The Atari Tutorial Pa.Rt 4: Display-List Interrupts

The Atari Tutorial Pa.rt 4: Display-List Interrupts

Chris Crawford 1272 Borregas Ave Sunnyvale CA 94086

The display-list interrupt is one of while it is being drawn. Of course, business. The interrupt to do this the most powerful features built into the computer must make each change must be precisely timed to occur at the Atari personal computer system. each time the screen is drawn, which exactly the same point during the It is also one of the least accessible happens 60 times per second. Also screen-drawing process. This spe features of the system, ' requiring of (and this is the tricky part), it must cially timed interrupt is provided by the programmer a firm understanding change the parameter in question at the ANTIC integrated circuit within of assembly language as well as all of exactly the same moment each time the Atari 400/ 800; it is called a the other characteristics of the the screen is drawn. That is, the cycle display-list interrupt (DLI). machine. Used alone, display-list in of changing screen parameters must The timing and execution of any in terrupts provide no additional be synchronized to the screen terrupt process can be intricate; there capabilities; they must be used in con drawing cycle. One way to do this fore, r shall first describe the sequence junction with the other features of the might be to lock the 6502 micro- of events in a properly working system, such as player-missile display-list interrupt. The process graphics, character-set indirection, or begins when the ANTIC chip en color-register indirection. With With display-list counters a display-list instruction display-list interrupts, the full power interrupts, many key having its interrupt bit (bit 07) set. of these features can be realized. Atari registers can be ANTIC waits until the last scan line Display-list interrupts take advan of the mode line it is currently tage of the sequential nature of the changed during the displaying. ANTIC then refers to its raster-scan television display. The drawing of a single NMIEN (nonmaskable interrupt television draws the screen image in a screen-display frame. enable) register (hexadecimal location time sequence, from the top of the D40E) to see if display-list interrupts screen to the bottom. This drawing have been enabled. If the enable bit process takes about 13,000 micro processor into a tight timing loop (bit 07) is cleared (to a logic 0), AN seconds and looks instantaneous to with an execution frequency of exact TIC ignores the interrupt and con the human eye. But that is a long time ly 60 hertz. This would make it very tinues its regular tasks. If the enable in comparison to the time scale the difficult for the computer to do bit is set (to a logic 1), ANTIC "pulls computer works in. The computer anything other than the screen down" the NMI (nonmaskable inter has plenty of time to change the display computations. It would also rupt) line on the 6502, signal ing an parameters of the screen display be a tedious job. A much better way interrupt. ANTIC then goes back to is to interrupt the 6502 just before the its normal display activities. The 6502 This article appears in slightly different time has come to change the screen starts executing an interrupt-service form in De Re Atari, a book published by parameters. The 6502 responds to the routine pointed to by the NMI vector Atari Inc, and is reproduced with its ex in terrup t, changes the screen in the operating system. This routine press permission. parameters, and returns to its normal first determines the cause of the inter-

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t u.s. Doilars rupt. If the interrupt is indeed a first thing you must do is write the memory. Page six (hexadecimal ad display-list interrupt, control is DLI routine itself. The routine must dresses 600 to 6FF) is an ideal place. transferred indirectly by means of the start by pushing any 6502 registers Set the vector at hexadecimal loca 16-bit address contained in hexa that will be altered onto the stack, tions 0200 and 0201 to point to your decimal locations 0200 and 0201 (low because the operating system routine. Determine the vertical point byte first) to a DLI-service routine. interrupt-poll routine itself saves no on the screen where you want the DLI The DLI routine changes one or more registers. (The 6502 status register is to occur, and then go to the cor of the graphics registers controlling automatically pushed onto the stack.) responding display-list instruction the display. The 6502 then returns The routine should be short and fast; and set bit D7 of the previous instruc from the interrupt routine to resume it should change only those registers tion. Finally, enable the DLI by set its mainline program. related to the display; and it should ting bit D7 of the NMIEN register at end by restoring any 6502 registers hexadecimal location D40E. The DLI Creating a Display-List Interrupt pushed onto the stack. will begin executing immediately. A number of steps are involved in Next, you must place the DLI As with any interrupt-service setting up a display-list interrupt. The service rou tine somewhere in routine, timing considerations can be critical. ANTIC does not send the in terrupt to the 6502 immediately upon encountering an interrupt instruction; it delays doing this until the last scan line of the interrupting mode line. The 6502 and the interrupt-service routine in the operating system together consume 33 machine cycles. Thus, the first instruction of your DLI-service routine will not be reached until 33 machine cycles have elapsed in the last scan line of the in terrupting mode line. Thirty-three D" machine cycles corresponds to 66 color clocks on the screen. Thus, your DLI-service routine will begin executing while the electron beam is partway across the screen in the last ~ scan line of the interrupting mode line. For example, if such a DLI WHY DOES OUR SPELLING CHECKER routine changes a color register, the COST LESS THAN THEIRS? old color will be displayed on the left half of the scan line and the 'new color will show up on the right half of the Maybe we goofed by not charging more The WORD gives you more! same scan line. Because of uncertain for The WORD. Our customers tell us we must be crazy, giving away this much The WORD gives you a 45,000 word timing in the response of the 6502 to software for only $75. dictionary that fits into less than 140K of an interrupt, the border. between the disk space. What's wrong with it? The WORD works with your favori te colors will not be sharp, but will Frankly, we're a little hurt when people text editor and marks mi stakes in your jiggle back and forth irritatingly. ask us this question. We guess everyone doc ument for easy, in'context, correc ti on. The solution to this problem is pro must be getting used to paying $200 to $300 The WORD's one·touch word review for decent software. An ything that costs lets you add new words to the dictionary vided in the form of the WSYNC less must be junk. Right? ... WRONG!!! with a single keypress. You can build your (wait for horizontal sync) register own custom di cti onaries too ! The WORD is not only cheaper, it's (hexadecimal address D40A). When better! The WORD wi ll look up the correct spelling of misspelled words. ever this register is addressed in any Available NOW for: The WORD anal yzes your writing, way, the ANTIC chip pulls down the 8" single CP/M" , CDOS™ counting words and showing you how XEROX 820 (8" only) oft en each word was used. RDY line on the 6502. This effectively Intertec Superbrain™ The WORD has a special homonym halts the 6502 until the WSYNC Apple™ with CP/M Soft card he/per feature to deal with these pesky register is reset by the next horizontal words. (35 .000 word dictionary) synch pulse. The result is that the North Sta?M Double/Qu ad with CP/M The WORD will find rh yming word s, CP / M is a registered trademark 01 Digital Research solve cro ssword pu zzles, and much more! 6502 freezes until the electron beam CALL TODAY! ' returns to the left edge of the screen. If you insert a ST A WSYNC instruc OASIS SYSTEMS (714) 291·9489 tion just before an instruction that 2765 REYNARD WAY, SAN DIEGO, CA 92103 stores a value into a color register, the color being displayed will change

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CP 1M and MP 1M are registered tractemaiks 01 Digital Research Corp. Listing 1: A simple Atari BASIC program to demonstrate display-list interrupts. This program changes the screen color from blue to pink and darkens the character set halfway down the video display. The complete BASIC program in listing la contains the assembly-language routine given in listing lb.

1a 10 DLIST=PEEK(560)+256*PEEK(561):REM find display list

20 POKE DLIST+15,130:REM insert interrupt Instruction 30 FOR 1=0 TO 19:REM loop for poking DLI service routine 40 READ A:POKE 1536+I,A:NEXt

50 DATA 72,138,72,169,80,162,88

60 DATA 141,10,212,141,23,208 70 DATA 141,24,208,104,170,104,64 80 POKE 512,0:POKE 513,6:REM poke In Interrupt vector

90 POKE 54286,192:REM enab I e DLI

1b while the beam is off the left edge of PHA save accumulator the screen. The color transition will occur one scan line lower, but it will TXA be neat and clean. The proper way to use a display list interrupt, then, is to set the DLI PHA save X-register bit on the mode line before the mode line for which you want the action to LDA #$50 dark color for characters occur. The DLI-service routine should first save the 6502 registers LDX #$58 pink onto the stack and then load the 6502 registers with the new graphics values STA WSYNC walt to be used. It should execute a STA WSYNC immediately before storing \ the new values into the appropriate STA COLPF1 store color ANTIC or CTIA registers. Finally, it should restore the 6502 registers and STX COLPF2 store color return from the interrupt. This pro cedure will guarantee that the PLA graphics registers are changed while the electronic beam is off the screen and that the new display parameters TAX take effect at the beginning of the desired line. PLA restore registers The program in listing 1 is a very simple DLI-service routine. It changes RTI done the background color from blue to pink. It also changes the color of the characters so that they show up as dark against the pink background. The upper half of the screen remains

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Attract mode is a YOUR blank period. The blue color comes feature provided by the operating from the operating system's shadow system. After nine minutes without a LOCAL register for that color register. Every keypress, the colors on the screen hardware color register is shadowed begin to cycle through random hues out to a RAM (random-access at lowered luminances. This insures DYSAN read/ write memory) location. You that a computer left unattended for may already know about these several hours does not burn an image OFFICE: shadow registers at decimal locations into the television screen. 708 through 712. For most purposes, It is easy to build attract mode into you can change colors by poking a DLI routine by inserting only two 1320 I Bel-Red Road values into the shadow registers (see lines of assembly code, as shown in Suite 205. C9200 last month's article for an explanation listing 2. Bellevue, Washington 98004 6f shadow registers). If you poke The implementation of attract (206) 455-4725 directly into the hardware registers, mode in display-list interrupts exacer 1242 Kifer Road the operating system shadow process bates an already difficult problem: Sunnyvale, California 94086 will wipe out your poked color within the shortage of execution time during (408) 730-2145 %0 second (ie: at the top of a new a DLI. A description of DLI timing screen display). For DLIs, however, will make the problem more obvious. #10 11th Avenue you must store your new color values Glendora, New Jersey 08029 directly into the hardware registers. DLI Timing (609) 939-4762 You cannot use a DLI to set the color DLI execution is broken into three of the first displayed line of the phases. Phase 1 covers the period 999 Plaza Drive, Suite 400 screen. The operating system takes from the beginning of the DLI to the Schaumburg, Illinois 60195 care of that line for you {and the first ST A WSYNC instruction. During (312) 882-8176 line is off the top of the screen, phase 1, the electron beam is drawing the last scan line of the interrupting 6520 Powers Ferry Road mode line. Phase 2 covers the period Suite 200 Atlanta, Georgia 30339 Listing 2: Restoring the Atari attract from the ST A WSYNC instruction to (404) 952-0919 mode to a display driven by display-list the appearance of the beam on the interrupts. Only two 6502 assembly television screen. Phase 2 cor 122 East 42nd Street language instructions have to be added to responds to the horizontal blank; all 17th Floor the DLI routine. DRKMSK and COLRSH graphics changes should be made dur New York, New York 10017 are page zero locations (hexadecimal 4E ing this phase. Phase 3 covers the (212) 687-7122 and 4F) set up and updated by the period from the appearance of the operatjng system during the vertical blank beam on the screen to the end of the interrupt. When the attract mode is not in 19600 Fairchild DLI-service routine. The timing of force , COLRSH takes a value of 00 and Suite 150 DRKMSK takes a value of hexadecimal phase 3 is not critical. INine, California 92713 FF. When attract mode is in force , One horizontal scan line takes 114 (714) 851-9462 COLRSH is given a new random value clock cycles of real time. A DLI every four seconds and DRKMSK holds a reaches the 6502 on or around cycle value of hexadecimal F6. Thus, COLRSH number 15 . The 6502 takes about 7 scrambles the color and DRKMSK lops cycles to respond to the interrupt. off the high-order luminance bit. The operating-system routine to ser vice the interrupt and turn control LDA NEWCOL LDA NEWCOL over to the DLI-service routine takes 11 machine cycles. Thus, the DLI STA WSYNC EOR COLRSH service routine does not gain control DDY§£!lJ· until about 33 clock cycles have 5440 Patrick Henry Drive STA COLPF2 AND DRKMSK elapsed. Furthermore, the STA Santa Clara, CA 95050 WSYNC instruction must begin by 408/988-3472 cycle number 103; this reduces the TLX: 171551 DYSAN SNTA STA WSYNC time available in phase 1 by 11 cycles. Finally, ANTIC's DMA (direct STA COLPF2 memory access) will steal some of the

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I@D):5(JII/ 13$1-"";' remaining clock cycles from the 6502. usable machine cycles are available to early in phase 3, if that color is not Nine cycles will be lost to memory phase 2. displayed on the left edge of the refresh DMA. This leaves an absolute The problems of DLI timing now screen. Similarly, a color not showing maximum of 61 cycles available for become obvious. To load, attract, up on the right side of the screen phase 1. This maximum is achieved and store a single color will consume could be changed during phase 1. only with blank-line mode lines. 14 ·cycles. Saving the 6502 A, X, and Another approach is to break one Character and map mode instructions Y registers onto the stack and then overactive DLI into two less am will result in the loss of one cycle for loading, attracti{lg, and saving three bitious DLIs, each doing half the each byte of display data. The worst colors into A, X, and Y registers will work of the original. The second DLI case arises with BASIC modes 0, 7, cost 47 cycles: most, if not all, of could be provided by inserting a and 8, which require 40 bytes per phase 1. Obviously, the programmer single-scan-line blank instruction line. Only 21 machine cycles are who wishes to use DLIs for extensive (with the DLI bit set) into the display available to phase 1 in these modes. graphics changes will expend much list just below the main interrupting Thus, a phase 1 routine will have effort on the timing of the DLI. For mode line. This will, of course, con from 21 to 61 machine cycles of tunately, the beginning programmer sume some screen space. execution time available to it. need not concern himself with exten Another partial solution is to per Phase 2, the critical phase, extends sive timing calculations. If only form the attract chores during over 24 clock cycles of real time. As single-color changes or simple vertical-blank periods. To do this, with phase 1, some of these cycles are graphics operations are to be per two tables of colors must be kept in lost to cycle-stealing DMA. Player formed, cycle counting and speed op memory. The first table contains missile graphics will cost 5 cycles if timiza tion are unnecessary. These color values intended to be displayed they are used. The display instruction considerations are only important for by the DLI routines. The second table will cost 1 cycle. Two more cycles high-performance situations. contains the attracted values of these will be stolen if the Load Memory No simple options are available to colors. During vertical blank, a user Scan option in the display list is used. the programmer'who needs to change supplied interrupt-service routine Finally, 1 or 2 cycles may be lost to more than three color registers in a fetches each color from the first table, memory refresh or display-data single DLI . It might be possible to attracts it, and stores the attracted retrieval. Thus, from 14 to 23 load, attract, and store a fourth color color in the second table. The DLI

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SAM'S BOOKS Listing 3a: An assembly-language routine which is included in the multiple display-list SAVE UP TO 250/0 interrupt program shown in listing 3b. BUY A SERIES FROM THIS LIST OR INDIVIDUAL TITLES USING THE 5 PHA DIGIT ORDER CODES. ORDER A CATALOG OF ALL SAM'S BOOKS AND SAVE! TXA SERIES 'A' COMPUTER COOKBOOKS PHA No. 21524 . The Cheap Video Cookbook o $7.95 No. 21723 · Son Of Cheap Video o $9.95 No. 21697 . 8085A Cookbook o $13.95 INC COUNTR No . 21313 . TV Typewriter Cookbook 0$11.95

SERIES 'B' TRS-80 LOX COUNTR

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180 December 1981 © BYTI Publications Inc Listing 3b: A simple Atari BASIC program to demonstrate multiple display-list interrupts. This program puts 80 different colors on the v ideo display. The complete BASIC program shown here contains the assembly-language routine given in listing 3a.

10 GRAPHICS 7

20 DLIST=PEEK(560)+256*PEEK(561):REM find display list 30 FOR J=6 TO 84:REM give every mode line a DLI 40 POKE DLIST+J,141:REM BASIC mode 7 with DLI bit set 50 NEXT J

60 FOR J=O TO 30

70 READ A:POKE 1536+J,A:NEXTJ:REM poke in DLI service routine 80 DATA 72,138,72,238,32,6,175,32,6 90 DATA 189,0,240,141,10,212,141,26,208 100 DATA 224,79,208,5,169,0 110 DATA 141,32,6,104,170,104,64 120 POKE 512,0:POKE 513,6:REM vector to DLI service routine 130 POKE 54286,192:REM enable DLI forI11ed after the time-critical portion puts from the keyboard. If keypresses putation, such as certain skill and of the DU, not before. are not acknowledged, the screen action games. For example, the jiggle does not occur. Basketball program for the Atari Keyboard-Click Routine 400/ 800 uses a kernel; the program The operating system keyboard Kernels requires little computation but much click routine interferes with the func The display-list interrupt was color. The multicolored players in tion of the DU. Whenever a key is designed to replace a more primitive this game could not be done with pressed and acknowledged, the on software/ hardware technique called a display-list interrupts because DLIs board speaker is clicked. The timing kernel. A kernel is a 6502 program are keyed to playfield vertical posi for this click is provided by several loop that is precisely timed to the tions, not player positions. STA WSYNC instructions. This can display cycle of a television set. By It is possible to extend the kernel throw off the timing of a DU routine monitoring the VCOUNT register idea right into a single scan line and and cause the screen colors to jump and consulting a table of screen change graphics registers on the fly. downward by one scan line for a frac changes catalogued as a function of In this way, a single color register can tion of a second. There is no easy VCOUNT values, the 6502 can arbi present several colors on a single scan solution to this problem. One pos trarily control all graphics values for line. The horizontal position of the sible remedy involves the VCOUNT the entire screen. A high price is paid color change is determined by the register (hexadecimal location D40B), for this power: the 6502 is not amount of time that elapses before a read-only register in ANTIC that available for computations during the the change goes in. Thus, by carefully tells what scan line ANTIC is display screen-display period, which is about counting machine cycles you can get ing. A DLI routine could examine this 75 percent of the time: Furthermore, more graphics onto the screen. Unfor register to decide when to change a no computation .may consume more tunately, this is extremely difficult to color. Another solution is to disable than the 4000 or so machine cycles achieve in practice. With ANTIC per the operating system keyboard available during vertical-blank and forming DMA on the 6502, it is very service routine (a tedious job) and overscan periods. This restriction difficult to know exactly how many provide your own keyboard routine. means that kernels can only be used cycles have really elapsed; a simple A third alternative is to accept no in- with programs requiring little com- count of 6502 cycles is not adequate.

December 1981 © BYTE Publications Inc 181 If ANTIC's DMA is turned off, the 6502 can assume full control of the display, but it must then. perform all the work that ANTIC normally does. For these reasons, horizontal kernels are seldom worth the effort. If the two images to be displayed in dif ferent colors are widely separated, however, say by 20 color clocks or more, the separation should cover up the timing uncertainties and render this technique feasible.

Using Display-List Interrupts The tremendous value of graphics indirection and all those modifiable registers in the hardware now becomes obvious. With display-list interrupts, everyone of those registers can be changed dynamically. You can put lots of c910r, graphics, and special effects onto the screen. The most obvious application of DLIs is to put more color onto the screen. Each color register can be changed as many times as you have DLIs. This applies to both playfield color registers and player color registers. Thus, you have up to nine color registers, each of which can display up to 128 different colors. Of course, a normal program could not effec 'Philoso,-~ieal fUD and games ... tively use all of those colors. Too from the authors of many DLIs start slowing down the of a very high order whole program, and sometimes the G6del, Escher, Bach and Brainstorms"* screen layout cannot accommodate all Pulitzer Prize"winner Douglas Hofstadter and the philosopher Daniel of them. In practice, displaying a Dennett take you on a mind-jolting exploration of art and science, dozen colors is easy, two dozen re humor and philosophy, to probe the meaning of self and conscious quires careful planning, and more ness. "The authors supply extracts from literature (Borges, Lem) and than that requires a contrived situa scientific reports (A.M. 'lUring, John Searle) that reveal the complex- tion. ities, idiosyncracies, and sheer brilliance (and folly) of man thinking about himself The quandary of human self-representation But DLIs can give more than color. ~~~~...... ~ is brilliantly explored in this cunning, interdisci They can also be used to extend the plinary banquet of food for thoughe' power of player-missile graphics by -ALA Booklist changing the horizontal position of a "Superb ... with equally wondrous player. In this way, a player can be commentaries~ ' repositioned partway down the -Martin Gardner screen. A single player can then have *Kirkus Reviews several incarnations on the screen. If you imagine a player as a vertical col umn with images drawn on it, a DLI becomes a pair of scissors with which you can snip the column and reposi An Alternate of the Book-of tion sections of it on the screen. Of the-Month Club (Science) course, no two sections of the player 70,000 copies in print $15.50 at bookstores can be on the same horizontal line, BASIC BOOKS, INC. and so two incarnations of the player 10 E. 53 Street, New York 10022 cannot be on the same horizontal line. If your display needs allow graphics objects that will never be on

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