z80
[Continued].
(C) WLODEK BLACK
00Ch. Special teams.
NOP (No Operation) - the command "no operation". No-op,
on which no action is performed. Command Code - 00.
Command is executed in 4 takta.Primenyaetsya when debugging
programs (Especially foreign) to tighten the real-time programs
time, for "nailing" of obsolete commands (example -
obessmerchivanie games).
HALT (stop) - stop the processor. Executing
HALT, the CPU stops. But the tire does not turn off!
The fact of execution of a stop command tells the processor to
other devices exhibiting a special signal "HALT" on the 18-m
output of its chips. HALT signal can be used by other devices
to permit or encourage any actions that may not be used at all.
At the MSX computer and "Spectrum" command HALT can be used
freely. Emerge from stopping the processor can only interrupt
(Read more about interruption - on) or reset (RESET). HALT
command is normally used to organize long-term cycles of
passive waiting, synchronizing interrupt (in the MSX and
Spectrum - with a refresh rate of 50 Hz).
00Dh. INTERRUPTION.
Russian title of "termination" and his original
"interruption" absolutely accurately determine the phenomenon
occurring in the microprocessor system after the 16 th leg of
the processor comes a short pulse of the zero logic level,
called interrupt request signal.
Interrupt request signal is denoted by "INT".
We assume that interrupts are enabled (you can disable the
processor to respond to the signal INT with the help of special
teams)
The presence of INT signal processor checks after each
command. Finding this signal, the processor instead of sample
of memory and executing the following command goes to service
interruption. Cycle entry in the interrupt service consists of
the following steps: 1. The processor acknowledges receipt of
the interrupt request. I Z80 for this special pin output is
not, so it generates interrupt acknowledge signal in a logical
combination of signals IORQ = 0, RD = 1, WR = 1, which clearly
regarded not as appealing to the port. Decrypt a combination of
signals device interrupt requests immediately removes the
interrupt request signal. Generate interrupt acknowledge signal
(INTA), plus provide priority service multiple devices capable
of simultaneous interrupt requests can special device - the
interrupt controller. But we will not go into the details - in
the Spectrum and MSX own chip priority interrupt controller is
not used. 2. Prohibited from receiving further requests to
interrupt, continue to resolve with the help of a special
command in the program. 3. In the stack send mail command,
before which there was an interruption. 4. The transition is
made to the interrupt service routine, address inputs which can
be defined by three fundamentally different ways (3 types of
interrupts).
As can be seen, interrupt actually interrupts the execution
of user programs, and make it can at any time and anywhere in
the main program. From this directly implies the need to ensure
a correct return from interrupt, so that the main program is
not sensed interrupts. Point (e) Return the CPU itself sends a
stack, but the preservation of registers should take care of
the programmer. Programmer to know the contents of interrupt
service routine, it knows which registers using this procedure.
Registers whose contents are changed when the interrupt service
routines must be saved in the stack: Entry point into the
service of interrupts: -> PUSH AF
PUSH HL
PUSH DE
PUSH BC
. . .
. . .
POP BC
POP DE
POP HL
POP AF
EI; sure you want to enable interrupts
before returning
RET; return to the main program
The system interrupt routines, which is in ROM, of course
correctly saves and restores information in the registers.
Why do we need an interrupt.
There are a number of events are rare, but
microprocessor system which must respond immediately. There is
also a need to regularly perform some actions that are linked
to real time. Enumerate those and others can be up to infinity,
so here we will only on what specifically do interrupt the
computer MSX.
So, with a frequency of 50 Hz:
polled keyboard - codes typed characters are placed in
buffer, fixed special cases (Ctrl + Stop, etc.) is
countdown timer, moving all the data for background
music (if the queue was specified) decrements the interval
for Beysikovogo operator "ON INTERVAL GOTO".
In the Spectrum interrupt polled keyboard and made the
countdown, keyboard input buffer in the Spectrum not, so the
code is only recorded one last key, keys.
On the usefulness of the mechanism of the interrupt can be
judged by the following rough life example: Suppose you are
sitting the computer and typing program, and the kitchen is
Time is heated kettle. If the kettle has a whistle, you have to
periodically run the kitchen and check to see if boiled He (the
survey states kettle). In the presence of a whistle, you can
not worry - the maker himself will give a signal (interrupt
request on kettle). But if this is also the phone rings, you
will have to decide what event requires a response priority
(priority interrupts).
[To be continued].
