Monster — VIC-20 Assembly Development System

Try Monster Online

Below you can try Monster for yourself in the browser (emulated with VICE). Try typing in some of the examples to get a feel for its operation.

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Program:

.ORG $1400
START
  LDX #$00
DISP
  LDA MSG,X
  BEQ DONE
  JSR $FFD2
  INX
  BNE DISP
DONE
  JMP DONE

MSG
.DB "HELLO WORLD!",0

Click the emulator screen to give it keyboard focus.
Press I to enter insert mode, then type one of the example programs. Press RETURN after each line so Monster can format and check it.
Press RUN/STOP to leave insert mode and return to command mode.
Press : to enter EX mode, type r HELLO.ASM, and press RETURN to name the buffer.
Press C= + A to assemble it.
Press C= + D to , or F7 to open the monitor.

After assembling successfully, press C= + D to start the source-level debugger.
Watch the lower part of the screen for the registers, current source line, effective address, and elapsed cycle count.
Press Z to step into one instruction. Press S to step over a JSR, or Y to finish the current subroutine and step out.
Press SPACE to see the program’s display, then press any key to return to the debugger.
Press C= + G to let the program run until it reaches a breakpoint or you press RESTORE.
Press C= + X and confirm the prompt when you are ready to leave the debugger.

Useful distinction: Z enters subroutines, while S runs a subroutine and stops on the instruction after it.