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191 lines
6.2 KiB
Plaintext
191 lines
6.2 KiB
Plaintext
anchor:cassette_tape[]
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=== Cassette Tape Interface
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The Apple ][ and Apple ][ plus machines had the ability to save and load binary
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data to and from cassette tape. The user would attach a standard cassette tape
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recorder to the jacks on the back of the Apple ][, and use the monitor +R+ and +W+
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commands, or the Applesoft BASIC commands +LOAD+ and +SAVE+, to read and write data
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on the cassette tape. The user would have to press the play and/or record buttons
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on the player at the right time.
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The Epple ][ emulates the cassette interface, using a file to hold the
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recorded portion of the tape. The file will grow in length as necessary
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to hold data that the emulated Apple is writing to the ``tape.''
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The emulator will not overwrite existing data on a tape image.
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The emulator will automatically ``press'' the play or record buttons that
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would have been necessary when using the original machine.
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==== Commands
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+cassette new <file-path>+
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This creates a new empty file (on the host computer) that represents a cassette tape image.
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The file must not already exist.
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+cassette load <file-path>+
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This loads an existing file (from the host computer) containing a cassette tape image.
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The tape is automatically positioned at its beginning (fully rewound).
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+cassette unload+
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This removes the file from the cassette tape. Note that you must manually save
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the file using the +cassette save+ command (described below).
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[WARNING]
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Unloading an unsaved file will lose any changes made to the file, without warning.
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+cassette rewind+
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This command ``rewinds'' the cassette tape, positioning it at the beginning
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of the tape (for subsequent reading). You do not need to rewind the tape
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before saving or unloading it, of course.
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+cassette save+
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This command saves the changed tape to the file. Note that the display will show
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an asterisk +*+ next to the file name if there are unsaved changes that need to
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be saved. Unsaved changes will be lost without warning if the file is unloaded
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or if you quit the program.
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==== Example of Saving to Tape
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Start up the emulator with Applesoft ROMs for this tutorial.
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Enter a simple Applesoft program, just as an example, that we
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are going to save to a cassette tape image file.
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[source,vbs]
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------------------------
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]NEW
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]10 PRINT "HELLO"
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]20 END
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]LIST
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10 PRINT "HELLO"
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20 END
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]RUN
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HELLO
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]
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------------------------
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We first need to load a tape image file into the cassette machine.
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Enter command mode by pressing +F5+, then make a new tape
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image file.
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------------------------
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command: cassette new hello.tap
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------------------------
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This will create a new, empty tape file image named +hello.tap+
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in the current default directory. (We could have specified a full path
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name for the file if we wanted to place it in a different directory.)
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Notice that the emulator now displays the name of the tape image file,
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along with the position and length of the tape image, which is now +0/0+.
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Next, we tell Applesoft to save the program to the cassette. For this,
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we just use the +SAVE+ command. Note that this is not the
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DOS +SAVE+ command; the DOS command has a file name after
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+SAVE+. We just use +SAVE+ with no file name.
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------------------------
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]SAVE
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]
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------------------------
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It will take 10 seconds or so for it to save. Notice that the
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current position of the tape is counting up as the Apple saves
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the program. When it is finished, you need to save the changes
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to the file. Press +F5+ and enter the emulator command to save
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the tape image file.
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------------------------
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command: cassette save
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------------------------
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We can now unload the file from the emulator (which is like ejecting
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the tape from the cassette player).
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------------------------
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command: cassette unload
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------------------------
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==== Example of Loading from Tape
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To load the saved program (from the previous section) into the Apple again,
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we will need to first load the tape image file back into the cassette machine.
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Press +F5+ to enter command mode and load the image file.
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------------------------
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command: cassette load hello.tap
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------------------------
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This will load hello.tap (in the current default directory). Notice the
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emulator now displays the name of the tape image file, along with the
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position and length of the tape image, which in this case is +0/33481+.
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Next, we tell Applesoft to load the program from the cassette. For this,
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we just use the +LOAD+ command. Note that this is not the
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DOS +LOAD+ command; the DOS command has a file name after
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+LOAD+. We just use +LOAD+ with no file name.
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------------------------
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]LOAD
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]
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------------------------
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It will take 10 seconds or so for it to load. Notice that the
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current position of the tape is counting up as the Apple loads
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the program. When it is finished, the program will be loaded.
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[source,vbs]
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------------------------
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]LIST
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10 PRINT "HELLO"
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20 END
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]RUN
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HELLO
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]
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------------------------
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==== Tape Image File Format
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The format of the tape image file is unique to the Epple ][
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It is stored in a low-level format that represents the waveform that the Apple writes
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to the cassette tape.
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The file is a binary format. Each byte in the file represents the length of one half of one cycle
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(of voltage level variation) written to the tape. The length is in 10-microsecond units.
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For example, a tape image file might have the following binary bytes (in decimal):
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+65 65 65 65 65 20 25 50 50 25 25 25 25 50 50+
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Since each byte represents a 10-microsecond unit, these bytes represent the following
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half-cycle lengths in microseconds:
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+650 650 650 650 650 200 250 500 500 250 250 250 250 500 500+
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The meaning of these half-cycle lengths to the Apple is as follows:
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------------------------
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|-------HEADER------|--sync-|-1-bit-|-0-bit-|-0-bit-|-1-bit-|
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| | | | | | |
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|650 650 650 650 650|200 250|500 500|250 250|250 250|500 500|
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------------------------
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where +HEADER+ is a header section the Apple writes (to skip any
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unrecordable leader section on a real cassette tape); +sync+ is a
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synchronization cycle; and the subsequent cycles are the actual
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bits of data saved on the tape. A 500-microsecond cycle (which
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is stored in the file as two 250 microsecond half-cycles)
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represents a *zero* bit, and a 1-millisecond cycle (which is
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stored in the file as two 500 microsecond half-cycles)
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represents a *one* bit.
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