TweetFollow Us on Twitter

VBL with Mach1
Volume Number:2
Issue Number:6
Column Tag:Threaded Code

Install a VBL Task with Mach1

By Jörg Langowski, EMBL, c/o I.L.L., Grenoble, Cedex, France, Editorial Board

Installing a Forth VBL Task - another CRT saver

Bob Denny's Article on vertical blanking tasks (MT V1#9) has been a constant challenge to me ever since it appeared. It seemed that there are certain things that one just could not do in Forth - the installation of an independently running task in the system heap being one of them. After all, where do you put the Forth runtime support when you leave Forth and want the task to run?

Another thing that would be difficult to do in Forth, for example, would be a desk accessory. In general, any tasks that will run independently and concurrently in the Mac operating system are almost impossible to handle with a language that needs a runtime interpreter to work.

After Mach1 came in, one could sort of see the light at the end of the tunnel. Here was a Forth that created real 68000 machine language output, and in principle its code could run anywhere without ever needing any runtime package. Maybe this makes some of the 'impossible' things that I mentioned less impossible.

Still, Mach1 does contain runtime support. Multitasking is the most important one, but by no means all, even the LOOP of a DO...LOOP structure is compiled as a JSR to a kernel routine.

The first rule for creating programs that are not only 'stand-alone applications', but self-contained code, is to avoid all references to kernel routines from within the program. This general statement will hold for any Forth system; given a Forth assembler, enough patience and a good set of macro definitions one will always be able to write code that is self-contained and won't need the runtime interpreter (e.g. using inline macros similar to the ones defined in MT V1#9, Forth column). Mach1 code doesn't contain too many references to its kernel anyway, so it should be much easier to accomplish what we try to do.

Our problem is to create a piece of code that does exactly what Bob Denny's CRT saver does: intercepts the GetNextEvent trap and updates a counter to keep track of when the last non-null event happened, and a VBL task that looks at that counter and blanks the screen whenever a certain time has elapsed after the last non-null event. Furthermore, the GetNextEvent intercept routine will repaint the screen after it has been blanked when a new non-null event occurs.

For the purpose of illustration, Listing 1 shows some Mach1 code that installs a very similar screen blanker as a background task under Mach1. This is the simple way to do it and will of course run only within the Mach1 system.

Short explanation of the program: the blankout routine opens a new GrafPort and paints its portRect (default size: whole screen) with the default pattern (black).

This routine is installed into one background task, CRTsaver, that checks continuously whether the time after the last 'relevant' event is larger than a preset value and blanks the screen if it is; thereafter, it unblanks the screen when a new event is received. The task's infinite loop contains a PAUSE to give control back to the scheduler.

The second background task, eventmonitor, checks for events by calling EventAvail. Whenever a 'relevant' event occurs, it is intercepted by this routine and the event tracking counter last.action updated.

Try and install these routines on your Mach1 system, and you'll have a fine CRT saver. Note that the multitasking system is not running while the screen is dark; I did this to keep it completely blank. You can insert a PAUSE in the event waiting loop of do.blank; in that case, all tasks will keep running, but you'll have a small white rectangle on the black screen (the cursor which is still active).

Installing the CRT saver as an independent VBL task

The example in Listing 1 contains many things that will just not work independently of the Mach1 runtime package:

- background tasks;

- variable definitions, which are kept separate from the main code block;

- JSR references to the kernel are not contained here, but could be easily in other tasks (as mentioned, a simple DO...LOOP);

- there are certain trap calls that are incompatible with the VBL task mechanism; as mentioned in IM, trap routines that move, purge or reallocate memory may not be called from a VBL task, the reason being that if the task happens to interrupt the memory manager and then calls the memory manager itself, very strange effects may result.

The list of 'forbidden' routines (IM, Addison-Wesley Vol. III in the appendix) is impressive, and EventAvail is one of them. The Quickdraw calls in blankout are also forbidden.

-The Mach1 data stack is maintained through the A6 register. In self-contained code, this register will have to be set up to point to a local stack area when the code is first entered. Also, for safety reasons the complete register file should be saved on entering and restored on exit.

An impressive list of restrictions; however, the example in Listing 2 shows that it is not that bad after all.

First, we may not use variables anymore. Any variable storage space should be defined within our piece of code. This can be done using create or header. References to variables thus defined must be through ['] (in colon definitions) or ' (in direct execution). If a create variable is referenced by name in a colon definition, a JSR to its code is compiled. create's own code again references the kernel and therefore cannot be used for self-contained programs.

If you use variables in this way, you can't call them directly by name after storing something there, because the create execution code will be overwritten. You'll always have to 'tick' their addresses on the stack.

Second, the screen blanker has to be rewritten; we store the black pattern into the screen area directly instead of calling Quickdraw routines, which may not be used. This is a little slower (Quickdraw really deserves its name), making the blanking less 'instantaneous'. Also, we may not use a DO...LOOP (reference to the kernel), so the blanking loop is implemented using BEGIN...UNTIL, a little more cumbersome, but still readable; and self-contained. See the definition of blankout in Listing 2. Note that HideCursor is not in the list of 'forbidden' traps.

Third, we cannot intercept events from within the VBL task using EventAvail. This is the reason why in Bob Denny's example from V1#9 a GetNextEvent filter procedure was used. We'll have to do the same thing.

The GNEFilter procedure

A short review of the method to install a GetNextEvent hook:

There is an (undocumented) system global at $29A which contains a location that GetNextEvent jumps to right after removing an Event from the queue. Through this hook, one can install a routine that will be called whenever GetNextEvent receives an event. A pointer to the event record is contained in A1. At the end of this routine, of course, one will have to jump to the location that was contained in $29A.

The GNE hook routine that we are going to install will, each time a non-null event is received, update the last.action counter with the current number of system ticks and repaint the screen if it was blank. In order to keep the definition short, we call traps directly instead of going through the Mach1 'glue' mechanism.

The GNEintfc routine saves most of the registers and restores them after exiting. A local stack (100 bytes) is set up for the Mach1 A6 stack pointer. Some inline assembly code is used to move arguments between the A7 and A6 stacks, and to setup a jump vector at the end of the routine.

The vertical blanking task

one.run is the heart of the vertical blanking task that will be the other part of the CRT saver. It checks whether one minute has passed since the last non-null event and blanks the screen in that case. Furthermore, it then sets a global flag, dark, to indicate to GNEintfc that the screen is dark. At the end of each run, screen blanked or not, the task reschedules itself by resetting the counter in its VBL queue element. Registers are saved and restored, and the routine also uses the local stack. (Note: writing this I realize that in the case that the GetNextEvent filter is interrupted by the vertical blanking and one.run is run during that same interrupt, it will use the same local stack. This might create a problem; so far the CRT saver has not crashed on me. You might think of duplicating the stack area so that the two routines use independent stacks.)

Installation of the CRT saver in the system heap

Of course, the code that we defined so far is local to the Mach1 system and will disappear as soon as Mach1 is exited. If we happened to install the CRT saver before, too bad! Most certainly the system won't survive this kind of abuse. Therefore it remains to copy the routines to a safe place in memory; we'll move them to the system heap before installing. The code that we have to copy is marked by the two headers START and END. The word install.blanker gets a pointer to a chunk of system heap (END - START) bytes long and copies the code to it. The offset is placed into the variable (yes, here we may use a good old variable) blockoffset. All references to the copied routines during installation (for initialization of flags and counters, and for the address passed to the VBL queue element) are made through the original addresses offset by this value.

After the installation (install.blanker and install.GNEfilter), you may leave Mach1, wait one minute and see your screen go dark. Any keypress or mouseclick will make reappear whatever was there.

Turnkeying the CRT saver installer

The word CRTsaver installs the tasks and quits Mach1. This word is used for turnkeying the program:

turnkey CRTsaver CRT

will create an application file CRT, 19K long (Mach1 runtime overhead of 16K) which installs the CRT saver.

Listing 1: CRT saver background task to run in the Mach1 system
( CRT saver task, © 1986 JL for MacTutor)
only forth definitions
also assembler also mac

hex
904 constant currentA5
9EE constant grayRgn
16A constant Ticks
 74 constant screenbits
 10 constant portrect

decimal

( first define port structure )
header screenport
     2 allot ( device )
    14 allot ( bitmap )
     8 allot ( portrect   )
    84 allot ( remaining bytes )


( *** now define background task that does the blanking *** )

variable last.action
variable max.ticks
3600 max.ticks !  ( 1 minute in ticks )

header myevents
    2 allot ( code )
    4 allot ( message )
    4 allot ( when )
    4 allot ( where )
    2 allot ( modifiers )

: relevant.action
    138 ( disk + key + mouse ) 
    ['] myevents call EventAvail
;

: redraw
    call drawmenubar
    call frontWindow
    grayRgn @ call paintBehind
    call showcursor
;

: blankout
    call hidecursor
    ['] screenport call openport
    ['] screenport portrect + call paintrect
    BEGIN relevant.action UNTIL
    ticks @ last.action !
    ['] screenport call setport
    redraw 
;

: monitor.events
    activate
    BEGIN
    PAUSE
    relevant.action 
        IF ticks @ last.action ! THEN
    AGAIN
;

: do.blank
    activate
    BEGIN
    PAUSE
    ticks @ last.action @ - max.ticks @ >
        IF blankout THEN
    AGAIN
;

400 1000 background CRTsaver
CRTsaver build
400 1000 background eventmonitor
eventmonitor build

: saver.start
    ticks @ last.action !
    eventmonitor monitor.events
    CRTsaver do.blank
;

Listing 2: CRT saver, written in Forth, for installation into system 
heap and Mach1 - independent execution

( CRT saver task for installation into VBLTask queue, © 1986 JL for MacTutor 
)
only forth definitions
also assembler also mac

hex
29A constant JGNEFilter
824 constant screenbase
904 constant currentA5
9EE constant grayRgn
16A constant Ticks
 74 constant screenbits
 10 constant portrect
FFFFFFFF constant minusone

.TRAP   _drawmenubar    $A937
.TRAP   _frontwindow    $A924
.TRAP   _paintbehind    $A90D
.TRAP   _newptr,sys     $A51E
.TRAP   _showcursor     $A853

CODE save.regs
    MOVE.W  SR,-(A7)
    MOVEM.L A1-A6/D0-D7,-(A7)
    RTS
END-CODE    MACH

CODE restore.regs
    MOVEM.L (A7)+,A1-A6/D0-D7
    MOVE.W  (A7)+,SR
    RTS
END-CODE    MACH

CODE getA1
    MOVE.L  A1,-(A6)
    RTS
END-CODE    MACH

decimal

header START

( *** we need a local stack after the relocation *** )    
 
header local.stack 100 allot

CODE setup.local.stack
    LEA -8(PC),A6 ( stack grows downward from here )
    RTS
END-CODE

( *** define port structure + some global variables *** )

header screenport
     2 allot ( device )
    14 allot ( bitmap )
     8 allot ( portrect   )
    84 allot ( remaining bytes )

header myevents
    2 allot ( code )
    4 allot ( message )
    4 allot ( when )
    4 allot ( where )
    2 allot ( modifiers )

( *** VBL queue element to be installed *** )

header VBLqelem
    4 allot ( qLink )
    2 allot ( qType )
    4 allot ( vblAddr )
    2 allot ( vblCount )
    2 allot ( vblPhase )

 1 constant vType
 4 constant qType
 6 constant vblAddr
10 constant vblCount
12 constant vblPhase
     
( *** GetNextEvent filter proc definitions *** )

header SavedJGNEFilter 4 allot
header dark  4 allot     
header last.action 4 allot
header max.ticks   4 allot
   3600 ' max.ticks !  ( 1 min in ticks )


: GNEIntfc
    save.regs
    setup.local.stack
    
    getA1 w@ ( event received? )
      IF ticks @ ['] last.action !
        ['] dark @
        IF 
            0 ['] dark !
            _drawmenubar
            CLR.L    -(A7)
            _frontwindow 
            grayrgn @
            MOVE.L   (A6)+,-(A7)
            _paintbehind
            _showcursor
        THEN
      THEN

    ['] SavedJGNEFilter @
    MOVE.L  (A6)+,A0
    restore.regs
    JMP     (A0)
;
    

( *** definitions for VBLtask that does the blanking *** )

: blankout 
    call hidecursor
    screenbase @ 21888 + 
    screenbase @
    BEGIN
        minusone over !
        4 + 2dup < 
    UNTIL 2drop 
    1 ['] dark !
;

: one.run
    save.regs
    setup.local.stack
    ticks @ ['] last.action @ - ['] max.ticks @ >
    ['] dark @ 0=
    AND
    IF  blankout  THEN
    60 ['] VBLqelem vblCount + w! ( reschedule )
    restore.regs
;

header END


( *** install CRT blanker task 'one.run' into VBL queue *** )

variable blockoffset

: get.sys.block  
    ['] end ['] start - 
    MOVE.L (A6)+,D0
    _newptr,sys ( get memory block in system heap )
    MOVE.L A0,-(A6)
;
    
: install.blanker   { | pointer offset -- }
    get.sys.block   -> pointer
    pointer IF
        0 ['] dark !
        ticks @ ['] last.action !
        pointer ['] start -  -> offset
        offset blockoffset !
        ['] start pointer ['] end ['] start - cmove
    ( now make all the moves on the relocated block )
        ['] VBLqelem offset +
        dup qtype + vtype swap w!
        dup vblAddr + ['] one.run offset + swap !
        dup vblCount + 60 swap w!
        dup vblPhase + 5 swap w!
        call Vinstall drop
    ELSE ." Not enough system heap for installation." cr
    THEN
;

: install.GNEfilter
    JGNEFilter @ ['] SavedJGNEFilter blockoffset @ + !
    ['] GNEIntfc blockoffset @ + JGNEFilter !
;

: remove.blanker
    ['] VBLqelem blockoffset @ + call Vremove drop
;

: remove.GNEfilter
    ['] SavedJGNEFilter blockoffset @ +  JGNEFilter ! 
;

: CRTsaver 
    install.blanker
    install.GNEfilter
    bye
;
 

Community Search:
MacTech Search:

Software Updates via MacUpdate

How to build a successful civilisation i...
GodFinger 2 grants you godlike powers, leaving you to raise a civilization of followers. In the spirit of games like Black & White, the GodFinger games will see you building bigger and better villages, developing more advanced technology and... | Read more »
How to get all the crabs in Mr Crab 2
Mr. Crab 2 may look like a cutesy platformer for kids, but if you're the kind of person who likes to complete a game 100%, you'll soon realise that it's a tougher than a crustacean's shell. [Read more] | Read more »
How to be a star in Britney Spears: Amer...
If you've ever wanted to be a star, baby, then you've probably already checked out Britney Spears: American Dream and are happily making your way up the charts. But fame doesn't come easy, and everyone needs a helping hand sometimes. So we've got... | Read more »
AppSpy is hiring a part time Staff Write...
| Read more »
How to save lives in ER Surgery Simulato...
A serious earthquake has struck a nearby town in ER Surgery Simulator - Emergency Doctor, and it’s up to you to save the victims. [Read more] | Read more »
Tips and tricks to get a high score in G...
Ketchapp Games loves the endless runner genre. And its newest game, Gravity Switch, is no exception. Gravity Switch takes a fresh approach, though, as you move a block, suspended in zero gravity, safely through a maze of shifting pillars. If the... | Read more »
Tips and tricks to get a high score in S...
Smash Fu is a high-paced tile-tapping game that requires quick reflexes and some practice. You’ll have to smash bricks with the skill of a seasoned black belt to get a high score. To raise the stakes a bit, you’ll also have to avoid tapping any... | Read more »
How to keep the ball rolling in Dropple
If you're new to the minimalist puzzler Dropple, you may find yourself struggling to make it beyond the first couple of steps before your ball falls into the endless abyss below. [Read more] | Read more »
Game Craft releases new Legend of War ti...
Set for release at the end of this month, real time strategy title Legend of War seems sure to delight with a veritable feast of sweet features to get stuck into. Developed by Game Craft, the game is due for release through both the App Store and... | Read more »
How not to die in Traffic Rider
Traffic Rider, an Out Run-esque game in which your ride a motorcycle recklessly into trffic, might not seem particularly complicated. [Read more] | Read more »

Price Scanner via MacPrices.net

Apple refurbished iMacs available for up to $...
Apple has Certified Refurbished 2015 21″ & 27″ iMacs available for up to $350 off MSRP. Apple’s one-year warranty is standard, and shipping is free. The following models are available: - 21″ 3.... Read more
Textkraft Professional Becomes A Mobile Produ...
The new update 4.1 of Textkraft Professional for the iPad comes with many new and updated features that will be particularly of interest to self-publishers of e-books. Highlights include import and... Read more
SnipNotes 2.0 – Intelligent note-taking for i...
Indie software developer Felix Lisczyk has announced the release and immediate availability of SnipNotes 2.0, the next major version of his productivity app for iOS devices and Apple Watch.... Read more
Pitch Clock – The Entrepreneur’s Wingman Laun...
Grand Rapids, Michigan based Skunk Tank has announced the release and immediate availability of Pitch Clock – The Entrepreneur’s Wingman 1.1, the company’s new business app available exclusively on... Read more
13-inch 2.9GHz Retina MacBook Pro on sale for...
B&H Photo has the 13″ 2.9GHz Retina MacBook Pro (model #MF841LL/A) on sale for $1599 including free shipping plus NY tax only. Their price is $200 off MSRP. Amazon also has the 13″ 3.9GHz Retina... Read more
Apple price trackers, updated continuously
Scan our Apple Price Trackers for the latest information on sales, bundles, and availability on systems from Apple’s authorized internet/catalog resellers. We update the trackers continuously: - 15″... Read more
Clearance 12-inch Retina MacBooks available s...
B&H Photo has dropped prices on leftover 2015 12″ Retina MacBooks with models now available starting at $999. Shipping is free, and B&H charges NY tax only: - 12″ 1.1GHz Gray Retina MacBook... Read more
Check Apple prices on any device with the iTr...
MacPrices is proud to offer readers a free iOS app (iPhones, iPads, & iPod touch) and Android app (Google Play and Amazon App Store) called iTracx, which allows you to glance at today’s lowest... Read more
New 2016 13-inch 256GB MacBook Air on sale fo...
B&H Photo has the new 13″ 1.6GHz/256GB MacBook Air (model MMGG2LL/A) on sale for $1149 including free shipping plus NY sales tax only. Their price is $50 off MSRP. Amazon has the 13″ 1.6GHz/256GB... Read more
Apple refurbished iPad Air 2s available start...
Apple has Certified Refurbished iPad Air 2 available starting at $339. Apple’s one-year warranty is included with each model, and shipping is free: - 128GB Wi-Fi iPad Air 2: $499 - 64GB Wi-Fi iPad... Read more

Jobs Board

*Apple* Nissan Service Technicians - Apple A...
Apple Automotive is one of the fastest growing dealer...and it shows. Consider making the switch to the Apple Automotive Group today! At Apple Automotive , Read more
ISCS *Apple* ID Site Support Engineer - APP...
…position, we are looking for an individual who has experience supporting customers with Apple ID issues and enjoys this area of support. This person should be Read more
Automotive Sales Consultant - Apple Ford Linc...
…you. The best candidates are smart, technologically savvy and are customer focused. Apple Ford Lincoln Apple Valley is different, because: $30,000 annual salary Read more
*Apple* Support Technician II - Worldventure...
…global, fast growing member based travel company, is currently sourcing for an Apple Support Technician II to be based in our Plano headquarters. WorldVentures is Read more
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.