C64-dev/examples/bitmap-demo.asm

; ============================================================
; C64 HI-RES BITMAP DEMO
; Switches to 320x200 hi-res bitmap mode, draws a diagonal
; line, then waits for a key before restoring text mode.
;
; Assemble to $C000 and run with:
;   LOAD"BITMAP",8,1 : SYS 49152
;
; Bitmap: $A000-$BFFF (8000 bytes, under BASIC ROM)
; Screen color RAM: $0400-$07E7 (1000 bytes)
; ============================================================

        * = $C000

; --- Zero-page pointers ---
PTR     = $FB       ; 2-byte pointer
BPTR    = $FD       ; 2-byte bitmap pointer

; --- Constants ---
BITMAP  = $A000     ; bitmap base
SCREEN  = $0400     ; color nybble RAM (foreground hi/background lo)

START:
        JSR INIT_BITMAP
        JSR DRAW_DIAGONAL
        JSR WAIT_KEY
        JSR RESTORE_TEXT
        RTS

; ============================================================
; INIT_BITMAP
; Sets up VIC-II for hi-res bitmap mode
; ============================================================
INIT_BITMAP:
        ; Turn off interrupts temporarily
        SEI

        ; Set VIC bank to 0 ($0000-$3FFF is default bank)
        ; Bitmap at $A000 requires bank 2 ($8000-$BFFF) — switch!
        LDA $DD00
        AND #$FC        ; clear bits 0-1
        ORA #$01        ; bank 2: VIC sees $8000-$BFFF
        STA $DD00

        ; VIC memory setup: $D018
        ; Bitmap at $A000 = offset $2000 from bank base $8000
        ; $2000 >> 13 = bit 3 set → bitmap pointer bit = 1
        ; Screen RAM at $8400 (color nybbles) — offset $0400 >> 10 = $0 → bits 7-4 = 0001
        LDA #%00011000  ; screen at $8400 (bits 7-4 = 0001), bitmap at $A000 (bit 3)
        STA $D018

        ; Enable bitmap mode: $D011 bit 5 = 1
        LDA $D011
        ORA #%00100000
        STA $D011

        ; Multicolor off (hi-res): $D016 bit 4 = 0
        LDA $D016
        AND #%11101111
        STA $D016

        ; Set border color to black
        LDA #0
        STA $D020

        ; Set background color to black
        STA $D021

        ; Clear bitmap (8000 bytes of zeros = black pixels)
        LDA #<BITMAP
        STA PTR
        LDA #>BITMAP
        STA PTR+1
        LDA #0
        LDY #0
        LDX #$1F        ; 32 pages = 8192 bytes (covers 8000)
CLRLOOP:
        STA (PTR),Y
        INY
        BNE CLRLOOP
        INC PTR+1
        DEX
        BNE CLRLOOP

        ; Set all color RAM nybbles: foreground=white(1), background=black(0)
        ; Each byte = $10 = white fg, black bg
        LDA #<SCREEN
        STA PTR
        LDA #>SCREEN
        STA PTR+1
        LDA #$10
        LDY #0
COLLOOP:
        STA (PTR),Y
        INY
        CPY #232        ; 1000 - 8 = 232? No: LDY loops 0-255 then second pass
        BNE COLLOOP
        ; Handle full 1000 bytes (4 pages)
        ; Simple approach: just fill $0400-$07E7
        LDX #3
        LDY #0
COLLOOP2:
        STA (PTR),Y
        INY
        BNE COLLOOP2
        INC PTR+1
        DEX
        BNE COLLOOP2

        CLI
        RTS

; ============================================================
; DRAW_DIAGONAL
; Draws a white diagonal line from (0,0) to (199,199)
; In hi-res bitmap, pixel (x,y) is in:
;   byte = bitmap + (y AND $F8)*40 + (x AND $F8) + (y AND 7)
;   bit  = 7 - (x AND 7)
; ============================================================
DRAW_DIAGONAL:
        LDX #0          ; X pixel (0-199)
DLOOP:
        TXA             ; Y = X (diagonal)
        JSR SETPIXEL
        INX
        CPX #200
        BNE DLOOP
        RTS

; ============================================================
; SETPIXEL — set pixel (.X=x, .A=y)
; ============================================================
SETPIXEL:
        ; Calculate byte address
        ; addr = $A000 + (Y>>3)*320 + (X>>3)*8 + (Y AND 7)
        ; Uses simple approach via table lookup or shift math
        STX XCOORD
        STA YCOORD

        ; Step 1: row base = (Y>>3) * 320
        ; Y>>3 * 320 = Y>>3 * 256 + Y>>3 * 64
        LSR             ; Y>>1
        LSR             ; Y>>2
        LSR             ; Y>>3  (char row 0-24)
        TAX             ; X = row number
        LDA ROWLO,X     ; low byte of row base
        STA BPTR
        LDA ROWHI,X
        STA BPTR+1

        ; Step 2: add (X>>3)*8 = column offset
        LDA XCOORD
        LSR             ; X>>1
        LSR             ; X>>2
        LSR             ; X>>3  (char col 0-39)
        ASL             ; *2
        ASL             ; *4
        ASL             ; *8
        CLC
        ADC BPTR
        STA BPTR
        BCC NOINC1
        INC BPTR+1
NOINC1:
        ; Step 3: add (Y AND 7) for sub-row
        LDA YCOORD
        AND #$07
        CLC
        ADC BPTR
        STA BPTR
        BCC NOINC2
        INC BPTR+1
NOINC2:
        ; Step 4: add bitmap base $A000
        LDA BPTR+1
        CLC
        ADC #>BITMAP
        STA BPTR+1
        ; (low byte already correct)

        ; Step 5: set bit (7 - (X AND 7))
        LDA XCOORD
        AND #$07
        TAX
        LDA BITMASK,X
        LDY #0
        ORA (BPTR),Y
        STA (BPTR),Y
        RTS

XCOORD: .BYTE 0
YCOORD: .BYTE 0

; Bit masks for bit positions 7..0
BITMASK:
        .BYTE %10000000  ; bit 7 (x mod 8 = 0)
        .BYTE %01000000
        .BYTE %00100000
        .BYTE %00010000
        .BYTE %00001000
        .BYTE %00000100
        .BYTE %00000010
        .BYTE %00000001  ; bit 0 (x mod 8 = 7)

; Row base address table (40*8 = 320 bytes per char row)
; Row N base = N * 320; precomputed for rows 0-24
ROWLO:
        .BYTE <(0*320),<(1*320),<(2*320),<(3*320),<(4*320)
        .BYTE <(5*320),<(6*320),<(7*320),<(8*320),<(9*320)
        .BYTE <(10*320),<(11*320),<(12*320),<(13*320),<(14*320)
        .BYTE <(15*320),<(16*320),<(17*320),<(18*320),<(19*320)
        .BYTE <(20*320),<(21*320),<(22*320),<(23*320),<(24*320)
ROWHI:
        .BYTE >(0*320),>(1*320),>(2*320),>(3*320),>(4*320)
        .BYTE >(5*320),>(6*320),>(7*320),>(8*320),>(9*320)
        .BYTE >(10*320),>(11*320),>(12*320),>(13*320),>(14*320)
        .BYTE >(15*320),>(16*320),>(17*320),>(18*320),>(19*320)
        .BYTE >(20*320),>(21*320),>(22*320),>(23*320),>(24*320)

; ============================================================
; WAIT_KEY — spin until any key pressed
; ============================================================
WAIT_KEY:
        JSR $FFE4       ; GETIN
        BEQ WAIT_KEY
        RTS

; ============================================================
; RESTORE_TEXT — return to standard text mode
; ============================================================
RESTORE_TEXT:
        ; Restore VIC bank 0
        LDA $DD00
        ORA #$03
        STA $DD00

        ; Restore VIC memory map ($D018)
        LDA #$15        ; screen at $0400, charset at $1000
        STA $D018

        ; Turn off bitmap mode
        LDA $D011
        AND #%11011111
        STA $D011

        ; Restore background/border
        LDA #14         ; light blue border (default)
        STA $D020
        LDA #6          ; blue background (default)
        STA $D021

        ; Clear screen
        LDA #147        ; PETSCII clear
        JSR $FFD2

        RTS