Commodore VIC-20 - Page 154

absolute address. VICMon allows you to lype in an absolute
address, and it calculates the correct offset. This is just one of the
"comforts11 of using an assembler.
Subroutines
In machine language (in the same way as using BASIC), you can
call subroutines. The instruction to call a subroutine is "JSR" (jump
to subroutine), followed by the specified absolute address.
Incorporated in the operating system is a machine language
subroutine that will PRINT a character to the screen. The CBM
ASCII code of the character should be in the accumulator beiore
calling the subroutine. The address of 1his subroutine ts SFFD2.
Therefore, to print 4'HI" to the screen, the following program
should be entered:
• A 1400 LDA #S48 load the CBM ASCII code of J'H"
• A 1402 JSR SFFD2 print it
• A 1405 LDA #S49 load the CBM ASCII code of T
• A 1407 JSR SFFD2 print that too
• A 140A LDA #$0D print a carriage return as well
-A 140C JSR SFFD2
• A 140F BRK return to VICMon.
• G 1400 will print "HI" and return to VICMon
The "PRINT a character1' routine we have jus1 used is part of the
KERNAL J1jump table." The Instruction similar to GOTO in BASICr
is "JMPT which means "jump to the specified absolute address/1
The KERNAL is a long list of "standardized" subroutines that
control ALL input and output of the VIC 20. Each entry in the
KERNAL JMP's to a subroutine in the operating system. This "jump
table" resides at SFF84 lo SFFF5 in the operating system. A full
explanation of the KERNAL is in the "KERNAL REFERENCE
SECTION1' in 1his manual, but certain routines will be used here to
show how easy, and effective, the KERNAL is.
We will now use these new principles in another program which
will help you to put these instructions into context:
This program will display the alphabet using a KERNAL routine.
The only new instruction introduced here is TXA "transfer the
contents of the X index register, into the accumulator."
• A 1400 LDX #S41 X-CBM ASCII of "A".
•A 1402 TXA A=X.
• A 1403 JSR SFFD2 print character.
• A 1406 INX bump count.
• A 1407 CPX #S51 have we gone past "Z" ?
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