- Memory map taken from the book Mapping The Atari - Revised Edition (article Memory Map)
System timer memory address map
|18, 19, 20
($12, $13, $14)
Internal realtime clock. Location 20 increments every stage one
VBLANK interrupt (1/60 second = one jiffy) until it reaches 255
($FF); then location 19 is incremented by one and 20 is reset to
zero (every 4.27 seconds). When location 19 reaches 255, it and
20 are reset to zero and location 18 is incremented by one (every
18.2 minutes or 65536 TV frames). To use these locations as a
timer of seconds, try:
TIME = INT((PEEK(18) * 65536 + PEEK(19) * 256 + PEEK(20) )/60)
To see the count in jiffies, eliminate the "/60" at the end. To see
the count in minutes, change "/60" to "/360." The maximum
value of the RT clock is 16,777,215. When it reaches this value, it
will be reset to zero on the next VBLANK increment. This value is
the result of cubing 256 (i.e., 256 * 256 * 256), the maximum
number of increments in each clock register. The RT clock is
always updated every VBLANK regardless of the time-critical
nature of the code being processed.
Attract mode timer and flag. Attract mode rotates colors on your
screen at low luminance levels when the computer is on but no
keyboard input is read for a long time (seven to nine minutes).
This helps to save your TV screen from "burn-out" damage suf-
fered from being left on and not used. It is set to zero by IRQ
whenever a key is pressed, otherwise incremented every four
seconds by VBLANK (see locations 18 - 20; $12 - $14). When the
value in ATRACT reaches 127 ($7F), it is then set to 254 ($FE) un-
til attract mode is terminated. This sets the flag to reduce the
luminance and rotate the colors when the Atari is sitting idle.
POKE with 128 ($80) to see this effect immediately: it normally
takes seven to nine minutes to enable the attract mode. The OS
cannot "attract" color generated by DLI's, although your DLI
routine can, at a loss of time.
Joysticks alone will not reset location 77 to zero. You will have to
add a POKE 77,0 to your program periodically or frequently call
in a subroutine to prevent the Atari from entering attract mode if
you are not using any keyboard input.
POKEY timer one interrupt vector, initialized to 59314 ($E7B2),
which is a PLA, RTI instruction sequence. Timer interrupts are
established when the POKEY timer AUDF1 (53760; $D200)
counts down to zero. Values in the AUDF registers are loaded
into STIMER at 53769 ($D209). IRQ.
POKEY timer two vector for AUDF2 (53762, $D202), initialized to
59314 ($E7B2). IRQ.
POKEY timer four vector for AUDF4 (53766, $D206), initialized
to 59314 ($E7B2). This IRQ is only vectored in the "B" version of
the OS ROMs.
System timer one value. Counts backwards from 255. This SIO
timer is decremented every stage one VBLANK. When it reaches
zero, it sets a flag to jump (JSR) through the address stored in
locations 550, 551 ($226, $227). Only the realtime clock
(locations 18-20; $12-14), timer one, and the attract mode
register (77; $4D) are updated when the VBLANK routine is cut
short because time-critical code (location 66; $42 set to non-zero
for critical code) is executed by the OS. Since the OS uses timer
one for its I/O routines and for timing serial bus operations
(setting it to different values for timeout routines), you should use
another timer to avoid conflicts or interference with the operation
of the system.
System timer two. Decremented at the stage two VBLANK. Can
be decremented every stage one VBLANK, subject to critical
section test as defined by setting of CRITIC flag (location 66;
$42). This timer may miss (skip) a count when time-critical code
(CRITIC equals non-zero) is being executed. It performs a JSR
through location 552, 553 ($228, $229) when the value counts
down to zero.
System timer three. Same as 538. Timers three, four, and five are
stopped when the OS sets the CRITIC flag to non-zero as well.
The OS uses timer three to OPEN the cassette recorder and to set
the length of time to read and write tape headers. Any prior value
in the register during this function will be lost.
||System timer four. Same as 538 ($21A)
System timer five. Same as 538 ($21A). Timers three, four, and
five all set flags at 554, 556 and 558 ($22A, $22C, $22E),
respectively, when they decrement to zero.
System timer one jump address, initialized to 60400 ($EBF0).
When locations 536, 537 ($218, $219) reach (count down to) zero,
the OS vectors through here (jumps to the location specified by
these two addresses). You can set your machine code routine
address here for execution when timer one reaches (counts down
to) zero. Your code should end with the RTS instruction.
Problems may occur when timer values are set greater than 255,
since the 6502 cannot manipulate 16-bit values directly (a
number in the range of zero to 255 is an eight-bit value; if a value
requires two bytes to store, such as a memory location, it is a
16-bit value). Technically, a VBLANK interrupt could occur
when one timer byte is being initialized and the other not yet set.
To avoid this, keep timer values less than 255. See the Atari OS
User's Manual, page 106, for details.
Since the OS uses timer one, it is recommended that you use
timer two instead, to avoid conflicts with the operation of the
Atari. Initialized to 60396 ($EBEA) in the old ROMs, 60400
($EBF0) in the new ROMs. NMI
System timer two jump address. Not used by the OS, available to
user to enter the address of his or her own routine to JMP to when
the timer two (538, 539; $21A, $21B) count reaches zero.
Initialized to zero; the address must be user specified. NMI
System timer three flag, set when location 540, 541 ($21C, $21D)
reaches zero. This register is also used by DOS as a timeout flag.
Software repeat timer, controlled by the IRQ device routine. It
establishes the initial 1/2 second delay before a key will repeat.
Stage two VBLANK establishes the 1/10 second repeat rate,
decrements the timer and implements the auto repeat logic.
Every time a key is pressed, STIMER is set to 48 ($30). Whenever
SRTIMR is equal to zero and a key is being continuously pressed,
the value of that key is continually stored in CH, location 764
System timer four flag. Set when location 542, 543 ($21E, $21F) counts down to zero.
System timer five flag. Set when location 558, 559 ($22E, $22F) counts down to zero.
Start the POKEY timers (the AUDF registers). You
POKE any non-zero value here to load and start the timers; the
value isn't itself used in the calculations. This resets all of the audio
frequency dividers to their AUDF values. If enabled by IRQEN
below, these AUDF registers generate timer interrupts when they
count down from the number you POKEd there to zero. The
vectors for the AUDF1, AUDF2 and AUDF4 timer interrupts are
located between 528 and 533 ($210 and $215). POKEY timer four
interrupt is only enabled in the new "B" OS ROMs.
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