dew2050

Thank you Andrew. It's all clear now.

The code snippet lost the format during the cut and pasting of the post. In my program is properly tabbed, like this: SEG.U VARS ORG $80 PFColor ds 1 PFTemp ds 1 SEG In this case there is a label after "seg.u". When compiling using the standard format we have been following so far, the file is 4K. However, if I omit the label "VARS" DASM creates a 64K file. As I understood it, Davie's lesson seems to say that the only thing you need to avoid such * huge * file (when declaring variables) is the SEG.U command without anything added to it.

I am confused. Does "ds" defines bytes or bits? Another question: When I tried for the time to use unitialized segment command, I forgot to declare the label name right after the seg.u command, like this: SEG.U ;<---- Notice no label here ORG $80 PFColor ds 1 PFTemp ds 1 SEG This compiled fine but the binary file inflated to 64K. When I noticed the label was missing, I put it there and the file shrinked to 4K. Why did DASM did this? Isn't the "seg.u" command enough for it to know?

I don't know if this is true, but from what I read anthology didn't save hiscores. They also had to cut out an internet feature that let you download more atari info and games. If a new anthology was to be released they should at least allow the save of hiscores. Back in those atari days, having the best score was everything...

Now that's more helpful. Thanks. So ASL sets the carry even if the dropped bit is 0? These subtle nuances are so interesting (programming-wise). Thanks Thomas. It's a lot more clear now.

I already have this information. I just wanted to make sure "ROL TEMP_VAR" uses Zero Page as the addressing mode.

Hint: Have a look at ROL. This will make your code much simplier. Thanks Thomas. With your help I was able to take this: CLC ASL PATTERN_RIGHT BCC SHIFTLEFT ASL PATTERN_LEFT INC PATTERN_LEFT BCC SCROLLCOMPLETE INC PATTERN_RIGHT JMP SCROLLCOMPLETE SHIFTLEFT ASL PATTERN_LEFT BCC SCROLLCOMPLETE INC PATTERN_RIGHT SCROLLCOMPLETE Into this: CLC ROL PATTERN_RIGHT ROL PATTERN_LEFT BCC SCROLLCOMPLETE INC PATTERN_RIGHT SCROLLCOMPLETE Wow, what a difference ROL makes! I am glad I did it the hard way first, otherwise I wouldn't had appreciated the difference. I have a newbie assembler question: How many machine cycles does the ROL commands above take? Is it 5 for zero page?

4. How would a game do horizontal scrolling? This is a difficult question - but I'm trying to get you to think about the implications of the odd playfield->pixel correspondence, and the implications for game writing. I modified the kernel code to use ASL commands to shift the bits in my playfield variable to the left. Since I didn't want the playfield to repeat itself twice on the screen, I added a second variable to the original kernel lesson sample. The code only uses PF0 to draw the screen. The result is that the playfield scrolls horizontally from left to right, jumping the center and edges of the screen (everything else not drawned by PF0). The scrolling feels a lot like Vanguard. It's not smoothy like say, Chopper Command. I think for that you would need to use the HMOVE register with something other than the playfield, but that's ok for now. On PCAEWIN it works fine, but on Z26 it draws some garbage at the top of the screen. Take a look at the code: ; 2600 for Newbies ; lesson 14 - playfield ; Question 4 - One way to do horizontal scrolling with the playfield. processor 6502 include "vcs.h" include "macro.h" ; OBJECTIVE: This variation of the lesson 14 kernel does horizontal scrolling with the playfield. ; Only PF0 is used. The program uses to variables to store the left and right side values ; of the playfield. These are declared below: PATTERN_LEFT = $80 PATTERN_RIGHT = $82 TIMETOCHANGE = 20 SEG ORG $F000 RESET ; Clear RAM and all TIA regs LDX #0 LDA #0 CLEAR STA 0,X INX BNE CLEAR LDA #$45 STA COLUPF LDY #0 ; Here we assign the pattern values to our left and right side of the playfield. ; Try using different values. LDA #$0 STA PATTERN_LEFT LDA #$01 STA PATTERN_RIGHT STARTOFFRAME LDA #0 STA VBLANK LDA #2 STA VSYNC STA WSYNC STA WSYNC STA WSYNC LDA #0 STA VSYNC ; 37 scanlines of vertical blank LDX #0 VERTICALBLANK STA WSYNC INX CPX #37 BNE VERTICALBLANK ; HANDLE A CHANGE IN THE PATTERN ONCE EVERY 20 FRAMES ; WRITE THE PATTERN TO PF1 INY CPY #TIMETOCHANGE BNE SCROLLCOMPLETE LDY #0 ; The following code uses arithmetic left shift commands (ASL) to make the scrolling happen. ; We shift the bits of the playfield variables to the left. Original bit 7 is shifted to the Carry. ; The program uses BCC to check if a bit was dropped. Dropped bits are inserted to the ; opposite pattern variable by the INC command. The INC command can mess up the pattern if ; the original bit 0 is turned on. To avoid this we shift everything to the left before incrementing. SHIFTRIGHT CLC ASL PATTERN_RIGHT BCC SHIFTLEFT ASL PATTERN_LEFT INC PATTERN_LEFT BCC SCROLLCOMPLETE INC PATTERN_RIGHT JMP SCROLLCOMPLETE SHIFTLEFT ASL PATTERN_LEFT BCC SCROLLCOMPLETE INC PATTERN_RIGHT SCROLLCOMPLETE ; DO 192 SCANLINES OF COLOR CHANGING (OUR PICTURE) LDX #0 PICTURE STX COLUPF ; Load the left pattern into PF1 register. LDA PATTERN_LEFT STA PF1 ; Wait enough cycles before drawing right pattern. SLEEP 20 ; Load right pattern to PF1. LDA PATTERN_RIGHT STA PF1 STA WSYNC INX CPX #192 BNE PICTURE LDA #%01000010 STA VBLANK ; 30 LINES OF OVERSCAN LDX #0 OVERSCAN STA WSYNC INX CPX #30 BNE OVERSCAN JMP STARTOFFRAME ORG $FFFA INTERRUPTVECTORS .WORD RESET .WORD RESET .WORD RESET END The tricky code is the bit shifting. Here is the pseudocode: [color=blue]; Pseudocode: ; ; Clear Carry ; Shift bits in variable PATTERN_RIGHT to the left. Original bit 7 is shifted into the Carry. ; ; if no bit was dropped from PATTERN_RIGHT (Carry Clear) then ; ; Shift the bits in PATTERN_LEFT ; if no bit was dropped from PATTERN_LEFT then ; ; Scroll is complete. Continue program flow. ; ; else ; ; Insert dropped bit from left pattern into PATTERN_RIGHT (Using the INC command). ; ; end ; ; else ; ; Shift bits in PATTERN_LEFT ; Insert dropped bit from right pattern into PATTERN_LEFT. ; If no bit was dropped from PATTERN_LEFT then ; ; Scroll is complete. ; ; else ; ; Insert dropped bit from left pattern into PATTERN_RIGHT. ; ; end ; ; end[/color] This translates to: SHIFTRIGHT CLC ASL PATTERN_RIGHT BCC SHIFTLEFT ASL PATTERN_LEFT INC PATTERN_LEFT BCC SCROLLCOMPLETE INC PATTERN_RIGHT JMP SCROLLCOMPLETE SHIFTLEFT ASL PATTERN_LEFT BCC SCROLLCOMPLETE INC PATTERN_RIGHT SCROLLCOMPLETE I am sure there is a better way of doing this without so many BCC's and ASL's (perhaps getting rid of the JMP altogether). question4.zip