Commodore Plus 4 - Page 213

Graphics Programming in BASIC
Copying the Standard Set If you want to add your own special characters to
the built-in characters and still use some of the standard characters, you need to
copy the patterns stored in the character ROM to an area of user memory. This
situation is somewhat complicated in BASIC. The PEEK function automatically
reads RAM. This means that if you PEEK the area of memory containing the
character ROM, you do not get the ROM patterns at all, but the contents of the
RAM "underneath" the ROM. (See the banking section in Chapter 5.) You can
defeat the BASIC switch to RAM with the following POKE:
POKE 1177,62
You can then copy the upper case/graphic ROM patterns to your RAM area (at
characters) with
FOR I = 0 TO 1023:P0KE c2iaracters+I,PEEK(53248+I):NEXT I
or copy the upper/lower case ROM patterns with
FOR I = 0 TO 1023:P0KE ciiaracters+I,PEEK(54272+I):]SrEXT I
You must then restore BASIC'S subroutine with
POKE 1177, 63
Because using this method actually changes the operation of BASIC while its
subroutine is altered, restore the switch to RAM before you perform any other
operations.
High-Resolution Characters You can create custom character patterns in
RAM and use them instead of the ROM characters in your program. First, you
need to define your new characters. For each custom character use an 8 * 8 grid
similar to the one shown in the beginning of this section. The top row of the grid
represents the top row of your character. Add up the bit values for the dots you
want on in this row. This sum is the first of eight data values for your character.
Do the same for each of the remaining rows.
Since your custom characters must appear in RAM, you must switch the
graphics chip to look at RAM. This is accomplished by clearing bit 2 of 65298
($FF12), that is, ANDing its current value with 251.
P0KE65298,PEEK(65298)AND251
To reselect ROM, you must set this bit, that is, OR its current value with 4.
P0KE65298,PEEK(65298)0R4