CPU I/O Registers - Plus/4 Encyclopedia



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CPU I/O Registers

Category
Programming/Assembly

Topic

Address	B7	B6	B5	B4	B3	B2	B1	B0	Description
$0000	0	0	0	0	1	1	1	1	Data direction register (PDIR)
$0001	1*	1*	0	0*	1	0	0	0	7 bit I/O port (PORT)

The port data direction register (PDIR: $0000) is set to $00 when RESET is activated. (Configured for all port bit inputs.) However, the KERNAL RESET routine sets it to $0F and never changes it afterwards. (B7, B6, B4 remain inputs, B3, B2, B1, B0 will be outputs.)

7 bit I/O port (PORT: $0001):

B7 / $80 : serial DATA in
B6 / $40 : serial CLOCK in
B5 / $20 : not implemented (always 0)
B4 / $10 : datasette read in
B3 / $08 : datasette motor out (0 = on, 1 = off)
B2 / $04 : serial ATN out
B1 / $02 : serial CLOCK out / cassette write
B0 / $01 : serial DATA out

Notes:

The values of the input bits also depend on the peripherals; the values marked with * are those that are readed in the "idle" state.
B5 port bit is not implemented! When reading, both the PDIR and PORT registers are guaranteed to have B5 = 0; when writing, the value set for the bit does not matter.
B4 is the datasette read signal. When the datasette motor is turned off, this bit usually reads 0. Warning: this port bit is only connected to the datasette! If no datasette is connected to the machine, this input is "floating." Normally, 0 is readed here, but this may change if the serial port is active!
B1 is the serial bus CLOCK line control, and it is also the write signal to the datasette.
B2/B1/B0 are used to control the serial port. These signals drive the serial port lines through an inverter. Therefore, a bit set to 1 switches the given line to low (0)! The current status of the serial lines can be read on B7/B6. (If the bit is 1, the given line is high.)
The B3/B2/B1/B0 bits are only useful as outputs; there is no point in switching them to inputs. (The status of serial lines cannot be read when switched to input for these bits; therefore, the status of the ATN line cannot be read back!)