16 V:

If the prompt is not visibiWon the bottom line of the screen, type X,
otherwise,

Type B 600B :

This will set a new Breakpoint after the BC and DE register pair

» have been incremented, but before the HL pair is incremented.

C — Breakpoint Continue

This command allows you to continue from a Breakpoint, and is
executed by typing C followed by ENTER. You can Escape to the
monitor by typing X before ENTER. The program being run will
continue as if the Breakpoint had never occurred.

The screen is cleared; the program Stack is reset; and the CPU
registers are re-loaded from their data block before the Breakpoint
address is put into the Program Counter, and execution is resumed.

Type C
Type ENTER

display "Press BREAK for Monitor". RA

When the prompt appears, after pressing 'BREAK';

Type K to restore the bytes occupied by the
- Breakpoint.
Type R to display the registers,

You can now verify that the Program Counter contains 60OB, the
BC and DE register pair contain 0001 , having been incremented, and
the HL pair still contains 0000.

When a routine encounters a Breakpoint, it returns control to the
MONITOR with a CALL operation, the return address being stored
On the Program Stack, for use by the Breakpoint Continue (C)
command, Having encountered a Breakpoint and studied the CPU
registers and/or memory locations, one of two situations will occur:

v
1. Everything will be as You expect, and the program is correct to
that point. In this case, you would normally restore the
Breakpoint bytes ('K' command) and use the Breakpoint
Continue ('C' command) to continue the program to a new
Breakpoint. :

" h 17
or , '

2). Anerror will become evident, in which case you would track
down the error and correct it, and then, leaving the current
Breakpoint set, use the 'J' command to re-run the program up

to the same Breakpoint, to check that your correction is
Successful.

The Program Stack operation of the MONITOR allows you to do

this providing that, at the Breakpoint, there have been an egual

number of PUSHes and POPs, or CALLs and RETSs, lf the Program

Stack is not balanced at the Breakpoint, you will have a cumulative

stack imbalance every time you use the 'J' command after a

Breakpoint (but not if you use the 'C' command). In this case to

restore the Stack to normal once you have traced an error,

RETURN to Basic ('Y' command) and re-access the monitor from

the beginning, then use the 'J' command to run your program up to

the Breakpoint again.

Having set a Breakpoint in a routine, you can either use the 'J'
command to run the routine, or you can use the RETURN
command to go back to Basic, and run the machine code via the
USR function, For example, if you have written some machine
code as part of a Basic program, which is accessed by the USR
function in the Basic program, you can set a Breakpoint using the
MONITOR then return to Basic and run the Basic program. When
the Breakpoint is reached in the machine code, the MONITOR will
be accessed as shown above, and the Breakpoint Continue command
wiH allow the machine code to resume, and eventually return to the
Basic program that called it.

The MONITOR has been carefully designed to allow this free
interchange between Basic and machine code, without upsetting
the Stack.

P — Printer

This command allows you to prŠduce a Hex dump of any section of
memory onto the Sinclair Printer, |t takes the form: 'P aaaa bbbb'
where P is the Printer command,.aaaa is the Hex address of the first
byte to be printed, and bbbb is the Hex address of the last byte you
want printed. The Printer produces a display in the format shown
below.

0000 FZ AF 11 FF FF C3 CB 11
O0OB 2A SD 5C 22 SF SC 18 43
0010 CS F2 15 FF FF FF FF FF