Apple IIe - Page 268

268 Apple lie Users Handbook
Once you have started the program, you can stop it by pressing
Ctrl-Reset.
This
program causes the speaker to buzz. This is accomplished
using the monitor bell subroutine . This is located at memory
address FBDD. The second instruction (JMP 300) causes the pro-
gram to repeatedly jump to the bell subroutine . This results in a
continuous buzzing being emitted from the speaker.
The various
1/0 subroutines are listed in Appendix I. These can
be
executed via a machine language program.
Creating a Custom Monitor Command
The
monitor 's CTRL-Y command causes program control to be
transferred
to the instruction at memory location 3F8. Addresses
3F8-3FF are available for usage by the programmer. Generally ,
because
only 8 bytes are available, a JMP instruction will be
stored at these addresses. However , a machine language pro-
gram could also be stored there.
One useful application of the CTRL-Y command is automatic
execution of the mini-assembler . By storing the following data
values at locations 3F8-3FA:
JMP
I
3F8: 4C
' Address
F666
,. ,...., '
66 F6
CTRL-Y will result in automatic execution of the mini-assembler .
The
programmer could avoid having to enter the preceding
command by adding the following program line to the DOS
"Hello" file. This can be accomplished by entering the following:
]UNLOCK HELLO
]LOAD HELLO
]5 POKE 1016, 76 : POKE 1017,102:POKE 1018,246
]SAVE HELLO
]LOCK HELLO