14

the Basic initialisation routines wHen the ZX Spectrum is switehed
on, must be reset before your program can be run. item (ii) above
does this for you. The use of two stacks helps make the MONITOR
invisible to your machine code program. . . :

Having run, the program will have encountered the Breakpoint at
address 6009, and have displayed ''Press BREAK for Monitor'"'.
Press tho BREAK/SPACE key (shifted or unshifted) to access the
MONITOR.

The first operation after a Breakpoint should always ba to replace
the correct byte values to the addresses where the break occurred. '

K — Break Restore s

This command restores the correct values into the three bytes
overwritten by the Breakpoint command.

Type K

The screen will show K 6009 and will serol! up one line, displaying
the prompt on the bottom line, There is no need to type ENTER.
Using the M command, verify that the original codes have been
replaced in addresses 6009 to 60OB,

Only one Breakpoint can be entered at any time, as there is only
enough room to store one address and three data bytes, so a Break
"Restore (K) command must be executed before the next Breakpoint
(B) is defined, and it is recommended that a Break Restore (K)
command is keyed immediately after a Breakpoint has been
encountered, h

If you enter an incorrect breakpoint with the B command, type K
im mediately afterwards to restore the original values to the
incorrect breakpoint address, and then re-type the Breakpoint.

The K command can only restore the last entered Breakpoint.

Let us now inspect the CPU registers, to make sure that the program
is working as we expect.

 R — Display values in CPU registers

1 the prompt is not visible on the bottom line of the screen, type X,
otherwise

e | mo

Type R

The screen scrolis up, automatically displaying the CPU register
contents thus: k

> ;

IR. "a ZFSA
a'F" i 0044
Bre"? / 1721
D'E"' " . 369B
BPA? 2758
AF OF5A
EC OODOO
DE OO00O
HL O00O
IX OZDA
IY SCZA
SE bE?3
PC 6009 k

There is no need to type ENTER.

As you will see, the Program Counter contains 6009, the address at
which the Breakpoint occurred. The BC, DE and HL register pairs
will all contain 0000. In this example, these are the oniy registers

that we are interested in.

The CPU registers are displayed with their contents shown in Hex,
and the values in the registers are stored in RAMat the addresses
shown in Appendix A.

When the MONITOR is entered from a Breakpoint, the values of the
registers immediately prior to the Breakpoint are stored in these
memory locations, so that the operation of your routine can be
checked, and corrections to the routine, or the register contents can
be made before continuing. J

Any changes to the CPU register values stored in RAM during a
Breakpoint only take effect after a Jump (J) or Breakpoint
Continue (C) command has been executed.

Having a) encountered one Breakpoint, b) restored the correct
values after the break, and c) verified that the CPU registers have
their correct values, we will now enter another Breakpoint, and
continue the routine. ;