c64-demo/common.asm

; -----------------------------------------------------------
; common.asm - routines, tables and variables shared by the
; slides: screen clearing, text printing, big 3x5 block font
; -----------------------------------------------------------

; ---- clear screen RAM, fill color RAM with white ----
clear_screen:
    ldx #0
cls_loop:
    lda #$20             ; space
    sta SCREEN,x
    sta SCREEN+$100,x
    sta SCREEN+$200,x
    sta SCREEN+$2e8,x
    lda #WHITE
    sta COLRAM,x
    sta COLRAM+$100,x
    sta COLRAM+$200,x
    sta COLRAM+$2e8,x
    inx
    bne cls_loop
    rts

; ---- print a text line: txtlo/hi, prow, pcol, pcolor, plen ----
print_text:
    ldx prow
    lda mul40lo,x
    clc
    adc pcol
    sta scrlo
    sta collo
    lda mul40hi,x
    adc #>SCREEN         ; carry still valid from the low-byte add
    sta scrhi
    clc
    adc #$d4             ; COLRAM - SCREEN high byte distance
    sta colhi
    ldy #0
pt_loop:
    lda (txtlo),y
    sta (scrlo),y
    lda pcolor
    sta (collo),y
    iny
    cpy plen
    bne pt_loop
    rts

; ---- draw a text line with big 3x5 block letters ----
; txtlo/hi = text (screen codes), bigrow/bigcol = top-left cell,
; bigcolor = color, biglen = char count; each letter is 3 cells
; wide + 1 gap, 5 cells tall
draw_big:
    lda #0
    sta bigi
db_char:
    ldy bigi
    lda (txtlo),y
    cmp #$20             ; space: leave the cell empty
    beq db_next
    tax
    lda bigmap,x         ; screen code -> glyph index
    sta glyphbase
    asl                  ; glyph base = index * 5
    asl
    clc
    adc glyphbase
    sta glyphbase
    lda bigi             ; cell x = bigcol + charindex * 4
    asl
    asl
    clc
    adc bigcol
    sta charx
    lda #0
    sta rowidx
db_row:
    lda bigrow           ; row pointers from the row*40 tables
    clc
    adc rowidx
    tax
    lda mul40lo,x
    sta scrlo
    sta collo
    lda mul40hi,x
    clc
    adc #>SCREEN
    sta scrhi
    clc
    adc #$d4
    sta colhi
    lda glyphbase
    clc
    adc rowidx
    tax
    lda bigglyphs,x
    asl                  ; 3-bit pattern up to bits 7-5
    asl
    asl
    asl
    asl
    sta curbits
    ldy charx
    lda #3
    sta bitcnt
db_bit:
    asl curbits          ; leftmost pixel first
    bcc db_nobit
    lda #$a0             ; inverse space = solid block
    sta (scrlo),y
    lda bigcolor
    sta (collo),y
db_nobit:
    iny
    dec bitcnt
    bne db_bit
    inc rowidx
    lda rowidx
    cmp #5
    bne db_row
db_next:
    inc bigi
    lda bigi
    cmp biglen
    beq db_done
    jmp db_char          ; too far for a relative branch
db_done:
    rts

; ---- shared tables ----

; row * 40 lookup tables (25 rows)
mul40lo:
    !byte $00, $28, $50, $78, $a0, $c8, $f0, $18, $40, $68
    !byte $90, $b8, $e0, $08, $30, $58, $80, $a8, $d0, $f8
    !byte $20, $48, $70, $98, $c0
mul40hi:
    !byte $00, $00, $00, $00, $00, $00, $00, $01, $01, $01
    !byte $01, $01, $01, $02, $02, $02, $02, $02, $02, $02
    !byte $03, $03, $03, $03, $03

; 3x5 block font for the big slide titles (5 bytes per glyph,
; low 3 bits used, MSB side = left)
bigglyphs:
    !byte 0, 0, 0, 0, 0          ;  0 space
    !byte 2, 5, 7, 5, 5          ;  1 A
    !byte 3, 4, 4, 4, 3          ;  2 C
    !byte 6, 5, 5, 5, 6          ;  3 D
    !byte 7, 4, 6, 4, 7          ;  4 E
    !byte 7, 4, 6, 4, 4          ;  5 F
    !byte 7, 2, 2, 2, 7          ;  6 I
    !byte 4, 4, 4, 4, 7          ;  7 L
    !byte 5, 7, 7, 5, 5          ;  8 M
    !byte 6, 5, 5, 5, 5          ;  9 N
    !byte 7, 5, 5, 5, 7          ; 10 O
    !byte 6, 5, 6, 4, 4          ; 11 P
    !byte 6, 5, 6, 5, 5          ; 12 R
    !byte 3, 4, 2, 1, 6          ; 13 S
    !byte 7, 2, 2, 2, 2          ; 14 T
    !byte 5, 5, 2, 2, 2          ; 15 Y
    !byte 3, 4, 5, 5, 7          ; 16 G

bigmap:                  ; screen code (a=1..z=26) -> glyph index
    !byte  0             ; (unused)
    !byte  1,  0,  2,  3,  4,  5, 16,  0,  6,  0   ; a-j
    !byte  0,  7,  8,  9, 10, 11,  0, 12, 13, 14   ; k-t
    !byte  0,  0,  0,  0, 15,  0                   ; u-z

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

fadetbl:                 ; fade-to-black: each color -> one shade darker,
                         ;   every chain reaches black in at most 4 steps
                         ;   (e.g. white -> lgrey -> mgrey -> dgrey -> black)
    ;      blk  wht  red  cyn  pur  grn  blu  yel
    !byte $00, $0f, $09, $0e, $06, $0b, $00, $08
    ;      org  brn  lrd  dgr  mgr  lgn  lbl  lgr
    !byte $02, $00, $02, $00, $0b, $05, $06, $0c

; ---- shared variables ----
framectr:  !byte 0       ; frame counter, driven by the slide updates
washphase: !byte 0       ; gold wash offset (slides 1 and 2)
curbits:   !byte 0       ; current bit pattern in the letter renderers
prow:      !byte 0       ; print_text: row, column, color, length
pcol:      !byte 0
pcolor:    !byte 0
plen:      !byte 0
bigrow:    !byte 0       ; draw_big: row, column, color, length
bigcol:    !byte 0
bigcolor:  !byte 0
biglen:    !byte 0
bigi:      !byte 0       ; draw_big internals
glyphbase: !byte 0
charx:     !byte 0
rowidx:    !byte 0
bitcnt:    !byte 0