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 Sub- Routine), followed by the specified absolute address.

Incorporated in the operating system, there 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 before calling the subroutine. The address of this subroutine is $FFD2.

Therefore, to print "Hi" to the screen, the following program should be entered:

   .A 1400 LDA #$48     - load the CBM ASCII code of "H"
   .A 1402 JSR $FFD2    -  print it
   .A 1405 LDA #$49     - load the CBM ASCII code of "I"
   .A 1407 JSR $FFD2    -  print that too
   .A 140A LDA #$0D     - print a carriage return as well
   .A 140C JSR $FFD2
   .A 140F BRK          - return to 64MON
   .G 1400              - will print "HI" and return to 64MON

The "PRINT a character" routine we have just used is part of the KERNAL jump table. The instruction similar to GOTO in BASIC is JMP, which means JUMP to the specified absolute address. The KERNAL is a long list of "standardized" subroutines that control ALL input and output of the Commodore 64. Each entry in the KERNAL JMPs to a subroutine in the operating system. This "jump table" is found between memory locations $FF84 to $FFF5 in the operating system. A full explanation of the KERNAL is available in the "KERNAL Reference Section" of this manual. However, certain routines are used here to show how easy and effective the KERNAL is.

Let's now use the new principles you've just learned in another pro- gram. It will help you to put the instructions into context: