full screen demo colorized

main.asm

@@ -1,5 +1,7 @@
 ; -----------------------------------------------------------
 ; main.asm - C64 demo: giant TTG logo + credits scroller
+;   effects: color wash on the logo, smooth pixel scroll,
+;   pulsing title - all timed by polling the raster beam
 ; Build: acme -f cbm -o main.prg main.asm
 ; -----------------------------------------------------------
 
@@ -22,6 +24,7 @@
 BORDER  = $d020          ; border color register
 BG      = $d021          ; background color register
 RASTER  = $d012          ; current raster line
+XCTRL   = $d016          ; horizontal fine scroll + 38/40 column mode
 SCREEN  = $0400          ; screen RAM (40x25 chars)
 COLRAM  = $d800          ; color RAM
 
@@ -40,7 +43,13 @@ TITLE_POS   = 12*40 + 13         ; "TELETYPE GAMES" (14 chars, centered)
 SCROLLROW   = 14                 ; scroller row
 WWW_POS     = 17*40 + 9          ; "WWW.TELETYPEGAMES.ORG" (21 chars)
 CITY_POS    = 19*40 + 14         ; "SZEGED, 2026" (12 chars)
-SCROLL_SPEED = 4                 ; frames per scroll step
+
+; ---- timing ----
+SCROLL_STEP = 2                  ; pixels per frame (8 px / 4 frames = 1 char)
+RAS_SCRTOP  = 160                ; just above the scroller row (row 14 = lines 162-169)
+RAS_SCRBOT  = 171                ; just below it (rows 13 and 15 are blank, so
+                                 ;   a line of jitter here is invisible)
+RAS_BOTTOM  = 248                ; lower border: safe time for screen updates
 
 ; ---- zero page pointers ----
 scrlo = $fb
@@ -49,6 +58,8 @@ collo = $fd
 colhi = $fe
 
 start:
+    sei                  ; no KERNAL IRQ - raster polling must not jitter
+
     lda #BLACK
     sta BORDER
     sta BG
@@ -173,47 +184,120 @@ scrollcol_loop:
     dex
     bpl scrollcol_loop
 
-    lda #SCROLL_SPEED
-    sta framecnt
+    lda #6               ; multiple of SCROLL_STEP -> clean 4-frame cycle
+    sta xscroll
     lda #0
     sta scrollidx
+    sta framectr
+    sta washphase
 
-; ---- main loop: scroll the credits line ----
+; ============================================================
+; main loop - one iteration per frame, driven by the raster
+; ============================================================
 main:
-wait_raster:             ; wait for raster line $fe (bottom of screen)
+    ; switch on fine scroll + 38-column mode for the scroller row only
+wait_scrtop:
     lda RASTER
-    cmp #$fe
-    bne wait_raster
-wait_raster2:            ; ...and wait until it passes -> exactly one frame
+    cmp #RAS_SCRTOP
+    bne wait_scrtop
+    lda xscroll          ; bits 0-2 = x scroll, bit 3 = 0 -> 38 columns
+    sta XCTRL
+
+wait_scrbot:
     lda RASTER
-    cmp #$fe
-    beq wait_raster2
+    cmp #RAS_SCRBOT
+    bne wait_scrbot
+    lda #$c8             ; back to default: 40 columns, no scroll
+    sta XCTRL
 
-    dec framecnt
-    bne main
-    lda #SCROLL_SPEED
-    sta framecnt
+    ; beam is in the lower border: do all updates now, invisibly
+wait_bottom:
+    lda RASTER
+    cmp #RAS_BOTTOM
+    bne wait_bottom
 
-    ; shift scroller row one char to the left
+    inc framectr
+
+    ; smooth scroll: move SCROLL_STEP pixels, shift chars on wrap
+    lda xscroll
+    sec
+    sbc #SCROLL_STEP
+    bcs xs_store         ; still 0..7: just store
+    adc #8               ; wrap (carry clear, so this adds exactly 8)
+    sta xscroll
+    jsr shift_row
+    jmp xs_done
+xs_store:
+    sta xscroll
+xs_done:
+
+    lda framectr         ; wash moves one column every 2nd frame
+    and #1
+    bne wash_hold
+    inc washphase
+wash_hold:
+
+    jsr do_wash
+    jsr do_pulse
+    jmp main
+
+; ---- shift the scroller row one char left, feed in the next char ----
+shift_row:
     ldx #0
-shift_loop:
+sr_loop:
     lda SCREEN+SCROLLROW*40+1,x
     sta SCREEN+SCROLLROW*40,x
     inx
     cpx #39
-    bne shift_loop
-
-    ; fetch next char of the scrolltext into the rightmost column
+    bne sr_loop
     ldx scrollidx
     lda scrolltext,x
     sta SCREEN+SCROLLROW*40+39
     inx
     cpx #scrolltext_end - scrolltext
-    bne no_wrap
+    bne sr_nowrap
     ldx #0
-no_wrap:
+sr_nowrap:
     stx scrollidx
-    jmp main
+    rts
+
+; ---- color wash: gold gradient running across the logo columns ----
+do_wash:
+    ldx #27              ; 28 logo columns
+dw_loop:
+    txa
+    clc
+    adc washphase
+    and #15
+    tay
+    lda washtbl,y
+    sta COLRAM + (BIGROW+0)*40 + BIGCOL,x
+    sta COLRAM + (BIGROW+1)*40 + BIGCOL,x
+    sta COLRAM + (BIGROW+2)*40 + BIGCOL,x
+    sta COLRAM + (BIGROW+3)*40 + BIGCOL,x
+    sta COLRAM + (BIGROW+4)*40 + BIGCOL,x
+    sta COLRAM + (BIGROW+5)*40 + BIGCOL,x
+    sta COLRAM + (BIGROW+6)*40 + BIGCOL,x
+    sta COLRAM + (BIGROW+7)*40 + BIGCOL,x
+    dex
+    bpl dw_loop
+    rts
+
+; ---- pulsing title: grey-white ramp, one step per 8 frames ----
+do_pulse:
+    lda framectr
+    lsr
+    lsr
+    lsr
+    and #7
+    tay
+    lda pulsetbl,y
+    ldx #txt_title_end - txt_title - 1
+dp_loop:
+    sta COLRAM+TITLE_POS,x
+    dex
+    bpl dp_loop
+    rts
 
 ; ---- data ----
 
@@ -229,6 +313,13 @@ bigfont:
 letter_off:              ; column offset of each letter (8 wide + 2 gap)
     !byte 0, 10, 20
 
+washtbl:                 ; gold glow ramp (16 entries, cyclic)
+    !byte $09, $02, $08, $0a, $07, $07, $01, $01
+    !byte $01, $01, $07, $07, $0a, $08, $02, $09
+
+pulsetbl:                ; grey-white pulse (8 entries, cyclic)
+    !byte $0b, $0c, $0f, $01, $01, $0f, $0c, $0b
+
 ; texts in screen codes (!scr); lowercase source shows as uppercase on C64
 txt_title:
     !scr "teletype games"
@@ -251,5 +342,7 @@ scrolltext_end:
 letter:    !byte 0
 rowcount:  !byte 0
 curbits:   !byte 0
-framecnt:  !byte 0
+framectr:  !byte 0
+xscroll:   !byte 0
+washphase: !byte 0
 scrollidx: !byte 0