66 lines
2.3 KiB
Plaintext
66 lines
2.3 KiB
Plaintext
From: http://support.apple.com/kb/TA40737
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For reading data, the cassette recorder uses a more complicated input circuit
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consisting of a 741 operational amplifier configured as a zero crossing
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detector. Zero crossing detection means that whenever the voltage at the input
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jack goes from positive to negative (or negative to positive) the output of
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the amplifier switches from a 1 to a 0 (or 0 to 1). The detector is accessed
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by any read to address $C060. The sign bit (most significant bit) of the byte
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read reflects the detector status. The read routines continually EXCLUSIVE
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ORs this bit with the value most recently read to detect a change in state.
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The amount of time required to change state indicates the incoming frequency
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which then is used to determine if a one or a zero has been received. After
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detecting the first zero crossing at the start of the header, the read routine
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uses HEADR to generate a 3.5 delay, and then the read routine waits for the
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sync bit. After HEADR generates the synchronous bit, the read routine reads
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the data and puts it in the specified memory range.
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In using the cassette interface to either read or write, all you need do is
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specify an address range and execute the read or write subroutine. The
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address range is stored in four bytes, two for the first address to be saved
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and two for the last to be saved. In both cases the least significant byte is
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first.
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Commanding the cassette interface:
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1. from the monitor:
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If the start is $800 and the end is $9FF, then
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800.9FFW will write the data to the cassette and
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800.9FFR will retrieve it.
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2. from machine language:
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Again, if the start is $800 and the end is $9FF then store the address
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range,
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LDA #$00
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STA $3C starting address low
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LDA #$08
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STA $3D starting address high
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LDA #$FF
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STA $3E ending address low
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LDA #$09
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STA $3F ending address high
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JSR $FEDC write to block to tape
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The JSR $FEDC will write to the cassette; JSR $FEFD will read from the
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cassette.
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3. from BASIC:
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First set up the address range. If S = the start and E = the end then from
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integer BASIC,
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POKE 60,S MOD 256
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POKE 61,S / 256
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POKE 62,E MOD 256
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POKE 63,E / 256
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4. from APPLESOFT,
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POKE 60,S - INT(S / 256) * 256
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POKE 61,S / 256
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POKE 62,E - INT(E / 256) * 256
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POKE 63,E / 256
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Then, to write out to cassette, use CALL -307; to read in from the cassette,
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use CALL -259.
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