Commodore Plus 4 - Page 333

Interfacing with BASIC
381
Adding a BASIC Wedge
Example:
BASIC retrieves each character from RAM by calling a routine referred to as
CHRGET, which is located at $0473. Because this is in RAM (initialized on
power up), it can be altered to recognize and process symbols as desired by the
programmer. It is possible to add BASIC commands in this way.
This example shows one way to add a direct-mode-only command to BASIC.
The command is the English pound symbol (£), and it increments the border
color. The first step is to enter the machine-language wedge through the machine-
language monitor.
2000 48
2001 A5 9A
S003 10 OB
S005 68
2006 C9 5C
PHA
LDA $9A
BPL $2010
PLA
CMP #$5C
. 2008 DO 07 BNE $2011
200A EE 19 PP IKrC $FP19
200D 4C 73 04 JMP $0473
2010 68 PIA
2011 8D 3E PP STA $PP3E
2014 4C 84 04 JMP $0484
Save the character on the stack.
Look at the message flag.
If positive, then this is not direct
mode so leave.
Get the character.
Compare with the English pound
symbol.
Not one, so leave.
Increment the border color.
Go to get the next character.
Get the character.
Back to ROM.
Go back to the operating system
routine.
After the wedge code is in place, change the statement at $0481 from STA
$FF3E to JMP $2000. Then exit to BASIC. When in direct mode, you can input
the English pound symbol to increment the border color. Everything else works
normally.
Relocating BASIC RAM
BASIC keeps track of the RAM it is permitted to use in a series of pointers,
$2B-$38. Some care must be taken when you change these pointers. The main
reason for moving BASIC is to reserve some of the RAM usually used by BASIC
for a machine-language routine.
Moving the top of BASIC down is the easiest way to restrict the RAM
available to BASIC. This can be done with the following sequence of commands: