Part 23
IN and OUT

Subjects covered...


The processor can read from (ROM and RAM) and write to (RAM) memory by
using PEEK and POKE. The processor itself does not really care whether
memory is ROM or RAM - it just thinks there are 65536 memory
addresses, and it can read a byte from each one (even if it's
nonsense), and write a byte to each one (even if it gets lost). In a
completely analogous way, there are 65536 of what are called I/O ports
(standing for input/output ports). These are used by the processor for
communicating with things like the keyboard or the printer, and also
for controlling the extra memory and the sound chip. Some of them can
be safely controlled from BASIC by using the IN function and the OUT
command, but there are locations to which you must not write from
BASIC as you will probably cause the system to crash, losing any
program and data.

IN is a function like PEEK. Its form is...

	IN address

It has one argument - the port address, and its result is a byte read
from that port.

OUT is a statement like POKE. Its form is...

	OUT address,value

...which writes the given value to the port with the given address.
How the address is interpreted depends very much upon the rest of the
computer. Quite often, many different addresses will mean the same. On
the +3 it is most sensible to imagine the address being written in
binary, because the individual bits (each of which can have the value
either 0 or 1) tend to work independently. There are 16 bits, which we
shall refer to (using A for address) as...

	A15, A14, A13, A12, A11, A10, A9, A8,
	A7, A6, A5, A4, A3, A2, A1, A0

Here, A0 is the 1s bit, A1 is the 2s bit, A2 is the 4s bit, and so on.
Bits A0, A1, A2, A3 and A4 are the important ones. They are normally
1, but if any one of them is 0, then this tells the computer to do
something specific. The computer cannot cope with more than one thing
at a time, so no more than one of these five bits should 0. Bits A6
and A7 are ignored, so if you are a wizard with electronics you can
use them yourself. The best addresses to use are those that are 1 less
than a multiple of 32, so that A0 to A4 are all 1. Bits A8, A9, and so
on are sometimes used to give extra information, but mostly for the
extra memory and sound.

The byte being written or read has 8 bits, and these are often
referred to (using D for data) as...

	D7, D6, D5, D4, D3, D2, D1, D0

Here follows a list of the port addresses used...

There is a set of input addresses that read the keyboard and the tape

The keyboard is divided up into 8 half-rows of 5 keys each, viz:

	IN 65278 (FEFEh)	reads the half-row CAPS SHIFT to V
	IN 65022 (FDFEh)	reads the half-row A to G
	IN 64510 (FBFEh)	reads the half-row Q to T
	IN 63486 (F7FEh)	reads the half-row 1 to 5 (and JOYSTICK 1)
	IN 61438 (EFFEh)	reads the half-row 6 to 0 (and JOYSTICK 2)
	IN 57342 (DFFEh)	reads the half-row P to Y
	IN 49150 (BFFEh)	reads the half-row ENTER to H
	IN 32766 (7FFEh)	reads the half-row (space) to B

(These addresses are 254+256x(255-2^n) as n goes from 0 to 7.)

Remember that digits followed by h signify hexadecimal numbers. If you
don't understand hexadecimal numbers, refer to part 32 of this

In the byte read in, bits D0 to D4 stand for the five keys in the
given half-row. D0 is for the outside key, and D4 is for the one
nearest the middle. The bit is 0 if the key is pressed, 1 if it is
not. D6 is set by the tape interface, and is effectively random if no
tape data is present.

For JOYSTICK 1, bit 0 is fire, bit 1 is up, bit 2 is down, bit 3 is
right and bit 4 is left. For JOYSTICK 2, bit 0 is left, bit 1 is
right, bit 2 is down, bit 3 is up and bit 4 is fire. From BASIC, these
read as the number keys.

Port address 00FEh (254 decimal) in output drives the sound (D4) and
the save signal to the tape interface (D3), and also sets the border
colour (D2, D1 and D0).

Port addresses 00FEh (254), 00F7h (247) and 00EFh (239) are reserved.

Port address 7FFDh (32765) drives the extra memory. Executing an OUT
to this port from BASIC will nearly always cause the computer to
crash, losing any program and data. There is a fuller description of
this port in part 24 of this chapter (under the heading `Memory
management'). This port is write only, i.e. you cannot determine the
current state of the paging by an IN instruction. This is why the
BANKM system variable is always kept up to date with the last value
output to this port.

Port address BFFDh (49149) drives the sound chip's data registers.
Port address FFFDh (65533) in output writes a register address, and in
input reads a register. Judicious use of these two registers can allow
sounds to be generated whilst BASIC gets on with something else, but
you should be aware that they also control RS232/MIDI and AUX

Port address 0FFDh (4093) is used for the parallel (Centronics)
interface (i.e. PRINTER). When read using an IN instruction bit 0
shows the state of the BUSY signal produced by the printer. If the
printer is off line or non-existent, then this bit will be 1. When
this port is written to using OUT, it acts as the parallel port data
register. In order to print a character it is necessary to wait until
BUSY is 0, write the character code to port 0FFDh (4093), and finally,
take the STROBE bit in port 1FFDh (8189) low than back high again.

Port address 1FFDh (8189) controls several aspects of the +3. Amongst
other things, this port controls the ROM that is switched into the
memory area from 0000h...3FFFh (0...16383). As the port is write only,
the +3 BASIC maintains a variable, BANK678, that holds the value last
output to this port. It is therefore very unwise to OUT values
directly to this port without first checking on the current state
(which holds its current state in BANKM). The bottom three bits (0..2)
of this port (1FFDh) are used to switch RAM/ROM - further details can
be found in part 24 of this chapter (under the heading `Memory
management'). Bit 3 controls the disk motor (0 is off, 1 is on),
though it should not be necessary to control the motor by writing to
this port as there are +3DOS routines that will achieve the desired
effect. Bit 4 is the parallel port STROBE which is active low - this
means that to print the character that has been output to port 0FFDh
(4093), the STROBE bit should be brought low and then returned to its
normally high state.

Port address 2FFDh (12285) can be used to read the disk controller
(uPD765A) chip's main status register. This is unlikely to be very
useful without an in-depth knowledge of how the chip operates.

Port address 3FFDh (16381) is the disk controller's data register.
This can be both read from and written to, but once again it is
unlikely to be useful to the BASIC programmer. Random OUTputting to
this port will probably confuse the poor disk controller chip to such
an extent that subsequent disk operations (like LOAD and SAVE) will be
unreliable. It is entirely possible that ill-informed experiments will
corrupt your disks and lose your data - you have been warned!

Run this program to see how the keyboard works...

	10 FOR n=0 TO 7: REM half-row number
	20 LET a=254+256*(255-2^n)
	30 PRINT AT 20,0; IN a: GO TO 30

...and play around by pressing keys (start with the half-row from CAPS
SHIFT to V). When you are finished with each half-row, press BREAK and
then type...


The control, data and address busses are all exposed at the back of
the +3 on the EXPANSION I/O socket. This means that you can do almost
anything with a +3 that you could with a raw Z80 chip (although
sometimes, the computer's internal workings may get in the way).

See chapter 10 for a diagram and pin-out of the EXPANSION I/O socket.

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