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Optron #35
25 апреля 2000 |
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Likbez - Interrupts ZX Spectrum.
Assembler - a view from afar
Continued.
Beginning at | | 20, 21, 24, 25, 28-30, 32, 33
{} Infarh, 2000
Hello enthusiasts! That, probably bored on vacation? Do not
worry, we will continue.
At this time we will talk about paper
with preemption. To begin to understand that this
is. So ...
Interrupt
You might not suspect that 50 times
a second processor is diverted from the processing of your
programs and deals not know what ... For example, interrogates
the keyboard, changing the system variable FRAMES etc. And it
does so, what do you and do not guess. In these
moments of the following happens: the processor
refers to podprograme at 56, as
if executes the command RST 56 or
CALL 56. The difference is that the transition is carried out
not by software and hardware. After completing the execution of
called procedure, processor continues to perform basic
program.
At first glance it might seem that
from such "chips" prosecutors no, because
software in the ROM will not change anything. K
Fortunately, the programmer has the ability to change the
address at which the procedure is an interrupt. For this
purpose the previously mentioned case I.
First of all, we must remember that there are three modes of
treatment interruptions. They are denoted by numbers 0
to 2. Standard mode - 1. It was his
exploit the operating sistama. Zero mode we do not particularly
interesting: the practice, it is no different from the first.
Note that it is in practice! Differences, of course, is, yes
only in the Spectrum they are not implemented. Thus, the most
interesting for us is the mode 2.
To begin with a few words about how it works and that at this
point in the computer going. When it comes to an interrupt
signal, processor to start with, determines the address
pointer to the procedure for processing interrupts. Aforesaid
consists of two bytes: the younger, read from the data bus
(also called the "interrupt vector"), and older, the readout
from the register I. The value obtained placed on the address
bus, and her previous contents are stored on the stack. All of
this, essentially a call option
subroutine.
"But what does this term - the" vector
Interrupt? " - You ask. The fact is
that this term is associated with the operation of computers,
which allow for receipt of several interrupt request
SIMULTANEOUSLY. Naturally, in this situation it is necessary to
determine which of interrupt has priority. And it means
that an interrupt request should be considered as "vector",
which, like any decent vector harakterizetsya at least two
values (in this case - Source and priority). When you receive
the interrupt request is processed or ignored, depending on the
precedence over the currently executing interrupt.
Well, for the Spectrum, even if it is used only one
interrupt, the term This, nevertheless, retained. That's all
long.
As a rule, the value of interrupt vectors
y Speccy is 255, so to determine the address pointer is enough
content I register multiplied by 256 and add
255. So, try to imagine the actions that must be produced to
use their own procedures for handling
Interrupt:
1) Disable interrupts. This is necessary
avoid starting a new procedure for interruption during the
current that is fraught with ...
2) Record at the address calculated in advance, a pointer to
the procedure for processing interrupts, and the address.
3) Set in the I register high byte of the address pointer to
the processing procedure.
4) Install a second mode of processing
interrupts.
5) re-enable interrupts.
Naturally, the process interrupt at this point should already
be in memory.
Here is a brief and all. Now - a few words about
means of implementation. More precisely, about the commands.
So:
DI interrupt prohibition
EI Interrupt Enable
IM 0
IM 1
IM 2 Installation of one of three modes
Interrupt
Now you have both teams. However, before setting its
procedures for processing think about whether it should be
switched off and return control to the standard procedure. To
disable as necessary:
1) Disable interrupts.
2) Restore the value of I, recorded in
him 63.
3) Assign a team of IM 1, the first mode
interrupts.
4) Enable interrupts.
But that's not all. It must be remembered
that some external devices may change the value of interrupt
vectors. Except Furthermore, if your Speccy collected on their
knees "Crooked pens" on splodgy beer circuit interrupt vector
can skip most unpredictable ways. Naturally, in
this case, the probability of a procedure call
treatment equal to 1 / 256. To avoid
glitches and headaches programmers have resorted to the
following trick: instead of writing two bytes at a specified
address table is built of at least 257 bytes in such a way that
in any case reads one address. Naturally, for
that all elements of the table must be
identical.
The most successful value to populate this table is 255. Do
not be alarmed that the address of the procedure will be equal
# FFFF. This one byte is just enough to accommodate the code 24
(the team JR). For it is followed by address # 0000, on which a
ROM is the value # F3. Thus
obtained by JR 65524. At this address is
You can add Komad JP with the arbitrary address of the
transition.
Consider the example of setting its procedures for handling
interrupts:
LD A, 24; command code JR
LD (65535), A
LD A, 195; instruction code JP
LD (65524), A
LD HL, ISR; address of the handler
LD (65525), HL
LD HL, # FE00
LD DE, # FE01
LD BC, # 0100
LD (HL), # FF
LD A, H
LDIR
DI
LD I, A
IM 2
EI
So we hooked up the procedure for processing
interrupts. We now consider the inverse
process, namely a return to the regime of IM 1.
DI
LD A, 63
LD I, A
IM 1
EI
And finally - an example. Suppose you want to play music
while working your program. If you use a melody, compiled in
one of the music editors and equipped with a standard player,
you have problems will not arise. To play every need 1 / 50
sekudny cause the player, and hence frequency of interruptions
you perfect. Before you start playing, you must initialize the
player, making CALL for sotvetstvuyuschemu address. Address is
initialized, as well as playing, depends on the editor and
Conditions compilation. However, it often happens that
initialization is situated in the very beginning of a melody
and playing music for a further six bytes further. If we assume
that the melody compiled under the address # C000, then the
initialization will be located at # C000, and play the notes -
at # C006.
Now we will correct our examples. In
procedure for installing a second interrupt mode, add CALL #
C000 for initialization. This should be done and the procedure
restore the previous interrupt mode. For what? So after you
have turned off the music, AY did not continue to pull the last
note. Initialization enables you quickly "to shut the throat
'dynamics.
And now posmotrm on procedure itself:
ISR DI
PUSH AF; save all registers
PUSH BC; for a normal return
PUSH DE; the main program
PUSH HL
CALL # C006; call the Player
POP HL; restore
POP DE; stored values
POP BC
POP AF
EI; permission to interrupt
RET; & Returns
If you did everything correctly and determine the address of
the initialization and playback, music should resound.
We mention one more team to facilitate
Life - HALT. When the processor to nabredaet
her, he pauses and waits
interrupt signal. After his admission
control is passed to the procedure for processing
interrupts. And after it is completed basic
program continues its work with the team following the HALT.
This allows the use of HALT to synchronize your program
interrupts.
Well, everything seems so far with the theory. Left
only, as usual, to present something about
flags and options for commands. Take a look:
Mnemonic Flags
operation
C Z P / V S N H
DI. . . . . .
EI. . . . . .
IM 0. . . . . .
IM 1. . . . . .
IM 2. . . . . .
LD A, I. x x x 0 0
LD I, A. . . . . .
HALT. . . . . .
That's all the flags. Now you can safely
ready to voice their programs.
Do not forget just what some players
have the habit to allow interrupts. Also
immediately after the transition to the procedure for
processing interrupts HALT interrupt are forbidden.
To be continued ...
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