12

The last line of the routine is a return instruetion which it is usual to
Vse at the end of a machine code routine, t2 return you to Basic.

When you are satisfied that you have entered the above program
correctly, type 'X' to restore the prompt and cursor.

B — Breakpoint

This command silows You to temporarily interrupt a machine code
program at any point, and return control to the MONITOR, so that
You can inspect the values in the CPU registers, and in RAM, and
make corrections as necessary.

It takes the form 'B aaaa', where B is the Breakpoint command
mode, and aaaa is the address of the instruction that the break will
replace. (aana must be the address of the first byte of a multi-byte
instruction),

The Op. codes in the three addresses aaaa, aaaa-1 and saaat2 are
automatically saved in data bytes within the MONITOR, and these
locations are then loaded with CD OF F7 (for the 48K version) or
CD OF 77 (for the 16K version), which constitutes a CALL to'the
entry point of the MONITOR, It must bea CALL to maintain
correct operation of the Stack,

On entering the MONITOR at this address, the values in the CPU
registers are stored in data bytes within the MONITOR at the
addresses shown in Appendix A; the Stack Pointer is set to the
MONITOR Stack; the message "Press Break for Monitor" is

»displayed on the bottom line of the screen, in addition to the sereen

display your program has created. The MONITOR will now wait
until you press 'Break' when it will display the prompt and cursor
on the bottom line of the screen.

You wili now be able to use any of the MONITOR commands to
check or alter the routine, before returning control to the routine at
the point at which the break occurred. As the MONITOR uses its
own integral Stack, separate from the Program Stack, there is no
danger of over-writing the Program Stack during'a Breakpoint.

Before running the example just entered, enter a Breakpoint at
address 6009. This will have the effect of stopping the program
after the registers BC, DE and HL have been cleagreci, but before
they are incremented.

H ž

CHE the prompt is not visible on the bottora line, type X, otherwise,

Type B the breakpoint command mode
Type 6009 the breakpoint address

There is no need to type ENTER, as the Breakpoint is set after
typing the fourth digit. The screen will seroll, and the prompt wili
appear on the bottom line...

Using the 'M' command, check that 6009 to 600B now contain CD
OF F7 (for 48K) or CD OF.77 (for 16K) in place of 03 13 23, the
latter having been stored for later replacement.

You are nowin a position to run the routine up to the breakpoint.

J — Jump and execute

The Jump command allows you to jump out of the control of the
MONITOR to the starting address of any routine that you write,
and it takes the form 'J aaaa' where J is the Jump command mode,
and aaaa is the start address of your program.

You can run your machine code programs either with the
MONITOR 'J' command, or by returning to Basic and using the
USR function. Either way, the MONITOR commands are available
to you after a Breakpoint.

In this exampla, we will use the 'J' command...

Type X to restore the prompt and cursor,
Type J to enter the Jump command. '
Type 6000 the start address,

Type ENTER "

The screen is cleared and the routine will run, and then return to the
MONITOR, with the "Press Break for Monitor'' message.

The seguence of events on executing the J command is:

i).'— the screen is cleared. — .:
ji). the Stack Pointer is set to the program Stack.
ii)

the start address is put into the Program Counter, and the
program is executed.

The MONITOR uses its own integral Stack, which is set on entry to
the monitor routine, therefore the program Stack, which is set by