Commodore Plus 4 - Page 240

228 Machine Language on the Commodore Plus/4
The Commodore Plus/4 is equipped with a built-in machine-language moni
tor. The monitor is accessed from BASIC with the MONITOR command. Using
this monitor, machine-language programs may be entered and executed. Several
other functions are also provided (see the next section), making it possible to
experiment in machine language without purchasing any additional software.
The monitor is also very useful for debugging machine-language programs
created using an assembler.
Machine-language programming generally requires a more detailed knowl
edge of the hardware devices used in the computer than BASIC programming
does. Information on using the various chips in the Plus/4 is provided in the
appropriate chapter in this book. Some of the subroutines in the ROM of the
Plus/4 are available to the machine-language programmer. Information on these
subroutines is included in this chapter. These subroutines are particularly useful
in performing I/O operations for which many programmers do not want to write
their own code.
Machine-Language Monitor Commands
The monitor is entered by issuing the MONITOR command from BASIC. When
the monitor is entered, the current contents of the program counter (PC), status
register (SR), accumulator (AC), X register (XR), Y register (YR), and stack
pointer (SP) are displayed in hexadecimal.
MONITOR
PC SR AC XR YR SP
; 0000 00 00 00 00 P9
The screen editor is fully functional while you are in the monitor. That is,
whenever a RETURN is entered, the line on which the cursor resides is processed,
and the screen scrolls normally.
Whenever the monitor encounters an error in a line it is attempting to process,
a question mark is displayed and no action is taken. For a list of monitor
input/ output errors, see Appendix A.
Note: The monitor normally accesses RAM up to $8000 and ROM thereafter.
If you want to access all RAM, change location $07F8 to $80 (see M command).
To switch back to ROM, change it back to $00.
Entering a Program—The A or. Command
, The A (Assemble) command is used to enter a line of machine code. The syntax is
as follows: