Optron #32 22 октября 1999

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Likbez - Assembler view from afar: Working with external devices. Reading data from port. Write commands to the port.

       Assembler - a view from afar


               Continued.

    Beginning at | | 20, 21, 24, 25, 28-30

{} Infarh, 1999



      Working with external devices


  These include the keyboard, tape recorder, a printer, a lot 
more then (depending on specific option). So how does a 
processor exchanges information with them? Obviously, they are 
not incorporated directly a data bus (SM). Between the external

unit (VU) and the processor is a
thing as a "port".

  Do not think that I'm talking about the sea
port to tell you I will! Actually
"Port" - a device that the command processor connects to a 
certain SM BY. Which - depending on its address. For example, 
the port has an address kempston joystick'a # 1F, beeper - # 
FE, etc. 

  Theoretically, the number of ports can
be 65536, but in practice this number is much smaller and 
depends on the hardware implementation of your Speccy. It is 
important method of decoding the port, ie, definition of what 
UW currently has be connected to the SM. For example, although 
the port joystick has the number # 1F, its decoding

usually occurs only in the category of A5
Address Bus (SHA).

  We consider this process in detail. Sign of the port, the low
signal IORQ. What process happens (read from the port or write 
to it) tell low level signals or RD

WR, respectively. And if the combination of signals per se 
(IORQ = 0; RD = 0) signal A5 Sha has a low level, will be 
selected joystick port. In this case, the values ​​read from 
it, appears in the SM. Record is similar, but uses

signal WR and the number of SM enters the port.

  And here is how to manage all these signals without the aid 
of a soldering iron and plenty of beer? To this end, the 
processor has special teams that I have now demonstrate.



          Reading data from port


  Certainly in BASIC you met the team IN. So, in the assembly 
have exactly the is the same. It is present in these ways:



  IN A, (n)

  IN r, (C)


  The first version of the command reads
from the port, low byte address is written in parentheses, and 
the senior - in the battery. Bytes read from the port will be 
placed in the "A". In order to help this command to read the 
state kempston joystick'a, we need to write it as follows:



                IN A, (# 1F)


  As I said earlier, decoding port # 1F
occurs, usually only one discharge IRA, so the contents of 
register "A" can be anything. Show off to all,

that your computer has 256 ports
joystick!

  Now turn to the second option.
As receiver for the read bytes
is "r", which in practice may be
one of the registers:


           A, B, C, D, E, H, L


  Port address is placed in the register pair BC. This option 
is most useful if you should use the full addressing

port.

  For example, consider reading the keyboard.
Keyboard port addresses differ only in
significant byte, and the younger all the time is
# FE. What poluryad currently being read, seen from the panel:



             Poluryad Address

                        port


            Space ... B 7FFE

            Enter ... H BFFE

            P. ...... Y DFFE

            0 ....... 6 EFFE

            1 ....... 5 F7FE

            Q. ...... T FBFE

            A. ...... G FDFE

            CS ...... V FEFE



  One of the features read from the port
the keyboard is that the lower bits
obtained values ​​refer to the extreme
poluryada keys. Narimer, if we make something like


  LD BC, # EFFE

  IN A, (C)

then the resulting byte will have the following
value:



  x x x 6 7 8 9 0 Key


  D7 D6 D5 D4 D3 D2 D1 D0 Bit



  Bits D7, D6, D5 are lucky - they had
the honor to remain unused. The fact
that some key is pressed, we learn
as the corresponding bit - he
will be cleared. We extend the above
example:


  LD BC, # EFFE; poluryad 6 ... 0

  IN A, (C); reading status

  RRCA; check

  JR NC, ZERO; pressed "0"
; Continue to work

       .....

       .....
ZERO


  That's how proshodit work with the keyboard. Does not that 
all that simple? 

  Now let's consider the reverse process -


          Write command to port


  They are very similar to a read command.
Here's a look:


  OUT (n), A

  OUT (C), r


  Entry to the port is similar to reading, to
At least, that relates to the transfer parameters.

  For example, consider the entry to the port # FE.
Is charged with not one problem, and immediately
several: lower three bits determine
Border color, bit D3 sends a pulse on the
MIC jack and the EAR, and bit D4 includes or
off the beeper.

  So, let's say you have to install red curb. To do this,
the following:


  LD A, 2

  OUT (# FE), A


  A few words about the bit D4. Whenever
you translate it into its opposite state (of course, sending 
the result port), the internal speaker produces a short

-clicking. Thus, by manipulating these
bit at a certain speed, you can
create simple sounds.


  So, today everything seems to be ...

  What? Ah, yes, I completely forgot about the impact on
Flags! Well, let's look at what we do
here ...



        Mnemonic Flags

        operation

                    C Z P / V S N H


        OUT (n), A. . . . . .

        OUT (C), r. . . . . .


        IN A, (n). . . . . .

        IN r, (C). x P x 0 x



  As you see, everything here is quite simple.
Well, now is really everything.

  So, until next time!


          To be continued ...