen. In both cases,
the VIC chip is doing exactly the same thing: taking characters from screen
memory and using them to look up dot patterns in character memory. Only
the description of the process is different. Recall how we built one player
from several custom characters in the last section. You can think of high-
resolution graphics as using the entire screen as a single "super-player."
High-resolution graphics have one serious drawback: They use a large
amount of memory. In fact, to completely map the screen in high-resolution
mode would take 4048 bytes of character memory. Since there are only
about 3500 bytes free on an unexpanded VIC, this does not leave much
room for your program. It is possible, however, to experiment with high
resolution without expanding memory: Simply use only a small portion of
the screen. The techniques we used for defining a custom character set can
also be used to set up a 64 X 64 dot area for high-resolution experimentation.
The steps in preparing this high-resolution work area are similar to
those for establishing a custom character set. The Setup program listed
below may seem familiar, because it was written by making some editing
changes to the program we used to set up our custom character work area.
11313 REM HIGH RESOLUTION "SETUP" PROGRAM
1113 REM PROTECT CHARACTER MEMORY
1213 POKE 52,24:POKE 56,24:CLR
1313 POKE 36869,254
140 REM COP' ' ' ' UNREVERSED CHARACTERS
1513 FOR 1=6144 TO 7167 : POKE I,PEEK(26624+I) NEXT
1613 REM CLEAR CHARACTER MEMORY
170 FOR 1=7168 TO 7679 : POKE 1,O : NEXT
1813 REM PREPARE SCREEN MEMORY
190 PRINT 1O:' l Il!HPX (138"
200 PRINT IO:flIQY!)19 1O
2113 PRINT "mlJRZ"; CHR$(34); CHR$(34) , : CHR$(20); "!lE2: "