By Darrell Spice, Jr. (adapted by Duane Alan Hahn, a.k.a. Random Terrain)
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Original Blog Entry
After getting a stable display, I like to implement the routines for displaying the score. You can see that in the first builds of Frantic, Medieval Mayhem, and Space Rocks. Even though we're not ready to show the player's score, the display is very useful for showing diagnostic information—such as this build of Frantic which uses it to show which sprites are colliding with the player.
To draw the score we're going to use the playfield graphics. The playfield pattern is comprised of 20 bits stored in 3 bytes of the TIA.
The 20 bits are either repeated or reflected to draw the 20 bits on the right half of the screen.
For our game, a two digit score and a two digit timer will meet our requirements. We could show a single digit each in PF1 and PF2, but due to the reversed output of PF2 that would mean we'd need to create both normal and mirrored digit graphics. Instead, we're going to create digit graphics that are 3 bits across, which will allow us to show two digits using PF1. You may have seen the graphics in one of my blog entries with the revised mode file for jEdit. Each digit appears twice as we can use a simple mask to get the tens (AND #$F0) and ones (AND #$0F) digits. If we only saved the image as the ones position we'd need to apply 4 shift instructions to create a tens position image.
PF1 is displayed twice on the screen and if we time it correctly we can change the contents of PF1 so that both sides of the screen are different. Andrew Davie posted another handy diagram that shows the timing:
In looking at the diagram we can figure out the update times required for each instance of PF1.
As mentioned before, the RIOT chip in the Atari 2600 stands for RAM, Input/Output and Timer. For this update we're going to look at the RAM and Input.
To show the score we need to keep track of a few things, so let's allocate some space in RAM:
ORG $80 ; Holds 2 digit score, stored as BCD (Binary Coded Decimal) Score: ds 1 ; stored in $80 ; Holds 2 digit timer, stored as BCD Timer: ds 1 ; stored in $81 ; Offsets into digit graphic data DigitOnes: ds 2 ; stored in $82-$83, DigitOnes = Score, DigitOnes+1 = Timer DigitTens: ds 2 ; stored in $84-$85, DigitTens = Score, DigitTens+1 = Timer ; graphic data ready to put into PF1 ScoreGfx: ds 1 ; stored in $86 TimerGfx: ds 1 ; stored in $87 ; scratch variable Temp: ds 1 ; stored in $88
Vertical Blank is now doing a little bit of work:
VerticalBlank: jsr SetObjectColors jsr PrepScoreForDisplay rts ; ReTurn from Subroutine
When the macro CLEAN_START initialized the Atari's hardware, it set all the colors to black. So we need to set the object colors if we want to see anything. This routine also reads the state of the console switches (via one of RIOT's Input registers) in order to determine if the player selected Color or Black & White:
SetObjectColors: ldx #3 ; we're going to set 4 colors (0-3) ldy #3 ; default to the color entries in the table (0-3) lda SWCHB ; read the state of the console switches and #%00001000 ; test state of D3, the TV Type switch bne SOCloop ; if D3=1 then use color ldy #7 ; else use the b&w entries in the table (4-7) SOCloop: lda Colors,y ; get the color or b&w value sta COLUP0,x ; and set it dey ; decrease Y dex ; decrease X bpl SOCloop ; Branch PLus (positive) rts ; ReTurn from Subroutine Colors: .byte $86 ; blue - goes into COLUP0, color for player0 and missile0 .byte $C6 ; green - goes into COLUP1, color for player1 and missile1 .byte $46 ; red - goes into COLUPF, color for playfield and ball .byte $00 ; black - goes into COLUBK, color for background .byte $0E ; white - goes into COLUP0, color for player0 and missile0 .byte $06 ; dark grey - goes into COLUP1, color for player1 and missile1 .byte $0A ; light grey - goes into COLUPF, color for playfield and ball .byte $00 ; black - goes into COLUBK, color for background
The score and timer will be stored using BCD (Binary Coded Decimal) but for now we'll treat and display them as hexadecimal values (that's why the digit graphics are 0-9 then A-F). This routine takes the upper and lower nybble (4 bits) of the Score and Timer and multiplies them by 5 in order to get the offset into the digit graphic data. The 6507 does not have a multiply command, though it does have a shift feature which is equivalent to *2. If a nybble is value X then X * 2 * 2 + X is the same as X * 5:
PrepScoreForDisplay: ; for testing purposes, change the values in Timer and Score inc Timer ; INCrement Timer by 1 bne PSFDskip ; Branch Not Equal to 0 inc Score ; INCrement Score by 1 if Timer just rolled to 0 PSFDskip ldx #1 ; use X as the loop counter for PSFDloop PSFDloop: lda Score,x ; LoaD A with Timer(first pass) or Score(second pass) and #$0F ; remove the tens digit sta Temp ; Store A into Temp asl ; Accumulator Shift Left (# * 2) asl ; Accumulator Shift Left (# * 4) adc Temp ; ADd with Carry value in Temp (# * 5) sta DigitOnes,x ; STore A in DigitOnes+1(first pass) or DigitOnes(second pass) lda Score,x ; LoaD A with Timer(first pass) or Score(second pass) and #$F0 ; remove the ones digit lsr ; Logical Shift Right (# / 2) lsr ; Logical Shift Right (# / 4) sta Temp ; Store A into Temp lsr ; Logical Shift Right (# / 8) lsr ; Logical Shift Right (# / 16) adc Temp ; ADd with Carry value in Temp ((# / 16) * 5) sta DigitTens,x ; STore A in DigitTens+1(first pass) or DigitTens(second pass) dex ; DEcrement X by 1 bpl PSFDloop ; Branch PLus (positive) to PSFDloop rts ; ReTurn from Subroutine
The Kernel's been modified so it now uses the data in DigitOnes and DigitTens to update PF1. Each line of graphic data is output twice so the digits are drawn over 10 scanlines which gives them a better appearance than if they had been drawn over 5 scanlines.
ldx #5 ScoreLoop: ; 43 - cycle after bpl ScoreLoop ldy DigitTens ; 3 46 - get the tens digit offset for the Score lda DigitGfx,y ; 5 51 - use it to load the digit graphics and #$F0 ; 2 53 - remove the graphics for the ones digit sta ScoreGfx ; 3 56 - and save it ldy DigitOnes ; 3 59 - get the ones digit offset for the Score lda DigitGfx,y ; 5 64 - use it to load the digit graphics and #$0F ; 2 66 - remove the graphics for the tens digit ora ScoreGfx ; 3 69 - merge with the tens digit graphics sta ScoreGfx ; 3 72 - and save it sta WSYNC ; 3 75 - wait for end of scanline ;--------------------------------------- sta PF1 ; 3 3 - @66-28, update playfield for Score dislay ldy DigitTens+1 ; 3 6 - get the left digit offset for the Timer lda DigitGfx,y ; 5 11 - use it to load the digit graphics and #$F0 ; 2 13 - remove the graphics for the ones digit sta TimerGfx ; 3 16 - and save it ldy DigitOnes+1 ; 3 19 - get the ones digit offset for the Timer lda DigitGfx,y ; 5 24 - use it to load the digit graphics and #$0F ; 2 26 - remove the graphics for the tens digit ora TimerGfx ; 3 29 - merge with the tens digit graphics sta TimerGfx ; 3 32 - and save it jsr Sleep12 ;12 44 - waste some cycles sta PF1 ; 3 47 - @39-54, update playfield for Timer display ldy ScoreGfx ; 3 50 - preload for next scanline sta WSYNC ; 3 53 - wait for end of scanline ;--------------------------------------- sty PF1 ; 3 3 - @66-28, update playfield for the Score display inc DigitTens ; 5 8 - advance for the next line of graphic data inc DigitTens+1 ; 5 13 - advance for the next line of graphic data inc DigitOnes ; 5 18 - advance for the next line of graphic data inc DigitOnes+1 ; 5 23 - advance for the next line of graphic data jsr Sleep12 ;12 35 - waste some cycles dex ; 2 37 - decrease the loop counter sta PF1 ; 3 40 - @39-54, update playfield for the Timer display bne ScoreLoop ; 2 42 - (3 43) if dex != 0 then branch to ScoreLoop sta WSYNC ; 3 45 - wait for end of scanline ;--------------------------------------- stx PF1 ; 3 3 - x = 0, so this blanks out playfield sta WSYNC ; 3 6 - wait for end of scanline
Score and Timer:
When TV Type switched to B&W:
You'll notice that the score and timer are different colors even though they're both drawn using the playfield. This is because I've modified Vertical Sync to turn on SCORE mode. SCORE mode tells the TIA to use the color of player0 for the left half of the playfield and the color of player1 for the right half.
VerticalSync: lda #2 ; LoaD Accumulator with 2 so D1=1 ldx #49 ; LoaD X with 49 sta WSYNC ; Wait for SYNC (halts CPU until end of scanline) sta VSYNC ; Accumulator D1=1, turns on Vertical Sync signal stx TIM64T ; set timer to go off in 41 scanlines (49 * 64) / 76 sta CTRLPF ; D1=1, playfield now in SCORE mode ... rts ; ReTurn from Subroutine
The ROM and the source are at the bottom of my blog entry.
Other Assembly Language Tutorials
Step 3: Score and Timer Display
This book was written in English, not computerese. It's written for Atari users, not for professional programmers (though they might find it useful).
This book only assumes a working knowledge of BASIC. It was designed to speak directly to the amateur programmer, the part-time computerist. It should help you make the transition from BASIC to machine language with relative ease.
The 6502 Instruction Set broken down into 6 groups.
Nice, simple instruction set in little boxes (not made out of ticky-tacky).
This book shows how to put together a large machine language program. All of the fundamentals were covered in Machine Language for Beginners. What remains is to put the rules to use by constructing a working program, to take the theory into the field and show how machine language is done.
An easy-to-read page from The Second Book Of Machine Language.
A useful page from Assembly Language Programming for the Atari Computers.
Continually strives to remain the largest and most complete source for 6502-related information in the world.
By John Pickens. Updated by Bruce Clark.
Below are direct links to the most important pages.
Goes over each of the internal registers and their use.
Gives a summary of whole instruction set.
Describes each of the 6502 memory addressing modes.
Describes the complete instruction set in detail.
Cycle counting is an important aspect of Atari 2600 programming. It makes possible the positioning of sprites, the drawing of six-digit scores, non-mirrored playfield graphics and many other cool TIA tricks that keep every game from looking like Combat.
Atari 2600 programming is different from any other kind of programming in many ways. Just one of these ways is the flow of the program.
The "bankswitching bible." Also check out the Atari 2600 Fun Facts and Information Guide and this post about bankswitching by SeaGtGruff at AtariAge.
Atari 2600 programming specs (HTML version).
Links to useful information, tools, source code, and documentation.
Atari 2600 programming site based on Garon's "The Dig," which is now dead.
Includes interactive color charts, an NTSC/PAL color conversion tool, and Atari 2600 color compatibility tools that can help you quickly find colors that go great together.
Adapted information and charts related to Atari 2600 music and sound.
A guide and a check list for finished carts.
A multi-platform Atari 2600 VCS emulator. It has a built-in debugger to help you with your works in progress or you can use it to study classic games. Stella finally got Atari 2600 quality sound in December of 2018. Until version 6.0, the game sounds in Stella were mostly OK, but not great. Now it's almost impossible to tell the difference between the sound effects in Stella and a real Atari 2600.
A very good emulator that can also be embedded on your own web site so people can play the games you make online. It's much better than JStella.
If assembly language seems a little too hard, don't worry. You can always try to make Atari 2600 games the faster, easier way with batari Basic.
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Use any example programs at your own risk. I am not responsible if they blow up your computer or melt your Atari 2600. Use assembly language at your own risk. I am not responsible if assembly language makes you cry or gives you brain damage.